In The Name of God, The Merciful, The Compassionate

In The Name of God, The Merciful, The Compassionate
Advanced Computer Networks Department of Computer Engineering Sharif University of Technology – Kish Campus Fall 2007 – CE 693 Dr. Hamid R. Rabiee Background Information Overview of Computer Networks

Introduction Basic concepts Terminology

Ubiquitous Computing Computers everywhere.
Also means ubiquitous communication Users connected anywhere/anytime. PC (laptop, palmtop) equivalent to cell phone. Networking computers together is critical!

Computer Network Provide access to local and remote resources.
Collection of interconnected end systems: Computing devices (mainframes, workstations, PCs, palm tops) Peripherals (printers, scanners, terminals). Applications: location transparency.

Computer Networks (cont’d)
Components: End systems (or hosts), Routers/switches/bridges, and Links (twisted pair, coaxial cable, fiber, radio, etc.).

Communication Model Network Source Destination

Example PTN Modem Modem Source Destination Source System
Destination System PTN: Public Telephone Network

Connecting End Systems
Dedicated link Multiple access / shared medium

Connecting End Systems (cont’d)
Router Switched network Router: switching element; a.k.a., IMPs (Interface Message Processors) in ARPAnet’s terminology.

Shared Communication Infrastructure
Shared medium: Examples: ethernet, radio. How to acquire channel: medium access control protocols. Switched networks: Shared infrastructure consisting of point-to-point links. Circuit- versus packet-switching.

Circuit Switching Establish dedicated path (circuit) between source and destination. Example: telephone network. +’s: dedicated resources(stream-oriented). -’s: lower resource utilization (e.g.,bursts).

Packet Switching S1 D1 S2 D2 Data split into transmission units, or packets. Routers: store packets briefly store packets and forward them: store-and-forward. Efficient resource use: statistical multiplexing. Ability to accommodate bursts.

(Switched) Network Topologies
Ring Tree Star Irregular

Protocol Set of rules that allow peering entities to communicate.
Example: 2 friends talking on the phone. Peering entities or peers: user application programs, file transfer services, services, etc.

Network Architecture Protocol layers: reduce design complexity.
Main idea: each layer uses the services from lower layer and provide services to upper layer. Higher layer shielded from the implementation details of lower layers. Interface between layers must be clearly defined: services provided to upper layer.

Example 1: ISO OSI Model ISO: International Standards Organization
OSI: Open Systems Interconnection. Application Presentation Session Transport Network Data link Physical

OSI ISO 7-Layer Model Physical layer: transmission of bits.
Data link layer: reliable transmission over physical medium; synchronization, error control, flow control; media access in shared medium. Network layer: routing and forwarding; congestion control; internetworking.

OSI ISO 7-Layer Model (cont’d)
Transport layer: error, flow, and congestion control end-to-end. Session layer: manages connections (sessions) between end points. Presentation layer: data representation. Application layer: provides users with access to the underlying communication infrastructure.

Example 2: TCP/IP Model Model employed by the Internet. TCP/IP
Application ISO OSI Application Presentation Session Transport Transport Internet Network Network Access Data link Physical Physical

TCP/IP Protocol Suite:
Physical layer: same as OSI ISO model. Network access layer: medium access and routing over single network. Internet layer: routing across multiple networks, or, an internet. Transport layer: end-to-end error, congestion, flow control functions. Application layer: same as OSI ISO model.

The Internet: Some History
Late 1970’s/ early 1980’s: the ARPANET (funded by ARPA). Connecting university, research labs and some government agencies. Main applications: and file transfer. Features: Decentralized, non-regulated system. No centralized authority. No structure. Network of networks.

The Internet (cont’d) Early 1990’s, the Web caused the Internet revolution: the Internet’s killer app! Today: Almost 60 million hosts as of Doubles every year.

Topics for Further Reading
Some Internet governing entities: IAB IETF IRTF The Internet’s standardization process. Other network standardization bodies. Other networks (Bitnet, SNA, etc).

Physical Layer Sending raw bits across “the wire”. Issues:
What’s being transmitted. Transmission medium.

Basic Concepts Signal: electro-magnetic wave carrying information.
Time domain: signal as a function of time. Analog signal: signal’s amplitude varies continuously over time, ie, no discontinuities. Digital signal: data represented by sequence of 0’s and 1’s (e.g., square wave).

Time Domain Periodic signals: Same signal pattern repeats over time.
Example: sine wave Amplitude (A) Period (or frequency) (T = 1/f) Phase(f)

Frequency Domain Signal consists of components of different frequencies. Spectrum of signal: range of frequencies signal contains. Absolute bandwidth: width of signal’s spectrum.

Example: Spectrum of S(f) extends from f1 to 3f1. Bandwidth is 2f1.

Bandwidth and Data Rate
Data rate: rate at which data is transmitted; unit is bits/sec or bps (applies to digital signal). Example: 2Mbits/sec, or 2Mbps. Digital signal has infinite frequency components, thus infinite bandwidth. If data rate of signal is W bps, good representation achieved with 2W Hz bandwidth.

Baud versus Data Rate Baud rate: number of times per second signal changes its value (voltage). Each value might “carry” more than 1 bit. Example: 8 values of voltage (0..7); each value conveys 3 bits, ie, number of bits = log2V. Thus, bit rate = log2V * baud rate. For 2 levels, bit rate = baud rate.

Data Transmission 1 Analog and digital transmission.
Example of analog data: voice and video. Example of digital data: character strings Use of codes to represent characters as sequence of bits (e.g., ASCII). Historically, communication infrastructure for analog transmission. Digital data needed to be converted: modems (modulator-demodulator).

Digital Transmission Current trend: digital transmission. Advantages:
Cost efficient: advances in digital circuitry (VLSI). Advantages: Data integrity: better noise immunity. Security: easier to integrate encryption algorithms. Channel utilization: higher degree of multiplexing (time-division mux’ing).

Transmission Impairments
Cause received signal to differ from original, transmitted signal. Analog data: quality degradation Digital data: bit errors. Types of impairments: Attenuation. Delay distortion. Noise.

Attenuation 1 Weakening of the signal’s power as it propagates through medium. Function of medium type Guided medium: logarithmic with distance. Unguided medium: more complex (function of distance and atmospheric conditions).

Attenuation 2 Problems and solutions:
Insufficient signal strength for receiver to interpret it: use amplifiers/repeaters to boost/regenerate signal. Error due to noise interference (level is not high enough to be distinguished from noise): use amplifiers/repeaters. Attenuation increases with frequency: special amplifiers to amplify high-frequencies.

Delay Distortion Speed of propagation in guided media varies with frequency. Different frequency components arrive at receiver at different times. Solution: equalization techniques to equalize distortion for different frequencies.

Noise Noise: undesired signals inserted anywhere in the source/destination path. Different categories: thermal (white), crosstalk, impulse, etc.

Decibel and Signal-to-Noise Ratio
Decibel (dB): measures relative strength of 2 signals. Example: S1 and S2 with powers P1 and P2. NdB = 10 log10 (P1/P2) Signal-to-noise ratio (S/N): Measures signal quality. S/NdB = 10 log10 (signal power/noise power)

Channel Capacity 1 Rate at which data can be transmitted over communication channel. Noise-free channel: Nyquist Theorem Limitation of data rate is signal’s bandwidth. Given channel bandwidth W, highest signal rate (or baud rate) is 2W. From receiver’s point of view: sampling at rate 2W can reconstruct signal.

Channel Capacity 2 Using data rate,
C = 2W log2V, where V is number voltage levels. Same bandwidth, increasing number of signal levels, increases data rate, but more complex signal recognition at receiver and more noise-prone. This is a theoretical upper bound, since channels are noisy.

Channel Capacity 3 Noisy channel: Shannon’s Theorem
Given channel with W (Hz) bandwidth and S/N (dB) signal-to-noise ratio, C (bps) is C = W log2 (1+S/N) Theoretical upper bound since assumes only thermal noise (no impulse noise, etc).

Transmission Media Physically connect transmitter and receiver carrying signals in the form electromagnetic waves. Types of media: Guided: waves guided along solid medium such as copper twisted pair, coaxial cable, optical fiber. Unguided: “wireless” transmission (atmosphere, outer space).

Guided Media: Examples 1
Twisted Pair: 2 insulated copper wires arranged in regular spiral. Typically, several of these pairs are bundled into a cable. Cheapest and most widely used; limited in distance, bandwidth, and data rate. Applications: telephone system (home-local exchange connection). Unshielded and shielded twisted pair.

Examples 2 Coaxial Cable
Hollow outer cylinder conductor surrounding inner wire conductor; dielectric (non-conducting) material in the middle. Applications: cable TV, long-distance telephone system, LANs. +’s: Higher data rates and frequencies, better interference and crosstalk immunity. -’s: Attenuation and thermal noise.

Examples 3 Optical Fiber
Thin, flexible cable that conducts optical waves. Applications: long-distance telecommunications, LANs. +’s: greater capacity, smaller and lighter, lower attenuation, better isolation,

Unguided, Wireless Media
Microwave: directional, LOS transmission. Satellite: directional, LOS, large delay, high bandwidth. Radio: omnidirectional (broadcast), single hop (cellular), multi-hop (ad hoc net’s). Infrared: directional, LOS transmission, cannot penetrate obstacles and used outdoors.

Data Encoding Transforming original signal just before transmission.
Both analog and digital data can be encoded into either analog or digital signals.

Digital/Analog Encoding
g(t) g(t) (D/A) Encoder Digital Medium Decoder Source Destination Source System Destination System Modulation: g(t) g(t) (D/A) Modulator Analog Medium Demodulator Source Destination Source System Destination System

Encoding Considerations
Digital signaling can use modern digital transmission infrastructure. Some media like fiber and unguided media only carry analog signals. Analog-to-analog conversion used to shift signal to use another portion of spectrum for better channel utilization (frequency division mux’ing).

Digital Transmission Terminology
Data element: bit. Signaling element: encoding of data element for transmission. Unipolar signaling: signaling elements have same polarization (all + or all -). Polar signaling: different polarization for different elements.

More Terminology Data rate: rate in bps at which data is transmitted; for data rate of R, bit duration (time to emit 1 bit) is 1/R sec. Modulation rate = baud rate (rate at which signal levels change).

Digital Transmission: Receiver-Side Issues
Clocking: determining the beginning and end of each bit. Transmitting long sequences of 0’s or 1’s can cause synchronization problems. Signal level: determining whether the signal represents the high (logic 1) or low (logic 0) levels. S/N ratio is a factor.

Comparing Digital Encoding Techniques
Signal spectrum: high frequency means high bandwidth required for transmission. Clocking: transmitted signal should be self-clocking. Error detection: built in the encoding scheme. Noise immunity: low bit error rate.

Digital-to-Digital Encoding Techniques
Nonreturn to Zero (NRZ) Multilevel Binary Biphase Scrambling

NRZ Techniques Use of 2 different voltage levels.
NRZ-L: positive voltage represents one binary value; negative voltage, the other. NRZI (Nonreturn to zero, invert on ones): transition (low-to-high or high-to-low) represents “1”; no transition, “0”. NRZI is an example of differential encoding: decoding based on comparing polarity of adjacent signal elements.

Multilevel Binary Use more than 2 signal levels.
Bipolar-AMI: “0”: no signal; “1”: positive and negative pulse; consecutive “1”s alternate in polarity: avoid synchronization loss. Pseudoternary: opposite representation. Long sequence of 0’s or 1’s still a problem for bipolar-AMI and pseudoternary respectively.

Biphase Manchester: transition in the middle of bit period.
Carries data and provides clocking. Low-to-high: “1”. High-to-low: “0”. Differential Manchester: Mid-bit transition only provides clocking. “0”: transition in the beginning of bit interval. “1”: no transition.

Scrambling Avoid long sequences of 0’s or 1’s.
Bipolar with 8-zeros substitution (B8ZS) Inserts transitions when transmitting 8 consecutive “0”s. High-density bipolar-3 zeros (HDB3) Inserts pulses when transmitting 4 consecutive “0”s. Receiver must recognize insertions and re-generate original signal.

Digital-to-Analog Encoding
Transmission of digital data using analog signaling. Example: data transmission of a PTN. PTN: voice signals ranging from 300Hz to 3400 Hz. Modems: convert digital data to analog signals and back. Techniques: ASK, FSK, and PSK.

Amplitude-Shift Keying
2 binary values represented by 2 amplitudes. Typically, “0” represented by absence of carrier and “1” by presence of carrier. Prone to errors caused by amplitude changes.

Frequency-Shift Keying
2 binary values represented by 2 frequencies. Frequencies f1 and f2 are offset from carrier frequency by same amount in opposite directions. Less error prone than ASK.

Phase-Shift Keying Phase of carrier is shifted to represent data.
Example: 2-phase system. Phase shift of 90o can represent more bits: aka, quadrature PSK.

Analog-to-Digital Encoding
Analog data transmitted as digital signal, or digitization. Codec: device used to encode and decode analog data into digital signal, and back. 2 main techniques: Pulse code modulation (PCM). Delta modulation (DM).

Pulse Code Modulation 1 Based on Nyquist (or sampling) theorem: if f(t) sampled at rate > 2*signal’s highest frequency, then samples contain all the original signal’s information. Example: if voice data is limited to 4000Hz, 8000 samples/sec are sufficient to reconstruct original signal.

PCM 2 Analog signal -> PAM -> PCM.
PAM: pulse amplitude modulation; samples of original analog signal. PCM: quantization of PAM pulses; amplitude of PAM pulses approximated by n-bit integer; each pulse carries n bits.

Delta Modulation (DM) Analog signal approximated by staircase function moving up or down by 1 quantization level every sampling interval. Bit stream produced based on derivative of analog signal (and not its amplitude): “1” if staircase goes up, “0” otherwise. Parameters: sampling rate and step size.

Analog-to-Analog Encoding
Combines input signal m(t) and carrier at fc producing s(t) centered at fc. Why modulate analog data? Shift signal’s frequency for effective transmission. Allows channel multiplexing: frequency-division multiplexing. Modulation techniques: AM, FM, and PM.

Amplitude Modulation (AM)
Carrier serves as envelope to signal being modulated. Signal m(t) is being modulated by carrier cos(2p fct). Modulation index: ratio between amplitude of input signal to carrier.

Angle Modulation FM and PM are special cases of angle modulation.
FM: carrier’s amplitude kept constant while its frequency is varied according to message signal. PM: carrier’s phase varies linearly with modulating signal m(t).

Spread Spectrum 1 Used to transmit analog or digital data using analog signaling. Spread information signal over wider spectrum to make jamming and eavesdropping more difficult. Popular in wireless communications

Spread Spectrum 2 2 schemes:
Frequency hopping: signal broadcast over random sequence of frequencies, hoping from one frequency to the next rapidly; receiver must do the same. Direct Sequence: each bit in original signal represented by series of bits in the transmitted signal.

Transmission Modes Assuming serial transmission, ie, one signaling element sent at a time. Also assuming that 1 signaling element represents 1 bit. Source and receiver must be in sync. 2 schemes: asynchronous and synchronous transmission.

Asynchronous Xmission 1
Avoid synchronization problem by including sync information explicitly. Character consists of a fixed number of bits, depending on the code used. Synchronization happens for every character: start (“0”) and stop (“1”) bits. Line is idle: transmits “1”.

Asynchronous Xmission 2
Example: sending “ABC” in ASCII … Timing requirements are not strict. But problems may occur. Significant clock drifts + high data rate = reception errors. Also, 2 or more bits for synchronization: overhead!

Synchronous Xmission 1 No start or stop bits. Synchronization via:
Separate clock signal provided by transmitter or receiver; doesn’t work well over long distances. Embed clocking information in data signal using appropriate encoding technique such as Manchester or Differential Manchester.

Synchronous Xmission 2 Need to identify start/end of data block.
Block starts with preamble (8-bit flag) and may end with postamble. Other control information may be added for data link layer. 8 -bit flag 8 -bit flag Control Data Control

Data Link Layer So far, sending signals over transmission medium.
Data link layer: responsible for error-free (reliable) communication between adjacent nodes. Functions: framing, error control, flow control, addressing (in multipoint medium).

Flow Control What is it? Ensures that transmitter does not overrun receiver: limited receiver buffer space. Receiver buffers data to process before passing it up. If no flow control, receiver buffers may fill up and data may get dropped.

Stop-and-Wait Simplest form of flow control. Good when few frames.
Transmitter sends frame and waits. Receiver receives frame and sends ACK. Transmitter gets ACK, sends other frame, and waits, until no more frames to send. Good when few frames. Problem: inefficient link utilization. In the case of high data rates or long propagation delays.

Sliding Window 1 Allows multiple frames to be in transit at the same time. Receiver allocates buffer space for n frames. Transmitter is allowed to send n (window size) frames without receiving ACK. Frame sequence number: labels frames.

Sliding Window 2 Receiver ack’s frame by including sequence number of next expected frame. Cumulative ACK: ack’s multiple frames. Example: if receiver receives frames 2,3, and 4, it sends an ACK with sequence number 5, which ack’s receipt of 2, 3, and 4.

Sliding Window 3 Sender maintains sequence numbers it’s allowed to send; receiver maintains sequence number it can receive. These lists are sender and receiver windows. Sequence numbers are bounded; if frame reserves k-bit field for sequence numbers, then they can range from 0 … 2k -1 and are modulo 2k.

Sliding Window 4 Transmission window shrinks each time frame is sent, and grows each time an ACK is received.

Example: 3-bit sequence number and window size 7
A B 1 2 RR3 3 4 5 RR4 6

Sliding Window (cont’d)
RR n acknowledges up to frame n-1. There is also RNR n, which ack’s up to frame n-1 but no longer accepts more frames. RNR shuts down the receive window and consequently the transmission window. Need subsequent RR to re-open window.

Piggybacking When both endpoints transmit, each keeps 2 windows, transmitter and receiver windows. Each send data and need to send ACKs. When sending data, transmitter can “piggyback” the acknowledgment information. When no data, send just the ACK.

Duplicate ACKs When no data, must re-send last ACK.
Duplicate ACKs: report potential errors.

Error Detection Transmission impairments lead to transmission errors: change of 1 or more bits in transmitted frame. Transmission errors defined using probabilities: transmission medium modeled as a statistical system.

Error Probabilities 1 Definitions:
Pb probability of single bit error (bit error rate); constant and independent for each bit. P1 probability frame received with no errors. P2 probability frame received with 1 or more undetected errors. P3 probability frame received with 1 or more detected bit errors, but no undetected ones.

Error Probabilities 2 If no error detection mechanism, P3 = 0.
P1 = (1 - Pb)F and P2 = (1- P1), where F is size of frame in bits. P1 decreases as Pb increases. P1 decreases as F increases.

Example 64-kbps ISDN channel’s bit error rate is less than User requirement of at most 1 frame with undetected bit error per day. Frame is 1000 bits. In a day, x 106 frames transmitted. Required frame error rate of 1/ x 106, or P2 = 0.18 x 10-6. But Pb = 10-6, so P1 = (1-Pb)F = and P2 = 1 - P1 = 10-3, which is >>> required P2

Error Detection Schemes
Transmitter adds additional bits for error detection. Transmitter computes error detection bits as function of original data. Receiver performs same calculation and compares results. If mismatch, then error. P3 probability error detection scheme detects error; P2 residual error rate or probability error goes undetected.

Parity Simplest error detection scheme.
Append parity bit to data block. Example: ASCII transmission 1 parity bit appended to each 7-bit ASCII character. Even parity: 8-bit code has even number of 1’s. Odd parity: 8-bit code has odd number of 1’s.

Parity Check Example: transmitting ASCII “G” ( ) using odd parity. Code transmitted is Receiver checks received code and if odd number of 1’s, assumes no error. Suppose it receives , then detects error. NOTE: If more than 2 bits in error, may not be detected.

Cyclic Redundancy Check
CRC is one of the most effective and common error detecting schemes. Let M be m-bit message, G (r+1)-bit pattern. Transmitter appends r 0’s to M, 2r*M. Divide 2r*M by G and add remainder to 2r*M forming T (m+r bits), which is transmitted. Receiver computes T/G; if remainder, then error.

CRC Example Frame M 1010001101 = x9+x7+x3+x2+x0. Pattern G 110101.
Dividing (frame*25) by pattern results in Thus T Receiver can detect errors unless received message Tr is divisible by G.

CRC Patterns are expressed as polynomials G(x). Example:
CRC-16 = x16+x15+x2+1 CRC-CCITT = X16+x12+x5+1

CRC-Based Detection If suitably selected polynomials, CRC can detect:
All single-bit errors. All double-bit errors, as long as P(X) has at least three 1’s. Any odd number of errors as long as P(X) contains factor (X+1). Any burst error whose length is <= sizeof(FCS).

Error Control Mechanisms to detect and correct transmission errors.
Consider 2 types of errors: Lost frame: frame is sent but never arrives. Damaged frame: frame arrives but in error. Error control: combination of error detection, feedback (ACK or NACK) from receiver, and retransmission by source. Coupled with flow control feedback.

ARQ ARQ: automatic repeat request.
Works by creating a reliable data link from an unreliable one. 3 versions: Stop-and-wait ARQ. Go-back-N ARQ. Selective-reject ARQ.

Stop-and-Wait ARQ Single outstanding frame at any time.
Simple but inefficient. Use of timers to trigger retransmission of data or ACKs. 2 types of errors: Damaged or lost frame. Damaged or lost ACK. Sequence numbers alternate between 0 and 1.

Stop-and-Wait ARQ: Example
Sender Frame 0 Receiver ACK1 Frame 1 ACK 0 Frame 0 Timeout Frame 0 Timeout ACK 1 Frame 0 B discards duplicate. ACK 1

Go-Back-N ARQ Variation of sliding window for error control.
Allows a window’s worth of frames to be in transit at any time. RR: ack’s receipt of frame. REJ: negative acknowledgment indicating the frame in error. Destination discards frame in error plus subsequent frames.

Go-Back-N ARQ Example S f0 R S R f7 f1 f0 f2 Time out rr0 rr3 f1 f3
rr(P bit =1) f4 rr4 f5 rr2 f6 Error f2 f7 rej5 Discarded f5 5, 6, 7 rexm. f6 rr6 f7

Go-Back-N ARQ Issues For k-bit sequence number, maximum window size is (2k-1). If window size is too large, ACKs may be ambiguous: not clear if ACK is a duplicate ACK (errors occurred). Example: 3-bit sequence number and 8 -frame window. Source transmits f0, gets back rr1, then sends f1--f0, and gets back another rr1. ???

Selective-Reject ARQ Only frames transmitted are the ones that are NACK’ed (SREJ) or that timeout. More efficient than Go-Back-N regarding amount of reXmissions. But, receiver must buffer out-of-order frames. More restriction on maximum window size; for k-bit sequence #’s, 2k-1 window.

Example Data Link Layer Protocol
High-Level Data Link Control (HDLC) Widely-used (ISO standard). Single frame format. Synchronous transmission.

HDLC: Frame Format Flag: frame delimiters (01111110).
Address field for multipoint links. 16-bit or 32-bit CRC. Refer to book (pages ) for more details. address flag flag control data FCS 8 bits 8 ext. 8 or 16 8 variable 16 or 32

Other DLL Protocols 1 LAPB: Link Access Procedure, Balanced.
Part of the X.25 standard. Subset of HDLC. Link between user system and switch. Same frame format as HDLC. LAPD: Link Access Procedure, D-Channel. Part of the ISDN standard.

Other DLL Protocols 2 LLC: Logical Link Control.
Part of the 802 protocol family for LANs. Link control functions divided between the MAC layer and the LLC layer. LLC layer operates on top of MAC layer. Dst. MAC addr Src. MAC addr LLC ctl. Dst. LLC addr Src. LLC addr MAC control Data FCS

Other DLL Protocols 3 SLIP: Serial Line IP
Dial-up protocol. No error control. Not standardized. PPP: Point-to-Point Protocol Internet standard for dial-up connections. Provides framing similar to HDLC.

Multiplexing Sharing a link/channel among multiple source-destination pairs. Example: high-capacity long-distance trunks (fiber, microwave links) carry multiple connections at the same time. MUX DEMUX . . . . . .

Multiplexing Techniques
3 basic types: Frequency-Division Multiplexing (FDM). Time-Division Multiplexing (TDM). Statistical Time-Division Multiplexing (STDM).

FDM 1 High bandwidth medium when compared to signals to be transmitted. Widely used (e.g., TV, radio). Various signals carried simultaneously where each one modulated onto different carrier frequency, or channel. Channels separated by guard bands (unused) to prevent interference.

TDM 1 TDM or synchronous TDM.
High data rate medium when compared to signals to be transmitted.

TDM 2 Time divided into time slots.
Frame consists of cycle of time slots. In each frame, 1 or more slots assigned to a data source. U1 U2 ... UN 1 2 ... N 1 2 ... N Time frame

TDM 3 No control info at this level. Flow and error control?
To be provided on a per-channel basis. Use DLL protocol such as HDLC. Examples: SONET (Synchronous Optical Network) for optical fiber. +’s: simple, fair. -’s: inefficient.

Statistical TDM 1 Or asynchronous TDM.
Dynamically allocates time slots on demand. N input lines in statistical multiplexer, but only k slots on TDM frame, where k < n. Multiplexer scans input lines collecting data until frame is filled. Demultiplexer receives frame and distributes data accordingly.

STDM 2 Data rate on mux’ed line < sum of data rates from all input lines. Can support more devices than TDM using same link. Problem: peak periods. Solution: multiplexers have some buffering capacity to hold excess data. Tradeoff data rate and buffer size (response time).

Local Area Networks 1 Interconnect devices over short distances.
Within same floor, Building, Campus. Characterized by low delays.

LANs 2 Typically use broadcast medium. LANs are characterized by:
Hosts share same communication medium. Also called multiple-access networks. LANs are characterized by: Topology. Transmission medium. Medium access control mechanism.

LAN Protocol Architecture
LAN protocol standards collectively known as IEEE 802 reference model. Application OSI Upper layer protocols Presentation Session Transport IEEE 802 Network LLC Data link MAC Physical Physical

LAN Protocols MAC sublayer: performs functions that control access to shared medium. LLC: performs flow and error control and provides services to upper layer.

802 standards 1 Text book page 367. LLC: IEEE 802.2
connectionless and connection oriented services. Reliable and unreliable.

802 standards 2 MAC + physical layers 802.3 802.5 802.4 FDDI 802.11
Bus/tree/star topologies Ring topology. CSMA/CD Token ring. FDDI Bus/tree/star topologies. Dual bus (optical). Token bus. Token ring. 802.11 Wireless. CSMA.

Encapsulation Application data TCP header IP header LLC header MAC MAC
trailer TCP segment IP datagram LLC PDU MAC frame

MAC Frame Format Dst. MAC addr Dst. LLC addr Src. MAC addr Src. LLC
control CRC LLC PDU MAC control: protocol information (protocol type, version #). Destination MAC address: physical address of LAN destination. Source MAC address: physical address of the LAN source.

LAN Topologies Star Ring Tree Central node Bus

Bus Topology Use of multipoint medium.
Stations attach to bus through tap. Full-duplex communication allows data to be sent to/received from bus. Transmission from any station propagates in both directions and is received by all. At each end, terminator absorbs and removes signal from bus.

Tree Topology Tree is generalization of bus.
Headend: start of 1 or more cables (branches). Transmission from one station propagates to all others.

Issues Inherently, broadcast. Multi-access. Frames to transmit data.
Need for specifying the destination. Addresses. Multi-access. Need for controlling access to medium. Avoid collisions. MAC protocol.

Ring Topology 1 Stations attach to repeaters.
Repeaters are linked to each other by point-to-point links forming a closed loop. Links are unidirectional. Repeaters: receive data from one link and repeat it on the other with no buffering.

Ring 2 Stations transmit/receive via repeater.
Frames circulate past all stations; destination copies frame as it goes by; source removes frame. Ring shared by multiple stations. Need MAC protocol. Determine when each station may insert frame.

Star Topology Each station directly connected to central node via point-to-point link. Central node’s modes of operation: Broadcast mode: node broadcasts received frame on all other links; logically works like bus. Switching mode: node sends frame out only on the link to the destination. Central node as single-point of failure.

Medium Access Control Control access to shared medium. Where and how?
Where: centralized versus decentralized. How: synchronous versus asynchronous.

Centralized versus Distributed MAC
Centralized approaches: Controller grants access to medium. Simple, greater control: priorities, qos. But, single point of failure and performance bottleneck. Decentralized schemes: All stations collectively run MAC to decide when to transmit.

Synchronous versus Asynchronous
Synchronous approaches: Static channel allocation. Examples: FDM, TDM. Simple but inefficient. Asynchronous or dynamic: Example: STDM. 3 categories: round-robin, reservation, and contention.

Round-Robin MAC Each station is allowed to transmit; station may decline or transmit (bounded by some maximum transmit time). Centralized (e.g., polling) or distributed control of who is next to transmit. When done, station relinquishes and right to transmit goes to next station. Efficient when many stations have data to transmit over extended period (stream).

Reservation Time divided into slots.
Station reserves slots in the future. Multiple slots for extended transmissions. Suited to stream traffic.

Contention No control. Stations try to acquire the medium.
Distributed in nature. Perform well for bursty traffic. Can get very inefficient under heavy load. NOTE: round-robin and contention are the most common.

Standardized MACs Topologies Bus Ring Techniques Token bus Token ring
(802.4) Polling (802.11) Token ring (802.5; FDDI) Round robin Reservation DQDB (802.6) Contention CSMA/CD (802.3) CSMA(802.11)

LLC for LANs Similar functions as general LLCs.
But it has to interface with MAC sublayer. LLC functions: Addressing: source and destination. LLC address versus MAC address. Control data exchange between 2 users. User as higher-layer protocol in the station.

LLC Services 3 different services:
Unacknowledged connectionless (type 1). No error or flow control. No delivery guarantees. Connection-mode (type 2). Logical connection established. Flow and congestion control provided. Acknowledged connectionless (type 3). No logical connection. Flow and error control.

LLC (802.2) Protocol Similar to HDLC (ISO standard). LLC PDU: 1 byte
1 or 2 bytes variable DSAP SSAP LLC control Information

Wireless LANs Use wireless transmission media.
Infrared (IR): limited to indoors and single room (IR light doesn’t penetrate walls). Radio Narrowband microwave. Spread Spectrum LANs. For wireless LAN technology comparison, see table on page 398.

Wireless LAN Applications
Nomadic access (e.g., users roaming around campus). LAN interconnection (e.g., across buildings). Ad Hoc Networks (e.g., disaster relief crew).

MAC Protocols Contention-based Round-robin : token-based protocols.
ALOHA and Slotted ALOHA. CSMA. CSMA/CD. Round-robin : token-based protocols. Token bus. Token ring.

The ALOHA Protocol Developed @ U of Hawaii in early 70’s.
Packet radio networks. “Free for all”: whenever station has a frame to send, it does so. Station listens for maximum RTT for an ACK. If no ACK, re-sends frame for a number of times and then gives up. Receivers check FCS and destination address to ACK.

Collisions Invalid frames may be caused by channel noise or
Because other station(s) transmitted at the same time: collision. Collision happens even when the last bit of a frame overlaps with the first bit of the next frame.

ALOHA’s Performance 1 t0+t t0+3t t0 t0+2t Time vulnerable

ALOHA’s Performance 2 S = G e-2G, where S is the throughput (rate of successful transmissions) and G is the offered load. S = Smax = 1/2e = for G=0.5.

Slotted Aloha Doubles performance of ALOHA.
Frames can only be transmitted at beginning of slot: “discrete” ALOHA. Vulnerable period is halved. S = G e-G. S = Smax = 1/e = for G = 1.

ALOHA Protocols Poor utilization.
Key property of LANs: propagation delay between stations is small compared to frame transmission time. Consequence: stations can sense the medium before transmitting.

Carrier-Sense Multiple Access (CSMA) 1
Station that wants to transmit first listens to check if another transmission is in progress (carrier sense). If medium is in use, station waits; else, it transmits. Collisions can still occur. Transmitter waits for ACK; if no ACKs, retransmits.

CSMA 2 Effective when average transmission time >> propagation time. Collisions can occur only when 2 or more stations begin transmitting within short time. If station transmits and no collisions during the time leading edge of frame propagates to farthest station, then NO collisions.

CSMA 3 Maximum utilization is function of frame size and propagation time. Longer frames or shorter propagation time, higher utilization.

CSMA Flavors 1-persistent CSMA (IEEE 802.3)
If medium idle, transmit; if medium busy, wait until idle; then transmit with p=1. If collision, waits random period to re-send. Non-persistent CSMA: after collision, node waits a random time before retransmitting. P-persistent: when channel idle detected, transmits packet in the first slot with p.

CSMA/CD 1 CSMA with collision detection.
Problem: when frames collide, medium is unusable for duration of both (damaged) frames. For long frames (when compared to propagation time), considerable waste. What if station listens while transmitting?

CSMA/CD Protocol 1. If medium idle, transmit; otherwise 2.
2. If medium busy, wait until idle, then transmit with p=1. 3. If collision detected, transmit brief jamming signal and abort transmission. 4. After aborting, wait random time, try again.

CSMA/CD Performance Wasted capacity restricted to time to detect collision. Time to detect collision < 2*maximum propagation delay. Rule in CSMA/CD protocols: frames long enough to allow collision detection prior to end of transmission.

IEEE 802.3 LAN Standards 802.3: 10 Mbps Ethernet.
802.3u: 100Mbps (Fast) Ethernet. 802.3z: 1Gbps (Gigabit) Ethernet.

Ethernet Most popular CSMA/CD protocol. 1-persistent.
Developed at Xerox Parc (1976). Different implementations (10Mbps): Notation: <bps><signaling><max seg size (100’s of meters)> Table page 409.

Ethernet Implementations
10Base5 (thick net): up to 500m segments and 100 stations; coaxial cable(10mm); baseband (Manchester); bus. 10Base2 (thin net): up to 200m segments and 30 stations; coaxial cable(5mm); baseband (Manchester); bus. 10BaseT: up to 100m segments; unshielded TP; baseband (Manchester); star.

Baseband and Broadband
Signaling techniques. Baseband: signals transmitted without modulation; digital signals represented by different voltages (e.g., using Manchester encoding). Broadband: analog signaling; if digital, modulation required.

Ethernet (cont’d) Multiple segments can be connected using repeaters.

Ethernet Frame Format 8 6 6 2 4 1 Data CRC
Preamble Type Data CRC DA SA Postamble Type: identifies upper layer protocol (for demux’ing) Data: bytes (min. is 46 bytes). DA and SA: destination and source addresses. Example: 6:2b:3e:0:0:1d Broadcast: all 1’s. Multicast: first bit is 1. Promiscuous mode: stations accept all frames.

Ethernet Transmission
If channel idle: Send frame immediately (p=1). Waits 2t between back-to-back transmissions. If channel busy: Wait till free, then transmit (p=1). If collision: Jam for 512 bits (for both ends to detect collision). Waits for 0-2t (1st try), 0-4t (2nd try),...

Token Bus 1 IEEE (1985). Token: special-purpose frame that circulates when all stations are idle. Physically, token bus is linear or tree-shaped topology; logically, it operates as ring. 4 5 3 token 1 2 6

Token Bus 2 In CSMA/CD (802.3) starvation may occur, i.e., stations can wait forever to transmit. In token bus, every station has a chance to transmit (token). No collisions! i.,e., contention-free.

Token Bus 3 Token passes around in pre-defined order.
Once station acquires token, it can start transmitting. When done, passes the token onto next station.

Token Bus 4 Limited efficient due to passing of the token. Issues:
Adding/removing stations. Lost token problem.

Token Ring 1 IEEE 802.5 and FDDI.
Most commonly used MAC protocol for ring topologies. Also uses special-purpose, circulating frame, or token (3 bytes). Station that wants to transmit waits till token passes by.

Token Ring 2 When station wants to transmit:
Waits for token. Seizes it by changing 1 bit and token becomes start-of-frame sequence. Station appends remainder of frame. When station seizes token and begins transmission, there’s no token on the ring; so nobody else can transmit.

Token Ring 3 Transmitting station inserts new token when:
Station completes frame transmission and Leading edge of frame returns to it after a round-trip. If ring length < frame length, 1st. condition implies 2nd. 2nd. condition ensures only 1 data frame at a time on the ring.

Token Ring 4 Under light load, inefficiency due to waiting for the token to transmit. Under heavy load, round-robin: fair and efficient. Issues: Token maintenance. Token loss or duplication. Monitoring station can be responsible for ring maintenance (removing duplicates, inserting token)

Token Ring Frame Format
1 1 1 1 1 2 or 6 2 or 6 4 SD DA FCS AC FC SA Data ED FS SD Token frame AC FC SD: starting delimiter; indicates starting of frame. AC: access control; PPPTMRRR; PPP and RRR priority and reservation; M monitor bit; T token or data frame. FC: frame control; if LLC data or control. DA and SA: destination and source addresses. FCS: frame check sequence. ED: ending delimiter; contains the error detection bit E; contains frame continuation bit I (multiple frame transmissions). FS: frame status.

Token Ring Revisited Single priority: priority and reservation bits = 0. Transmitter seizes token. Sets token bit to 1. Token’s SD and AC are first 2 fields. Station transmits 1 or more frames. Until done or token-holding timer expires. When AC of last frame returns, sets token bit to 0, appends ED: new token.

Detecting Errors Frame status bits (end delimiter).
A bit: address recognized. C bit: frame copied. A=0, C=0: destination non-existent or not active. A=1, C=0: destination exists but frame not copied. A=1, C=1: frame received.

Token Ring Priority Optional priority mechanism in 802.5.
3 priority bits: 8 priority levels. Service priority: priority of current token. Station can only transmit frame with priority >= service priority. Reservation bits allow station to influence priority levels trying to reserve next token.

Early Token Release Typically, station waits for frame to come back before issuing a new token. Problem: low ring utilization. ETR option: Station may release token as soon as it completes transmission.

Ethernet versus Token Ring
Efficient at heavy traffic. Guaranteed delay. Fair. Supports priorities. But, ring/token maintenance overhead. Centralized monitoring. Ethernet is simple!

High-Speed LANs FDDI 100VG-AnyLAN Fast Ethernet Gigabit Ethernet

FDDI 1 Fiber Distributed Data Interface.
Similar to with some changes due to higher data rates. 100Mbps, token ring LAN. Also suitable for MANs. Fiber or TP as transmission medium. Up to 100 repeaters and up to 2 Km (fiber) or 100m (TP) between repeaters.

FDDI 2 2 counter-rotating fiber rings; only one used for transmission; the other for reliability, i.e., self-healing ring. Normal operation Under failure Line failure

FDDI 3 Primary ring Secondary ring DAS: dual attachment
SAS DAS CON Secondary ring DAS: dual attachment SAS: single attachment CON: concentrator

FDDI 4 Basic differences to 802.5:
Station waiting for token, seizes token by failing to repeat it (completely removes it). Original technique impractical (high data rate). Station inserts new frame. Early token release by default.

FDDI 5 FDDI can also be implemented using twisted pair (copper): CDDI.
Cheaper. 100m. THT: token holding time. TRT: token rotation time.

100VG-ANYLAN 1 VG: voice grade; ANYLAN: support multiple frame types.
(uses new MAC scheme and not CSMA/CD). Intended to be 100Mbps extension to Ethernet like 100BASE-T. MAC scheme: demand priority (determines order in which nodes share network). Supports both and frames.

100VG-ANYLAN 2 Topology: hierarchical star. Level 1 hub Level 2

MAC Protocol 1 Single-hub network
Station issues request to central hub and waits permission to transmit. High- and low-priority requests. Hub scans its ports for requests in RR order, e.g., port 1, 2,…, n; it keeps 2 separate pointers for high- and low-priority traffic. Services high-priority requests in order; then low-priority ones.

MAC Protocol 2 Hierarchical topology 1.1 1.2 1.6 1.7 1.4 1.3.1 1.5.1
1.3.3 1.5.3 1.3.2 1.5.2

Fast Ethernet 100 Mbps Ethernet. IEEE 802.3u, 1995.
Medium alternatives: 100BASE-TX (twisted pair) 100BASE-FX (fiber). IEEE MAC and frame format. 10-fold increase in speed => 10-fold reduction in diameter (200m).

Gigabit Ethernet IEEE 802.3z (1996). Currently over fiber: 1000Base-F.
Modified MAC layer due to high data rates.

Wireless LANs IEEE Distributed access control mechanism (DCF) based on CSMA with optional centralized control (PCF). Contention-free Service (polling) MAC layer PCF Contention Service (CSMA) DCF Physical Layer

MAC in Wireless LANs Distributed coordination function (DCF) uses CSMA-based protocol (e.g., ad hoc networks). CD does not make sense in wireless. Hard for transmitter to distinguish its own transmission from incoming weak signals and noise. Point coordination function (PCF) uses polling to grant stations their turn to transmit (e.g., cellular networks).

Switched Ethernet Point-to-point connections to multi-port hub acting like switch; no collisions. More efficient under high traffic load: break large shared Ethernet into smaller segments. Switch Hub

LAN Interconnection Extend LAN coverage.
Interconnect different types of LAN. Connect to an internetwork. Reliability and security.

Interconnection Schemes
Hubs or repeaters: physical-level interconnection. Devices repeat/amplify signal. No buffering/routing capability. Bridges: link-layer interconnection. Store-and-forward frames to destination LAN. Need to speak protocols of LANs it interconnect. Routers: network-layer interconnection. Interconnect different types of networks.

Bridges 1 Operate at the MAC layer.
Interconnect LANs of the same type, or LANs that speak different MAC protocols. LAN A Frames for 5->8. B 1 4 Frames for 1->4 LAN B 5 8

Bridges 2 Function: Listens to all frames on LAN A and accepts those addressed to stations on LAN B. Using B’s MAC protocol retransmits the frames onto B. Does the same for B-to-A traffic.

Bridges 3 Behave like a station; have multiple interfaces, 1 per LAN.
Use destination address to forward unicast frames; if destination is on the same LAN, drops frame; otherwise forwards it. Forward all broadcast frames. Have storage and routing capability.

Bridges 4 No additional encapsulation.
But they may have to do header conversion if interconnecting different LANs (e.g., to frame). May interconnect more than 2 LANs. LANs may be interconnected by more than 1 bridge.

Bridge Protocol Architecture
IEEE 802.1D specification for MAC bridges. LLC LLC MAC MAC MAC LAN LAN PHY PHY PHY PHY Station Bridge Station

Routing with Bridges Bridge decides to relay frame based on destination MAC address. If only 2 LANs, decision is simple. If more complex topologies, routing is needed, i.e., frame may traverse more than 1 bridge.

Routing Determining where to send frame so that it reaches the destination. Routing by learning: adaptive or backward learning.

Note on Terminology: Repeaters and Bridges
Extend scope of LANs. Serve as amplifiers. No storage/routing capabilities. Bridges: Also extend scope of LANs. Routing/storage capabilities.

Bridges Operate at the data link layer.
Only examine DLL header information. Do not look at the network layer header.

Routing with Bridges 3 algorithms: Fixed routing. Spanning tree.
Source routing.

Fixed Routing Fixed route for every source-destination pair of LANs.
Does not automatically respond to changes in load/topology. Statically configured routing matrix (pre-loaded into bridge). If alternate routes, pick “shortest” one. Rij: first bridge on the route from i to j.

Fixed Routing: Example
1 2 3 Source LAN LAN A A B C D E F G 107 A 101 102 103 105 106 102 101 B 101 102 103 104 105 106 LAN B LAN C C 105 102 101 103 107 106 107 D 104 101 103 102 105 106 103 105 106 104 E LAN D E F 107 102 103 G 104 105 106 105 107 106 F 102 101 103 4 5 6 7 102 101 106 107 105 G 103 Ex: E-> F: 107; 102; 105.

Fixed Routing Each bridge keeps column for each LAN it attaches.
Table “From X” derived from column “x”. Every entry that has the number of the bridge results in entry. 101 From A From B Dest Next B B A A C A D E F A G A C D B E F G

Fixed Routing Simple and minimal processing.
Too limited for internets with dynamically changing topology.

Spanning Tree Routing Aka transparent bridges.
Bridge routing table is automatically maintained (set up and updated as topology changes). 3 mechanisms: Address learning. Frame forwarding. Loop resolution.

Address Learning 1 Problem: determine where destinations are.
Bridges operate in promiscuous mode, i.e., accept all frames. Basic idea: look at source address of received frame to learn where that station is (which direction frame came from). Build routing table so that if frame comes from A on interface N, save [A, N].

Address Learning 2 When bridges first start, all tables are empty.
So they flood: every frame for unknown destination, is forwarded on all interfaces except the one it came from. With time, bridges learn where destinations are, and no longer need to flood for known destinations.

Backward Learning Bridges look at frame’s (MAC) source address to find which machine is accessible on which LAN. LAN 4 A B C LAN 1 B2 LAN 2 B1 If B1 sees frame from C on LAN 2, RT entry (C, LAN2). Any frame to C on LAN1 will be forwarded. But, frame to C on LAN2 will not be forwarded. LAN 3

Address Learning 3 RT entries have a time-to-live (TTL).
RT entries refreshed when frames from source already in the table arrive. Periodically, process running on bridge scans RT and purges stale entries, i.e., entries older than TTL. Forwarding to unknown destinations reverts to flooding.

Frame Forwarding Depends on source and destination LANs.
If destination LAN (where frame is going to) = source LAN (where frame is coming from), discard frame. If destination LAN != source LAN, forward frame. If destination LAN unknown, flood frame. Special purpose hardware used to perform RT lookup and update in few microseconds.

Loops Alternate routes: loops. Example: LAN A, bridge 101,
LAN B, bridge 104, LAN E, bridge 107, LAN A. 1 2 LAN A 101 LAN B 107 103 104 E 4 5

Loop: Problems B LAN 1 B1 B2 LAN 2 A
1. Station A sends frame to B; bridges B1 and B2 don’t know B. 2. B1 copies frame onto LAN1; B2 does the same. 3. B2 sees B1’s frame to unknown destination and copies it onto LAN 2. 4. B1 sees B2’s frame and does the same. 5. This can go on forever.

Loop Resolution Goal: remove “extra” paths by removing “extra” bridges. Spanning tree: Given graph G(V,E), there exists a tree that spans all nodes where there is only one path between any pair of nodes, i.e., NO loops. LANs are represented by nodes and bridges by edges.

Definitions 1 Bridge ID: unique number (e.g., MAC address + integer) assigned to each bridge. Root: bridge with smallest ID. Cost: associated with each interface; specifies cost of transmitting frame through that interface. Root port: interface to minimum-cost path to root.

Definitions 2 Root path cost: cost of path to root bridge.
Designated bridge: on any LAN, bridge closest to root, i.e., the one with minimum root path cost.

Spanning Tree Algorithm 1
1. Determine root bridge. 2. Determine root port on all bridges. 3. Determine designated bridges.

Initially all bridges assume they are the root and broadcast message with its ID, root path cost. Eventually, lowest-ID bridge will be known to everyone and will become root. Root bridge periodically broadcasts it’s the root.

Directly connected bridges update their cost to root and broadcast message on other LANs they are attached. This is propagated throughout network. On any (non-directly connected) LAN, bridge closest to root becomes designated bridge.

Spanning Tree: Example
LAN 2 LAN 2 10 5 10 5 10 B3 B4 10 B3 B4 B1 10 5 B1 10 5 10 LAN 5 10 LAN 5 5 5 B5 B5 5 5 LAN 1 LAN 1 10 10 5 5 5 B2 5 B2 LAN 3 LAN 4 LAN 3 LAN 4

Spanning Tree: Example
B1 . Only designated bridges on each LAN allowed to forward frames. . Bridges continue exchanging info to react to topology changes. LAN 2 LAN 1 B4 B3 B5 LAN 5 B2 LAN 3 LAN 4

Source Routing 1 Route determined a priori by sender.
Route included in the frame header as sequence of LAN and bridge identifiers. When bridge receives frame: Forward frame if bridge is on the route. Discard frame otherwise.

Source Routing 2 Route: sequence of bridges and LANs. LAN 3
X->Z: L1,B1,L3,B3,L2. X->Z: L1,B2,L4,B4,L2 B3 LAN 2 B1 LAN 1 Z B4 B2 LAN 4 X

Source Routing 4 No need to maintain routing table.
Frame has all needed routing information. However, stations need to find route to destination.

Route Discovery 1 Finding all routes. Problem: frame explosion.
If destination is unknown, source sends broadcast route discovery frame. Frame reaches every LAN. When reply comes back, intermediate bridges record their id. Source gets complete route information. Problem: frame explosion.

Route Discovery 2 Alternative: single route request frame forwarded according to spanning tree. X LAN 1 LAN 3 B3 B1 LAN 2 Single-route broadcast Z Z X LAN 4 B4

Route Discovery 3 L2, B3, L3, B1, L1 X LAN 1 LAN 3 B3 B1 LAN 2 Z

Route Selection Select minimum-cost route, e.g., minimum-hop route.
If tie, choose the one that arrived first. Routes are cached with a TTL; when TTL expires, re-discover route.

Routers Operate at the network layer, i.e., inspect the network-layer header. Usually main router functionality implemented in software. Store-and-forward. Ability to interconnect heterogeneous networks: address translation, link speed and packet size mismatch.

The Network Layer

Goals Get data from source to destination.
May require traversing many hops and involving intermediate routers. In contrast with data link layer: frames from one end of a wire to the other. Network layer as lowest end-to-end transmission layer: multiple hops.

Routing and Internetworking
Based on knowledge of network topology, choose appropriate paths from source to destination. Load balancing across routers and links. Avoid congestion. Network interconnection: internetworking. Source and destination in different networks.

Design Issues Services provided to transport layer.
Design/implementation of the subnet. Router End system Router Router Router Subnet

[Circuit- versus Packet- Switching]
Circuit Switching Physical circuit (physical connection) is establish between source and destination throughout the network (involving switches and links). This happens before any data can be sent.

Circuit Switching

Packet Switching Special case of message switching.
No physical path establishment ahead of time. As data moves from source to destination, route is formed one hop at a time: store-and-forward. On-demand resource acquisition as opposed to circuit switching where resources reserved statically beforehand.

Context We are talking about packet switching networks!

Services Provided to Transport Layer
Network/transport layer interface: typically interface between carrier (netwrk service provider) and end user. NSP has control over protocols up to network layer. Network/transport interface needs to be very well defined. Types of service: connection-less versus connection-oriented

Connection-less service
Internet. E2E argument. Push functionality closer to users. Error and flow control at higher layers. No delivery or ordering guarantees. Every packet must carry full destination address (each packet independent of the other).

Connection-oriented Telephone and ATM networks.
Network-layer connection: Logical connection between network-layer processes at sender and receiver. Connection ID used to identify PDUs. Connection set up (QoS, cost negotiation) and tear down. Full duplex communication. Reliable and ordered delivery.

Internet over ATM Source first establishes ATM network-layer connection to destination; then send IP packets over it. Inefficient: duplicate functionality. Example: ordered delivery guarantees at the ATM network layer and TCP packet re-ordering mechanism.

Network Layer Design Connection-oriented versus connection-less infrastructure. Connection-oriented: virtual circuit Connection-less: datagrams.

Virtual Circuit Analogy to physical circuits used by telephone networks. At connection establishment time, path from source to destination is selected and used throughout connection lifetime. When connection is over, virtual circuit terminated.

Datagram No logical connection.
Each packet (datagram) routed independently; successive packets may follow different routes. More work at intermediate routers, but more robust and adaptive to failures and congestion.

Routers For VCs, routers keep a table with (VC number, outgoing interface) entries. Packets only need to carry VC number. For datagrams, routing table. (destination, outgoing interface) entries. Each packet must carry destination address.

Combinations of Service and Subnet Structure
Datagram Virtual Circuit UPD over IP over ATM UDP over IP Connection- less TCP over IP ATM over Connection- oriented

Routing Algorithms 1 Routing is main function of network layer.
Routing algorithm: decides which route a packet should take from source to destination. For router: which interface a packet should be forwarded.

Routing Algorithms 2 If datagram network, decision is made for every packet. If VC, decision is made only once when VC is setup.

Routing Metrics Routing algorithms can use different metrics when building/selecting routes. Example: Number of hops. Delay. Bandwidth.

Adaptive and Non-adaptive Routing
Fixed routing, static routing. Do not take current state of the network (e.g., load, topology). Routes are computed in advance, off-line, and downloaded to routers when booted. Adaptive routing: Routes change dynamically as function of current state of network. Algorithms vary on how they get routing information, metrics used, and when they change routes.

Optimality Principle General statement about optimal routes (topology, routing algorithm independent). If router J is on optimal path between I and K, then the optimal path from J to K also falls along the same route. Proof by contradiction. Corollary: Set of optimal routes from all sources to destination form a tree rooted at destination. Sink tree.

Adaptive and Non-adaptive Routing
Fixed routing, static routing. Do not take current state of the network (e.g., load, topology). Routes are computed in advance, off-line, and downloaded to routers when booted. Adaptive routing: Routes change dynamically as function of current state of network. Algorithms vary on how they get routing information, metrics used, and when they change routes.

Optimality Principle General statement about optimal routes (topology, routing algorithm independent). If router J is on optimal path between I and K, then the optimal path from J to K also falls along the same route. Proof by contradiction. Corollary: Set of optimal routes from all sources to destination form a tree rooted at destination. Sink tree.

Static Algorithms Shortest-path routing. Flooding.

Shortest Path Routing 1 Dijkstra (1959).
Network represented by graph G(V, E), where V is set of nodes and E is set of links connecting nodes. What is “shortest”? Different metrics. Example: number of hops (static), geographic distance (static), delay, bandwidth (raw versus available), combination of a subset of these.

Dijkstra’s Shortest Path
Nodes labeled with distance to source through best known path. At start, no known paths so all nodes labeled with infinity. As algorithm progresses, nodes are labeled; “tentative” labels may change, while “permanent” labels don’t change. Label made permanent when it’s known to be in the shortest path to source.

Dijkstra’s Algorithm: Example
B 7 C (2,A) B 7 C 2 3 2 3 2 A D 2 3 2 3 2 1 2 11 E F A D 6 2 2 1 E F 4 6 2 H G 4 (6,A) H G (2,A) B 7 (9,B) C (2,A) B 7 (9,B) 2 3 C (4,B) 3 2 A D 2 (4,B) 3 2 2 1 E F A (6,E) D 6 2 1 E F 4 6 2 G (6,A) H 4 G(5,E) H B 7 (9,B) C B 7 (9,B) C 2 (4,B) 3 2 (4,B) 3 2 A (6,E) D 2 A (6,E) D 1 E F 1 E F 6 2 6 2 4 4 G(5,E) H(9,G) G(5,E) H(8,F)

Flooding Every incoming packet forwarded on every outgoing link except the one it arrived on. Problem: duplicates. Constraining the flood: Hop count. Keep track of packets that have been flooded. Robust, shortest delay (picks shortest path as one of the paths).

Dynamic Routing Algorithms
Distance vector routing. Link state routing.

Distance Vector Routing 1
Each router keeps routing table (or routing vector) giving best known distance to each destination and the corresponding outgoing interface. Routing tables are updated by exchanging routing information with neighbors. Aka, Bellman-Ford, Ford-Fulkerson. Original ARPANET routing; also used by Internet’s RIP.

Distance Vector 2 Routing table at each router:
One entry per participating router. Each entry contains outgoing interface and distance to corresponding destination. Metric: number of hops, delay, queue length. Each router knows distance to its neighbors. Old ARPANET algorithm: DV where cost metric is outgoing link queue length.

Routing Updates Every T interval, routers exchange routing updates.
Routing update from router X consists of a vector with all destinations and the corresponding distance from X to them. When router Y receives an update from X, it can estimate its distance to router Z through X as Dyz = Dyx + Dxz. Router Y receives update from all its neighbors; discards its RT and builds a new one.

Distance Vector: Example
2 3 3 Node Distance Next 5 3 1 2 1 3 7 5 9 1 2 6 4 2 1 1 7 9 2 3 2 5 4 2 2 Node Distance Next 3 1 1 1 5 3 3 T=T0 T=T1 T=T2

Problems Routing loops. Slow convergence. Counting to infinity.

Count-to-Infinity 1 Good news propagates faster. A B C D E
Initially, A down: A comes up: infinity 1 infinity infinity (after 2 exchanges) infinity (after 3 exchanges) (after 4 exchanges) infinity infinity infinity infinity (after 1 exchange)

Count-to-Infinity 2 But, bad news propagate slower! A B C D E ….
Initially, all up: A goes down: (after 1 exchange) (after 2 exchanges) (after 3 exchanges) (after 4 exchanges) (after 5 exchanges) (after 6 exchanges) …. infinity

Count-to-Infinity 3 Gradually routers work their way up to infinity.
Number of exchanges depends on how large is infinity. To reduce number of exchanges, if metric is number of hops, infinity=maximum path+1.

Solution Routing loops: Count-to-infinity:
Path vector: record actual path used in the DV. Previous hop tracing: records preceding router. Count-to-infinity: Split horizon: router reports to neighbor cost “infinity” for destination if route to that destination is through that neighbor.

Split Horizon Tries to make bad news spread faster.
A node reports infinity as distance to node X on link packets to X are sent on. Example, in the first exchange, C tells D its distance to A but tells B its distance to A is infinity. So B discovers its link to A is down and C’s distance to A is infinity; so it sets its distance to A to infinity.

Link State Routing 1 DV routing used in the ARPANET until 1979, when it was replaced by link state routing. Used by the Internet’s OSPF.

Link State Routing 2 Link state routing is based on:
Discover your neighbors and measure the communication cost to them. Send updates about your neighbors to all other routers. Compute shortest path to every other router.

Finding Neighbors When router is booted, its first task is to find who its neighbors are. Special single-hop “hello” packets. Cost metric: Number of hops: in this case, always 1. Delay: “echo” packets and measure RTT/2. Load?

Generating Link State Updates
Link state packets (LSP). Sender identity. Sequence number. TTL. List of (neighbor, cost). When to send updates? Proactive: periodic updates; how often? Reactive: whenever some significant event is detected, e.g., link goes down. Where to send them? Everywhere: flood.

Processing Updates When LSP is received: Check sequence number.
If higher than current sequence number, keep it and flood it; otherwise, discard it. Periodically decrement TTL. When TTL=0, purge LSP.

Computing Routes Routers have global view of network.
They receive updates from all other routers with their cost to their neighbors. Build network graph. Use Dijkstra’s shortest-path algorithm to compute shortest paths to all other nodes.

DV versus LS DV: Node tells its neighbors what it knows about everybody. Based on other’s knowledge, node chooses best route. Distributed computation. LS: Node tells everyone what it knows about its neighbors. Every node has global view. Compute their own routes.

Hierarchical Routing For scalability:
As network grows, so does RT size, routing update generation, processing, and propagation overhead, and route computation time and resources. Divide network into routing regions. Routers within region know how to route packets to all destinations within region. But don’t know how to route within other regions. “Border” routers: route within regions.

Hierarchical Routing Example
Dest. Next Hops 1A 1B B 1 1C C 1 2A B 2 2B B 3 2C B 3 2D B 4 3A C 3 3B C 2 4A C 3 4B C 4 4C C 4 5A C 4 5B C 5 5C B 5 5D C 6 5E C 5 1A 1B 2A 2B 1C 1A 2D 2C 4A 5B 5A 5C 3A 3B 4B 4C 5D 5E

Hierarchical Routing Example
Dest. Next Hops 1A 1B B 1 1C C 1 B 2 C 2 C 3 C 4 A 1B 2A 2B 1C 1A 2D 2C 4A 5B 5C 5A 3A 3B 4B 4C 5D 5E

Hierarchical Routing Optimal paths are not guaranteed.
Example: 1A->5C should be via 2 and not 3. How many hierarchical levels? Example: 720 routers. 1 level: each router needs 720 RT entries. 2 levels: 24 regions of 30 routers: each router’s RT has entries. 3 levels: 8 clusters of 9 regions with 10 routers: each router’s RT

Many-to-Many Routing Support many-to-many communication.
Example applications: multi-point data distribution, multi-party teleconferencing.

Broadcasting Simplistic approach: send separate packet to each destination. Simple but expensive. Source needs to know about all destinations. Flooding: May generate too many duplicates (depending on node connectivity).

Multidestination Routing
Packet contains list of destinations. Router checks destinations and determines on which interfaces it will forward packet. Router generates new copy of packet for each output line and includes in packet only the appropriate set of destinations. Eventually, packets will only carry 1 destination.

Spanning Tree Routing Use spanning tree (sink tree) rooted at broadcast initiator. No need for destination list. Each on spanning tree forwards packets on all lines on the spanning tree (except the one the packet arrived on). Efficient but needs to generate the spanning tree and routers must have that information.

Reverse Path Forwarding
Routers don’t have to know spanning tree. Router checks whether broadcast packet arrived on interface used to send packets to source of broadcast. If so, it’s likely that it followed best route and thus not a duplicate; router forwards packet on all lines. If not, packet discarded as likely duplicate.

Multicasting Special form of broadcasting: Multicast group management:
Instead of sending messages to all nodes, send messages to a group of nodes. Multicast group management: Creating, deleting, joining, leaving group. Group management protocols communicate group membership to appropriate routers.

Multicast Routing Each router computes spanning tree covering all other participating routers. Tree is pruned by removing that do not contain any group members. 2 2 1 1 1,2 1,2 1,2 1,2 2 2 2 2 1 1 1 1 2 1 1 2 1 2 2 2 1 1

Shared Tree Multicasting
Source-rooted tree approaches don’t scale well! 1 tree per source, per group! Routers must keep state for m*n trees, where m is number of sources in a group and n is number of groups. Core-based trees: single tree per group. Host unicast message to core, where message is multicast along shared tree. Routes may not be optimal for all sources. State/storage savings in routers.

Congestion Control Ideal network behavior: Maximum capacity Packets
delivered Packets sent

Network Congestion What is network congestion?
Too many packets in the network. Router queues are always full. Routers start dropping packets. Congestion can fuel itself. Packet drops lead to retransmissions. More traffic! May result in congestion collapse! Close to 0 throughput!

Infinite-Buffer Routers
Intuition says add more memory to routers and that’ll avoid congestion. Nagle (1987) showed that infinite buffers actually make congestion worse. More packets enqueued for long time; they time out and are retransmitted; but still transmitted by router. Therefore, more traffic.

Causes of Congestion Mismatch in capacity among different parts of the system. Mismatch in link speeds. Mismatch in router processing capability. Table lookup and update. Queue management. Congestion in one point of network tends to propagate backwards toward sender. R

Congestion versus Flow Control
Congestion control tries to ensure the network is able to carry offered traffic. Involves hosts and intermediate routers. Flow control ensures that the communication end-points are able to keep up with one another. Involves only the end-points.

Congestion and Flow Control
Often mixed because tend to use same feedback mechanisms. Example: “slow down” message received at host may be caused by receiver not being able to keep up with sender host or by network not being able to handle additional traffic.

Congestion Control Principles
From control theory point of view: Open and closed loop solutions. Open loop solutions: Avoidance approach. Tries to make sure problem doesn’t happen. Doesn’t take current network state into account. Closed loop solutions: Feedback loop.

Closed Loop Solutions 3 components: Monitoring metrics: Monitoring.
Feedback generation. Operation adjustment. Monitoring metrics: Packet loss. Average queue length. Number of retransmitted packets. Average packet delay.

Feedback Send information about the problem once it’s detected.
Router that detects problem sends packet to traffic source(s). Special-purpose bit in every packet that router sets when it detects congestion above certain level to warn neighbors. Special probe messages to detect congested areas so they can be avoided. Stability: avoid oscillations.

Congestion Control Taxonomy
Open loop algorithms: Act at source. Act at destination. Closed loop algorithms: Explicit feedback. Implicit feedback.

Open Loop Approaches Traffic Shaping
Avoid traffic burstiness by forcing packets to be transmitted at more predictable rate. Used in ATM networks. Regulates average transmission rate. In contrast to sliding window protocols which regulate amount of data in transit. Service agreement between user and carrier. Important to real-time traffic such as audio, video.

Leaky Bucket 1 Host Unregulated flow Network interface Regulated
1. No matter the rate water enters bucket, the outflow is constant. 2. Once bucket full, water spills and lost. Regulated flow Network

Leaky Bucket 2 Equivalent to a single-server queuing system with constant service time. Same size packets (e.g., ATM cells): use packets as unit. Variable-sized packets: use numbr of bytes per clock tick.

Token Bucket More flexible.
Allows packets to go out as fast as they come in provided there are enough tokens. Leaky bucket holds tokens generated every T sec. Allows hosts to save up for later. Hosts can accumulate up to n tokens, when n is bucket size.

Leaky and Token Bucket Token bucket throws away tokens but never packets. Can be used between host and network and between routers. Token bucket can still produce bursts. Insert leaky bucket after token bucket.

Flow Specifications Way for user/application to specify traffic patterns and desired quality of service. Before connection established or data is sent, source provides flow spec to network. Network can accept, reject, or counter-offer. Example: flow spec language by Partridge (1992). Traffic spec: maximum packet size, maximum transmission rate. Service desired: maximum acceptable loss rate, maximum delay and delay variation.

Closed Loop Approaches
Virtual circuit networks: Admission control: Once congestion is detected, no more virtual circuits are set up until problem is gone. Avoid congested areas. Resource reservation based on service agreement. Resources include space (table, buffer) in routers, link bandwidth.

Choke Packets 1 Closed loop approach.
Can be used in both VC and DG networks. Main idea: Routers detect congestion. Example: routers measure utilization of its output lines; if it goes above threshold, congestion warning. New packet using line in warning state will be forwarded normally (tagged for no more choke packets), but generates choke packet back to source with destination.

Choke Packets 2 Hosts receiving choke packets: Reducing traffic:
Decrease their traffic to the problematic destination. Ignore other choke packets for the same destination for some period of time. After that period, if more choke packets for same destination, reduce traffic even more, etc. Reducing traffic: Adjust window size, leaky bucket rate, etc.

Hop-by-Hop Choke Packets
Goal is to provide quick relief at congestion point. Choke packet takes effect at every hop it passes through. Intermediate nodes reduce traffic on corresponding output line. More buffers since input traffic stays the same until choke packet reaches previous hop.

Fair Queuing Problem with choke packets:
Route sends signal, but it’s up to host to react. Well-behaved hosts loose! Fair queuing makes compliance attractive. Routers have multiple queues per output line. One queue per source. Router scans queues in round robin, transmitting first packet on next queue.

Weighted Fair Queuing Enable different priorities.
Different queues may have different priorities. Handle various types of traffic differently.

Load Shedding 1 If everything else fails, routers simply drop packets.
Choosing packets to drop: Randomly. Some packets are worth more than others. Application dependent Data distribution: old packets more important than new. Real-time applications: new more important than old. Applications need to mark packets with their priority

Load Shedding 2 Marking packets required special bits in packet header. ATM cells have 1 bit in the header reserved for this purpose. When routers sense some congestion build up, better to start dropping packets early rather than waiting until it becomes completely swamped.

Internetworking Interconnection of 2 or more networks forming an internetwork, or internet. LANs, MANs, and WANs. Different networks man different protocols. TCP/IP, IBM’s SNA, DEC’s DECnet, ATM, Novell and AppleTalk (for LANs). Also, satellite and cellular networks.

Example Internet LAN-WAN- LAN R R B R LAN- WAN LAN-LAN R
802.5 LAN R 802.3 LAN 802.3 LAN 802.4 LAN X.25 WAN B R R LAN- WAN LAN-LAN R Gateway: device connecting 2 or more different networks. SNA WAN

Gateways Repeaters: operate at physical layer (bits); amplify/regenerate signal. Bridges: store-and-forward frames; data link layer devices. Routers: operate at network layer. Transport gateways: connect networks at the transport layer. Application gateways: connect 2 parts of an application at application layer.

Half-Gateways Gateway is split in two: each half owned and operated by one of the network providers. Common protocol between the 2 halves. Half-gateway N2 N1

How do networks differ? Service offered: connection-oriented versus connection-less. Protocols: IP, IPX, AppleTalk, DECnet. Addressing: flat (802) versus hierarchical (IP). Maximum packet size. Quality of service. Error control: reliable, ordered, unordered delivery. Flow control: sliding window versus rate-based. Congestion control: leaky bucket, choke packets. Security: privacy rules, encryption. Parameters: different timeouts.

Types of Internetworks
Connection-oriented concatenation of VC subnets. VC between source and router closest to destination network. Router builds V to gateway to other subnet. Gateway keeps state about that VC. Builds VC to router in the next subnet, etc. Every packet traverses same path. Ordered delivery. Routers convert between packet formats.

Connection-oriented concatenation
VC between source and router closest to destination network. Router builds VC to gateway to other subnet. Gateway keeps state about VC. Gateway builds VC to router in the next subnet, etc. Every packet traverses same path. Ordered delivery. Routers convert between packet formats.

Connectionless Internetworking
Datagram model. Different packets may take different routes. Separate routing decision for each packet. No ordered delivery guarantees.

Datagram versus VC Internets
Plus’s: resources reserved in advance, ordered delivery, short headers. Minus’s: vulnerability to failures, less adaptive, hard if involving datagram subnet. Datagram: Plus’s: more robust and adaptive, can be used over datagram subnets (many LANs, mobile networks). Minus’s: Longer headers, unordered delivery.

Tunneling Interconnecting through a “foreign” subnet. Tunnel
Ethernet 2 Ethernet 1 G G WAN IP IP IP Ethernet frame Ethernet frame IP packet inside payload field of WAN packet.

Internetwork Routing 1 2-level hierarchy:
Routing within each network: interior gateway protocol. Routing between networks: exterior gateway protocol. Within each network, different routing algorithms can be used. Each network is autonomously managed and independent of others: autonomous system (AS).

Internetwork Routing 2 Typically, packet starts in its LAN. Gateway receives it (broadcast on LAN to “unknown” destination). Gateway sends packet to gateway on the destination network using its routing table. If it can use the packet’s native protocol, sends packet directly. Otherwise, tunnels it.

Fragmentation 1 Network-specific maximum packet size.
Width of TDM slot. OS buffer limitations. Protocol (number of bits in packet length field). Maximum payloads range from 48 bytes (ATM cells) to 64Kbytes (IP packets).

Fragmentation 2 What happens when large packet wants to travel through network with smaller maximum packet size? Fragmentation. Gateways break packets into fragments; each sent as separate packet. Gateway on the other side have to reassemble fragments into original packet. 2 kinds of fragmentation: transparent and non-transparent.

Transparent Fragmentation
Small-packet network transparent to other subsequent networks. Fragments of a packet addressed to the same exit gateway, where packet is reassembled. OK for concatenated VC internetworking. Subsequent networks are not aware fragmentation occurred. ATM networks (through special hardware) provide transparent fragmentation: segmentation.

Problems with Transparent Fragmentation
Exit gateway must know when it received all the pieces. Fragment counter or “end of packet” bit. Some performance penalty but requiring all fragments to go through same gateway. May have to repeatedly fragment and reassemble through series of small-packet networks.

Non-Transparent Fragmentation
Only reassemble at destination host. Each fragment becomes a separate packet. Thus routed independently. Problems: Hosts must reassemble. Every fragment must carry header until it reaches destination host.

Keeping Track of Fragments 1
Fragments must be numbered so that original data stream can be reconstructed. Tree-structured numbering scheme: Packet 0 generates fragments 0.0, 0.1, 0.2, … If these fragments need to be fragmented later on, then 0.0.0, 0.0.1, …, 0.1.0, 0.1.1, … But, too much overhead in terms of number of fields needed. Also, if fragments are lost, retransmissions can take alternate routes and get fragmented differently.

Another way is to define elementary fragment size that can pass through every network. When packet fragmented, all pieces equal to elementary fragment size, except last one (may be smaller). Packet may contain several fragments.

Header contains packet number, number of first fragment in the packet, and last-fragment bit. Last-fragment bit 1 byte A B C D E F G H I J (a) Original packet with 10 data bytes. Number of first fragment Packet number A B C D E F G H I J (b) Fragments after passing through network with maximum packet size = 8 bytes.

Firewalls 1 Analogy: ditch around medieval castles.
To enter or exit castle, must pass over single bridge. Firewalls force traffic to and from company through single point. Firewalls typically consist of: Packet filters (one for incoming, other for outgoing packets). Application gateway.

Firewalls 2 Packet filter: router equipped with capability of inspecting packets. Packets that meet criteria are forwarded; others discarded. Application gateways operate at application level; e.g., mail gateway. Application gateway Corporate network Outside world

The Internet Network Layer
The Internet as a collection on networks or autonomous systems (ASs). Hierarchical structure. Transcontinental US backbone Transcontinental links links European backbone Regional network National network

IP (Internet Protocol)
Glues Internet together. Common network-layer protocol spoken by all Internet participating networks. Best effort datagram service: No reliability guarantees. No ordering guarantees.

IP Transport layer breaks data streams into datagrams; fragments transmitted over Internet, possibly being fragmented. When all packet fragments arrive at destination, reassembled by network layer and delivered to transport layer at destination host.

IP Versions IPv4: IP version 4. IPv6: IP version 6 (aka, IPng).
Current, predominant version. 32-bit long addresses. IPv6: IP version 6 (aka, IPng). Evolution of IPv4. Longer addresses (16-byte long).

IP Datagram Format IP datagram consists of header and data (or payload). Header: 20-byte fixed (mandatory) part. Variable length optional part.

IP Header 32 bits U D M Header length Type of service Version
Total length Identification U D M Fragment offset TTL Protocol Header checksum Source address Destination address Options

IP Header Fields 1 Version: which IP version datagram uses.
Header length: how long (in 32-bit words) is header; minimum=5; maximum=15 (options=40 bytes). Type of service: precedence (priority), 3 flags (delay, throughput, reliability). In practice, routers ignore type of service. Total length: length of total datagram, i.e., header + data (max = 64Kbytes).

IP Header Fields 2 Identification: which datagram fragment belongs to.
U: unused bit. D: don’t fragment. M: more fragments. Fragment offset: position of fragment in datagram. TTL: datagram lifetime.

IP Header Fields 3 Protocol: number of the transport protocol that generated the datagram. Header checksum: verifies header integrity; computed at each hop. Source and destination address: IP addresses of source and destination. Options: way of extending the protocol.

Addressing Required for packet delivery.
Each network may use different addressing scheme. Addresses must be unique. Flat addresses: physical addresses (e.g., Ethernet address). Hierarchical addresses: use hierarchy scheme like postal addresses (e.g., IP).

Address Types Unicast: uniquely distinguishes a single node.
Multicast: shared by a group of nodes. Broadcast: shared by all nodes.

IP Addresses Every host and router on the Internet must have an IP address. 2-level hierarchy: Network number. Host number. Notations: Binary: Dotted decimal:

IP Address Formats 1 4 different classes: Network Host Class A:
128 nets. 16M hosts/net. Class B: 16K nets. 64K hosts/net. Class C: 2M nets. 256 hosts/net. Class D: Multicast. 0XXXXXXX 10XXXXXX XXXXXXXX 110XXXXX XXXXXXXX XXXXXXXX 1110XXXX XXXXXXXX XXXXXXXX XXXXXXXX

IP Address Formats 2 Class A: 1~127. Class B: 128~191.
Class C: 192~223. Class D: 224~239.

Multi-addresses A router usually has more than one IP address.
Multi-homed host: host with multiple network interfaces each of which has different IP address.

Management and Scalability 1
Network numbers assigned by single authority: NIC (network information center). All hosts in a network must have same network number. What if networks grow?

Management and Scalability 2
Example: company starts with 1 class C LAN, thus can connect up to 256 hosts. It might grow to more than 256 hosts. It might get more LANs. For every new LAN, need new network number from NIC. Moving machines between LANs needs address change.

Subnetting 1 Split address space into several “internal” subnets.
Still act like single network to outside world.

Subnetting 2 Routing: hierarchical. With subnetting:
(network, -) entries: distant networks hosts. (this network, host) entries: local hosts. Routers only need to keep track of other networks and local hosts. With subnetting: (this network, subnet, -). (this network, this subnet, host). Adds extra hierarchical level

Subnet Mask Used to compute the subnet number; i.e., gets rid of the host number. Facilitates routing table look-up. IP address AND subnet mask = subnet # Example: 10XXXXXX XXXXXXXX SSSSSSHH HHHHHHHH Ex: AND subnet mask =

Internet Control Protocols
IP carries data. There are other network layer protocols that carry control information. Example: ICMP

ICMP Internet Control Message Protocol. Report specific events.
Generated by routers. Encapsulated in IP packets.

ICMP Messages Destination unreachable Packet couldn’t be delivered
Time exceeded TTL field hit 0 Parameter problem Invalid header field Source quench Choke packets Redirect Route problem Echo request Check if destination is up Echo reply Destination responds Timestamp request Same as echo request + TS Timestamp reply Same as echo reply + TS

Mapping IP to DLL Address
Internet applications refer to hosts by their IP addresses; once packet gets to destination LAN, node needs to figure out the destination address. One solution is to have configuration file. Hard to maintain/update. Address Resolution Protocol (ARP): Run by every node to map IP to DLL address (RFC 826).

ARP Advantage: Easy to administer, less human intervention.
Example: 2 hosts on the same Ethernet want to communicate. Host 1 must figure out host 2’s Ethernet address. Host 1 broadcasts ARP packet on Ethernet asking for the Ethernet address of host 2. Host 2 receives the ARP request, and replies with its Ethernet address.

ARP Optimizations Caching of ARP replies.
Entries may have large TTLs. When sending ARP request, piggyback its own IP-DLL address mapping. Every machine broadcasts its mapping at boot time. No response is expected. Other machines cache that information.

Proxy ARP What if host 1 wants to send data to host 3 on a different LAN? Router connecting the 2 LANs can be configured to respond to ARP requests for the networks it interconnects: proxy arp. Another solution is for host 1 to recognize host 3 is on remote network and use default LAN address that handles all remote traffic; that could be the router’s Ethernet address.

RARP Reverse Address Resolution Protocol.
Given LAN address, what’s the IP address? Usually for booting diskless workstation. Gets the OS image from remote file server. Same image for all machines. Machine broadcasts its LAN address. Remote RARP server responds with machine’s IP address.

BOOTP RARP broadcasts are not forwarded by routers.
Need RARP server on every network. BOOTP uses UDP messages that are forwarded by routers. Also provides additional information such as IP address of file server holding OS image, subnet mask, etc.

Internet Routing IGPs and EGPs IGPs: routing within ASs.
EGPs: routing between ASs.

IGPs Original Internet IGP was RIP. New IGP: OSPF. Distance vector.
OK for small ASs but not efficient as ASs got larger. New IGP: OSPF. Open Shortest Path First. Became standard in 1990. Link state algorithm. RIP is still running but OSPF is taking over.

OSPF 1 Design requirements: Open implementation.
Support for various distance metrics: delay, hops, etc. Dynamic: automatically adapt to topology changes. QoS Routing: real-time versus other traffic using IP’s type of service field. Load balancing across multiple lines. Security and tunneling.

OSPF 2 Abstracts collection of networks, routers and lines into a directed graph where edges are assigned a cost proportional to the routing metric. It then computes shortest path. Hierarchical routing within ASs. Areas: collection of contiguous networks. Area 0: AS backbone; all areas connected to it.

OSPF 3 Type of service routing: Routing updates:
Uses different graphs labeled with different metrics. Routing updates: Adjacent routers exchange routing information. Adjacent routers are on different LANs. Reliable link state updates with sequence #’s.

EGPs Routing protocol between ASs. Take policy into account.
An AS may not be willing to carry traffic originating and destined to foreign ASs. Example: phone companies are willing to carry traffic for their customers but not for others.

Routing Policy Examples
No transit traffic through certain ASs. Traffic source restricts ASs through which its traffic crosses. Same for destination.

BGP 1 Border Gateway Protocol.
Policies are manually configured into BGP routers. BGP abstracts networks as a collection of BGP routers and the their links. 2 BGP routers are connected if they share a common network. BGP routers communicate reliably using TCP.

BGP 2 3 types of networks: Stub networks: have a single connection in the BGP graph; cannot carry transit traffic. Multi-connected networks: have multiple connections but refuse to carry transit traffic. Transit networks: agree to carry transit (3rd. party) traffic possibly with some restriction; e.g., backbones.

BGP 3 BGP is a distance vector protocol.
Routing table entries keep whole path to destination + distance. BGP routers can discard the paths containing itself: avoiding loops and counting to infinity. Routers compute distance associated to a route taking policy into account. If policy is violated, distance = infinity.

Internet Multicasting
IP supports multicasting using class D addresses. Each class D address identifies a group of hosts. 28 bits define over 250 million groups. Best-effort delivery.

Group Membership Hosts (single or multiple processes) may join and leave group. Special, multicast routers perform multicast routing and packet forwarding. Hosts belonging to multicast groups periodically send messages to the closest multicast router. Multicast routers and hosts use IGMP (Internet Group Management Protocol) to exchange membership information.

IP Multicast Routing Use spanning trees.
Modified distance vector protocol using unicast routing information. Build one spanning tree per source, per group. Or, one shared spanning tree per group. Use pruning to remove parts of the tree that don’t have any multicast group members. Use tunneling to cross regions that are not multicast capable.

Mobile IP 1 Support for mobile users. Problem: IP addressing scheme.
“Last hop” mobility. Problem: IP addressing scheme. Class+network number+host number. If host moves and attaches itself to foreign network, packets destined to it will still go to its home network. Assigning hosts new IP address? Too much hassle.

Mobile IP 2 Solution: Home agent: runs at the home network.
Foreign agent: runs at foreign network. When mobile host connects itself to foreign network, registers with foreign network’s foreign agent. Foreign agent assigns host care-of address, and informs home agent.

Mobile IP 3 Sending packets: mobile host uses its care-of address.
Receiving packets: When packet arrives at home network, router that gets it sends ARP request for that IP address. Home agent replies with its own Ethernet address. It gets the packet, and tunnels it to foreign agent. Foreign agent delivers packet to mobile host. Home agent sends care-of address to sender, so future packets are sent directly to foreign network.

Mobile IP 4 Locating foreign agents: Unregistration: Security:
Foreign agents periodically broadcast their address and service provided (e.g., home, foreign, or both). Mobile host can announce its presence and wait for response from foreign agent. Unregistration: If host leaves without unregistering, its registration expires after some time. Security: Authentication issues.

Scaling IP Addresses 1 Exponential growth of the Internet!
32-bit address fields are getting too small. Early predictions: it’d take decades to achieve 100,000 network mark. 100,000th. network was connected in 1996! Internet is rapidly running out of IP addresses! Waste due to hierarchical address.

IP Address Formats 4 different classes: Network Host Class A:
128 nets. 16M hosts/net. Class B: 16K nets. 64K hosts/net. Class C: 2M nets. 256 hosts/net. Class D: Multicast. 0XXXXXXX 10XXXXXX XXXXXXXX 110XXXXX XXXXXXXX XXXXXXXX 1110XXXX XXXXXXXX XXXXXXXX XXXXXXXX

Scaling IP Addresses 2 Class A addresses: 16M hosts is usually too much. Class C addresses: 254 hosts is usually too small. Class B addresses provide room for 64K hosts. Organizations usually request class B addresses but more than 50% of them only have up to 50 hosts!

Scaling IP Addresses 3 Class C addresses should have 10-bit host numbers instead of only 8-bit numbers. Would allow for 1022 hosts instead of just 254. More Class C networks: network number can grow up to 0.5M. But, could result in routing table explosion. Routers will have to know about many more networks.

CIDR 1 Classless Interdomain Routing: RFC 1519.
No longer uses classes A, B, and C addresses. Allocate remaining Class C addresses in variable-sized blocks. Example: if an organization needs 2000 addresses, it’s given a block of 2048 addresses, or 8 contiguous class C networks and not a full class B address.

CIDR 2 New allocation rules for class C addresses.
World partitioned into 4 zones and each one was given portion of class C address space (192~223). ~ : Europe. ~ : North America. ~ : Central and South America. ~ : Asia and Pacific.

CIDR 3 Each region is allocated ~ 32M class C addresses.
Addresses ~ reserved for future use. Advantages: Less waste. Routers can keep only one RT entry per region, i.e., 32M addresses compressed into one.

CIDR 4 Once packet gets to its destination region, need more detailed routing information. One possibility is to keep 131,072 (32M/28) entries for all “local” networks. Explosion problem. Instead, use of 32-bit masks: only need to keep start address of block.

CIDR - Example 1 Cambridge University has 2048 addresses from ~ and mask Oxford University: 4096 addresses ~ with mask U of Edinburgh: 1024 addresses ~ and mask

CIDR - Example 2 Routing tables in Europe contain base address and mask: Address Mask When packet to ( ) arrives, it’s ANDed with Cambridge U’s mask yielding which does not match Cambridge U’s base. When it’s ANDed with Oxford’s mask, it matches Oxford’s base, so packet sent to Oxford’s router.

IP Evolution CIDR bought IPv4 a few more years.
Because of its addressing limitations and to accommodate next-generation Internet applications, IP must evolve. In 1990, IETF started work on IP next generation, or IPng. Several proposals were considered. SIPP (Simple Internet Protocol Plus) was selected and became IPv6.

IPv6 1 RFCs 1883~1887. Features: Longer addresses (16 bytes versus only 4 in IPv4). Header simplification (only 7 fields versus 13 fields in IPv4): faster processing by routers. Better option support since fields that were previously required are now optional. Improved security and QoS support.

IPv6 Header 32 bits Source address (16 bytes) Destination address
Version Priority Flow label Payload length Next header Hop limit Source address (16 bytes) Destination address (16 bytes)

IPv6 Header Fields 1 Version = 6.
During transition period, routers will examine this field to decide what kind of packet it is. Priority: handling different kinds of traffic. 0~7: data that can be flow controlled, e.g., data distribution services. 8~15: real-time traffic (e.g., audio, video) Within each group, lower values have lower priority than higher values (e.g., 1 for news, 4 for ftp and 6 for telnet)

IPv6 Header Fields 2 Flow label (experimental): allows source and destination to set up pseudo-connection. Try to have some kind of service guarantees. Example: assign flow number to a stream of packets that need reserved bandwidth. Flow number: src+dst+flow #. Payload length: length of data. Different from IPv4 which specified total length of datagram.

IPv6 Header Fields 3 Next header: specifies what is present in the options field (extension headers). Hop limit: equivalent to IPv4’s TTL. Source and destination addresses: 16-byte addresses (fixed length). Address space is divided by using prefixes.

IPv6 versus IPv4 No more IHL (header length); why?
No more protocol field: next header field. No more fragmentation-related fields. All IPv6 hosts and routers must support 576-byte packets. Fragmentation is less likely to occur. Router sends error messages back to source when packet is too big so source breaks it down. No more checksum: rely on more reliable networks and DLL and transport checksums.

IPv6 Addressing 1 Separate prefixes for provider-based and geographic-based addresses. Ability to accommodate 2 ways of address assignment: Addresses allocated to ISP companies. Prefix 010. Each ISP assigned portion of address space. First 5 bits following prefix defines registry where provider is registered. Remaining 15 bytes are allocated by each provider. Example: 3-byte provider number.

IPv6 Addressing 2 Geographic-based addresses: Multicast addresses:
Prefix 100. Same model as current Internet. Multicast addresses: Prefix 4-bit flag + 4-bit scope fields bit group id. Flags: 1 bit defines whether group is permanent or not. Scope: limit reach of multicast packet.

IPv6 Address Notation 8 groups of 4 hexadecimal digits separated by colons. Example: 8000:0000:0000:0000:0123:4567:89AB:CDEF Optimizations: Leading zeros within group can be omitted. Groups of zeros can be replaced by pair of colons. 8000::123:4567:89AB:CDEF. IPv4 addresses: ::

Extension Headers 1 Equivalent to IPv4 options.
6 types of extension headers: Hop-by-hop options Misc. info for routers Routing Full or partial route included Fragmentation Management of fragments Authentication Verification of source’s id Encrypted payload Information about encryption Destination options Information for destination

Extension Headers 2 Fixed format and variable-sized headers.
(type, length, value). Type: 1 byte specifying which option this is. First 2 bits tell option-uncapable routers what to do: skip option, discard packet, discard packet with ICMP message, discard packet without ICMP packet for multicast addresses. Length: how long value field (0~255 bytes). Value: information.

Hop-by-Hop Header Convey information all routers along path must examine. Jumbograms: datagrams > 64KBytes. Next header: what option this is. Length of hop-by-hop header excluding the first 8 (mandatory) bytes. Defines option, in this case datagram size. Next Header Jumbogram payload length

Routing Header Lists one or more routers that must be visited on the way to the destination. Strict source routing: full path is supplied. Loose source routing: only selected routers are listed.

Fragment Header Allows source to fragment datagram.
In IPv6, routers are not allowed to fragment. If a router receives packet that is too big, it discards it and sends back a ICMP message to source. Source uses this option to fragment packet, and resend it. Contains datagram id, fragment number, and “last fragment” bit.

Authentication Header
Supports verification of sender’s identity. Contains authentication key and cryptographic checksum of the whole datagram. Receiver uses key number to find secret key. Computes checksum using secret key and checks whether it matches with received datagram.

Destination Options Supports options that need only be interpreted by destination host.

Network Layer in ATM Networks
ATM layer: connection oriented. Provides connection-oriented service. Uses virtual circuits, or virtual channels. No ACKs. Intended for fiber networks. Intended for real-time traffic. Ordering guarantees.

ATM Networks Virtual path: group of virtual circuits.
When re-routed, all VCs are re-routed together.

ATM Cells 53 bytes! 2 different formats: UNI: user-network interface.
Between host and ATM network (carrier). NNI: network-network interface. Between 2 ATM switches (ATM for routers).

Cell Formats UNI Header: GFC VPI VCI PTI P HEC 4 bits 8 bits 16 bits
NNI Header: VPI VCI PTI P HEC PTI: Payload type C: Cell loss priority HEC: Header error control GFC: General flow control VPI: Virtual path id VCI: Virtual channel id

Cell Fields 1 GFC: only in UNI cells.
No e2e significance. First switch overwrites it. Not currently used. VPI: specifies virtual path (up to 256 VPs). VCI: specifies virtual circuit (up to 64K VCs).

Cell Fields 2 PTI: type of payload.
Cell type defined by user, congestion info by network. Payload Type Meaning User data, no congestion, cell type 0 User data, no congestion, cell type 1 User data, congestion, cell type 0 User data, congestion, cell type 1 Control info adjacent switches Control info between src and dst Resource management (ABR CC) Reserved

Cell Field 3 CLP bit may be set by host to differentiate high- from low-priority traffic when choosing cell to discard if congestion. HEC: header checksum. Payload: 48 bytes.

Connection Setup Permanent and switched VCs.
Permanent: always present (like leased lines). Switched: need to be established (like phone calls). How are switched VCs established? Separate protocol called Q.2931.

VC Setup Source Switch 1 Switch 2 Destination Setup Setup
Call processing Setip Call processing Connect Connect Connect ack Connect Connect ack Connect ack

VC Tear-down Release Release Release Release complete Release complete

Routing and Switching Routing using VPs and VCs.
Route on VPIs except at the final hop. Advantages: Once VP established, all VCs between src-dst can follow the same path: no new routing decisions. Cell switching only needs to look at the VP (12bits) instead of VP (12 bits) + VC (16 bits). Easier to re-route whole group of VCs. Easier for carriers to offer private networks.

Network Layer in ATM Networks
[Continuation]

Service Categories 1 Types of traffic carried by ATM networks and types of services required by users. Constant-bit rate (CBR): No error or flow control. Constant-rate, synchronous bit transmission. Accommodate traffic carried by current telephone system: T1 lines, voice-grade lines.

Service Categories 2 Variable bit rate (VBR):
RT-VBR: variable bit rates and real-time requirements. Example: interactive compressed video (videoconferencing applications). Compression schemes: base frame+differences between current and base frames: transmission rate varies over time. Cell delay and cell delay variation must be controlled: image quality. But occasional loss is tolerable.

Service Categories 3 Variable bit rate (VBR):
NRT-VBR: services with variable bit rates and non real-time requirements. Example: multimedia (stored in disk; eliminates delay variation).

Service Categories 4 Available bit rate (ABR): Targets bursty traffic.
Guarantees average demand and will try to provide peak demand. Network provides feedback to sender: request sender to slow down if congestion. If senders are well-behaved, low loss rate.

Service Categories 5 Unspecified bit rate (UBR):
No guarantees: best effort. Suited to IP traffic. Potential applications: file transfer, , news.

Quality of Service Service offered by the network (carrier) to customer (end user): service agreement. Service agreement: offered traffic, offered service, compliance requirements. If customer and carrier don’t agree: VC will not be set up. Different requirements for each direction. E.g., VOD application: required bandwidth user->server <> server->user.

Quality of Service Parameters 1
Peak cell rate PCR Max. cell transmission rate Sustained cell rate SCR Average cell rate Minimum cell rate MCR Min. acceptable cell rate Cell delay variation tolerance CDVT Max. acceptable cell jitter Cell loss ratio CLR Fraction of lost cells Cell transfer delay CTD Time to deliver Cell delay variation CDV Delivery delay variation Cell error rate CER Fraction of correct cells

QoS Parameters 2 PCR, SCR, MCR, and CVDT: specified by sender.
CLR, CTD, and CDV describe network conditions and are measured at receiver.

Traffic Policing Checking whether each cell conforms to service agreement parameters. 2 parameters: Maximum allowed arrival rate (PCR). Or minimum inter-arrival time. Amount of acceptable variation (CDVT). Enforcing service agreement: Non-conforming cells are dropped.

Congestion Control Admission control: Congestion avoidance strategy.
New flow specifies offered traffic and expected service. Before setting up VC, network checks whether requested resources are available without affecting other flows. If no routes satisfy request, call is rejected. Prevent starvation by dividing users into classes.

Resource Reservation Resources can be reserved at call setup time.
Reserve peak bandwidth along each hop. Reserving peak versus average bandwidth.

Rate-Based Congestion Control 1
CBR and VBR: sender cannot slow down due to real-time nature of traffic. UBR: extra cells are simply dropped. ABR: network can signal congestion asking sender(s) to slow down. ACR: actual cell rate. For each sender. MCR < ACR < PCR

Rate-Based Congestion Control 2
Resource management (RM) cell: Transmitted after a certain number of data cells traveling along same path. Carry the explicit rate (ER), which is rate at which sender would currently like to transmit. Congested switches may reduce ER. When RM cell comes back, sender knows acceptable rate and adjusts ACR accordingly.

The Transport Layer

The Transport Layer End-to-end.
Communication from source to destination host. Only hosts run transport-level protocols. Under user’s control as opposed to network layer which is controlled/owned by carrier.

The Transport Service Service provided to application layer.
Transport entity: process that implements the transport protocol running on a host. At OS kernel, user-level process, or network card.

The Transport Layer Source host Destination host Application
address Application/ transport interface TPDU Transport Entity Transport Entity Transport/ network interface Network Address Network Layer Network Layer

Types of Transport Services
Connection-less versus connection-oriented. Connection-less service: no logical connections, no flow or error control. Connection-oriented: Based on logical connections: connection setup, data transfer, connection teardown. Flow and error control.

Transport versus Network Layer
Transport layer is “controlled” by user. Ability to enhance network layer quality of service. Example: transport service can be more reliable than underlying network service. Transport layer makes standard set of primitives available to users which are independent from the network service primitives, which may vary considerably.

Quality of Service User may specify QoS parameters at then transport layer. At connection setup time, user may define preferred, acceptable, and minimum values for various service parameters. Transport layer determines whether it’s possible to provide required service based on available network service(s).

Transport-Layer QoS Parameters 1
Connection establishment delay: time to establish connection. Connection establishment failure probability: probability connection is not established within maximum establishment time. Throughput: bytes transferred per second measured over a time interval.

Transport-Layer QoS Parameters 2
Transit delay: time between sending a message and receiving it on the other side (measured by the transport entities). Residual error ratio: ratio of messages in error to total messages sent. Priority: way for user to indicate that some connections are more important. Resilience: probability connection is terminated due to congestion, etc.

Transport Layer QoS Only few transport protocols provide QoS parameters. Most just try to minimize residual error rate. QoS parameters specified by transport user when connection is setup. Desired and minimum acceptable values can be specified. Service negotiation.

Transport Service Primitives
Allow transport users (e.g., application programs) to access transport service. Example: connection-oriented transport service primitives. PRIMITIVE TPDU Sent Meaning LISTEN (none) listen for connection CONNECT Connection Req. try to establish connection SEND DATA send data RECEIVE (none) waits for data DISCONNECT Disc. Req. try to release connection

TPDU Transport protocol data unit.
Messages sent between transport entities. TPDUs contained in network-layer packets, which in turn are contained in DLL frames. Frame header Packet header TPDU header TPDU payload

Connection Management State Machine
SERVER CLIENT Connection req. received Connect executed Idle Passive establishment pending Active establishment pending Connect executed Established Connection Accept Disc. req. received s Disconnect execute Active disconnect pending Passive disconnect pending Idle Disconnect executed Disc. accept. received

Berkeley Sockets 1 Set of transport-level primitives made available by Berkeley UNIX. Server side: SOCKET: create new communication end point. BIND: attach local address to socket (once server binds address, clients can connect to it). LISTEN: listen for connection. ACCEPT: accept new connection. SEND, RECEIVE: send and receive data. CLOSE: release connection.

Berkeley Sockets 2 Client side: SOCKET: create socket.
CONNECT: try to establish connection. SEND, RECEIVE: send and receive data. CLOSE: release connection.

Transport Protocol Issues: Addressing
Address of the transport-level entity. TSAP: transport service access point (analogous to NSAP). Internet TSAP: (IP address, local port). Internet NSAP: IP address. There may be multiple TSAPs on one host. Typically, only one NSAP.

Example 1 Finding the time of day from a time-of-day server.
Time-of-day server process on host 2 attaches itself to TSAP 122 and waits for requests (e.g., through LISTEN). Application process (TSAP 6) on host 1 wants to find out the time-of-day; issues CONNECT specifying TSAP 6 as source and TSAP 122 as destination.

Example 2 Transport entity on host 1 tries to establish transport connection between its TSAP 6 and the TSAP 122 on host 2. Transport entity on host 2 contacts process on TSAP 122; if it agrees, transport connection established.

Finding Services 1 Well-known TSAP.
Time-of-day server has been using TSAP 122 forever so every users know it. Initial connection protocol: special process server that proxies for less well-known services. Process server listens to set of ports at the same time. Users CONNECT to a TSAP, and if there are no servers, process server is likely to be listening. It them spawns requested server.

Finding Services 2 Name or directory service.
Name server listens to well-known TSAP. User sends service name and name server responds with service’s TSAP. New services need to register with name server. Finding the server’s network address. Hierarchical addresses solve this problem, i.e., the NSAP is part of the TSAP.

Connection Establishment
CONNECTION REQUEST and CONNECTION ACCEPTED TPDUs. Problem: delayed duplicates. Duplicates can re-appear and be taken as the real messages. Solution: messages age and are discarded after some time; need to discard ack’s. Maximum hop count. Timestamp.

Avoiding Duplicates 1 2 identically numbered TPDUs are never outstanding at the same time. Bounded packet lifetime. Each host has its clock. Clock as a counter that increments itself. #bits(counter)>= #bits(sequence number). Clocks don’t “crash”.

Avoiding Duplicates 2 When connection setup, low-order k bits of clock used as initial sequence number. Each connection starts numbering its TPDUs with different sequence number. Sequence number space need to be such that by the time sequence numbers wrap around, old TPDUs with same sequence numbers have aged.

Sequence Numbers versus Time 1
. Linear relation between time and initial sequence number. Time

Sequence Numbers versus Time 2
. Host crash: when it comes up, it doesn’t know where it ere in the sequence # space. Forbidden region . Example: T=60 sec and clock ticks once per second. . At t=30s, TPDU on connection 5 gets seq.# 80. Time . Host crashes and comes up. . At t=60s, reopens connections 0~4. . At t=70s, reopens connection 5 and at t=80s, sends TPDU 80. . Old TPDU 80 still valid, and one would look like a duplicate. . To prevent this, check if it’s in the “forbidden region” and delay sequence number.

Three-Way Handshake Solves the problem of getting 2 sides to agree on initial sequence number. 2 1 CR: connection request. CR (seq=x) ACK(seq=y,ACK=x) DATA(seq=x, ACK=y)

3-Way Handshake: Duplicates 1
2 1 . Old duplicate CR. . The ACK from host 2 tries to verify if host 1 was trying to open a new connection with seq=x. . Host 1 rejects host 2’s attempt to establish. Host 2 realizes it was a duplicate CR and aborts connection. * CR(seq=x) ACK(seq=y, ACK=x) REJECT(ACK=y)

3-Way Handshake: Duplicates 2
1 . Old duplicate CR and ACK to connection accepted. * CR(seq=x) ACK(seq=y, ACK=x) DATA(seq=x, ACK=z) REJECT(ACK=y)

Connection Release Asymmetric release: telephone system.
When one party hangs up, connection breaks. May cause data loss. Symmetric release: Treats connection as 2 separate unidirectional connections. Requires each to be released separately.

Symmetric Release How to determine when all data has been sent and connection could be released? 2-army problem: Blue army 2 Blue army 1 . White army larger than either blue armies. . Blue army together is larger. . If each blue army attacks, it’ll be defeated. They win if attack together. White army

2-Army Problem 1 To synchronize attack, they must use messengers that need to cross valley: unreliable. Is there a protocol that allows blue army to win? No. Blue army 1 sends message to blue army 2. Blue army 2 sends ACK back. Blue army 2 is not sure whether ACK was received.

2-Army Problem 2 Use 2-way handshake. Applying to connection release:
Blue army 1 ACKs back but it’ll never know if the ACK was received. Applying to connection release: Neither side is prepared to disconnect until convince other side is prepared to disconnect. In practice, hosts are willing to take risks.

Connection Release Protocol
Send DR+ start timer DR: disconnection request. DR DR Send DR+ start timer Release connection ACK Send ACK Release connection

Connection Release Scenarios 1
Send DR+ start timer DR: disconnection request. DR DR Send DR+ start timer Release connection ACK Send ACK Timeout: Release connection

Connection Release Scenarios 2
Send DR+ start timer DR: disconnection request. DR DR Send DR+ start timer Timeout: send DR+ start timer DR Send DR+ start timer DR Release connection ACK

The Internet Transport Protocols: TCP and UDP
UDP: user datagram protocol (RFC 768). Connection-less protocol. TCP: transmission control protocol (RFCs 793, 1122, 1323). Connection-oriented protocol.

UDP Provides connection-less, unreliable service. Low overhead.
No delivery guarantees. No ordering guarantees. No duplicate detection. Low overhead. No connection establishment/teardown. Suitable for short-lived connections. Example: client-server applications.

UDP Segment Format 0 15 31 Destination port Source port Length
Source port Destination port Length Checksum Data Source and destination ports: identify the end points. Length: 8-byte header+ data. Checksum: optional; if not used, set to zero.

UDP Checksum Computed over a pseudo-header+ UDP header+data+padding (to even number of bytes if needed). Pseudo-header: 31 Source IP address Destination IP address Protocol Segment length

TCP Reliable end-to-end communication. TCP transport entity:
Runs on machine that supports TCP. Interfaces to the IP layer. Manages TCP streams. Accepts user data, breaks it down and sends it as separate IP datagrams. At receiver, reconstructs original byte stream from IP datagrams.

TCP Reliability Reliable delivery. Ordered delivery. ACKs.
Timeouts and retransmissions. Ordered delivery.

TCP Service Model 1 Obtained by creating TCP end points.
Example: UNIX sockets. TSAP address: IP address + 16-bit port number. Multiple connections can share same port pair. Port numbers below 1024: well-known ports reserved for standard services. List of well-known ports in RFC 1700.

TCP Service Model 2 TCP connections are full-duplex and point-to-point. Byte stream (not message stream). Message boundaries are not preserved e2e. A B C D A B C D 4 512-byte segments sent as separate IP datagrams 2048 bytes of data delivered to application in single READ

TCP Byte Stream When application passes data to TCP, it may send it immediately or buffer it. Sometimes application wants to send data immediately. Example: interactive applications. Use PUSH flag to force transmission. URGENT flag. Also forces TCP to transmit at once.

TCP Protocol Overview 1 TCP’s TPDU: segment. 20-byte header + options.
Data. TCP entity decides the size of segment. 2 limits: 64KByte IP payload and MTU. Segments that are too large are fragmented. More overhead by addition of IP header.

TCP Protocol Overview 2 Sequence numbers.
Reliability, ordering, and flow control. Assigned to every byte. 32-bit sequence numbers.

TCP Segment Header Source port Destination port Sequence number
Acknowledgment number Header length U A P R S F Window size Checksum Urgent pointer Options (0 or more 32-bit words) Data

TCP Header Fields 1 Source and destination ports identify connection end points. Sequence number. Acknowledgment number specifies next byte expected. TCP header length: how many 32-bit words are contained in header. 6-bit unused field.

TCP Header Fields 2 6 1-bit flags:
URG: indicate urgent data present; urgent pointer gives byte offset from current sequence number where urgent data is. ACK: indicates whether segment contains acknowledgment; if 0, acknowledgement number field ignored. PUSH: indicates PUSHed data so receiver delivers it to application immediately.

TCP Header Fields 3 Flags (cont’d):
RST: used to reset connection, reject invalid segment, or refuse to open connection. SYN: used to establish connection; connection request, SYN=1, ACK=0. FIN: used to release connection. Window size: how many bytes can be sent starting at acknowledgment number.

TCP Header Fields 4 Checksum: checksums the header+data+pseudo-header.
Options: provide way to add extra information. Examples: Maximum payload host is willing to accept; can be advertised during connection setup. Window scale factor that allows sender and receiver to negotiate larger window sizes.

TCP Connection Setup 3-way handshake. Host 2 Host 1 SYN (SEQ=x)
SYN(SEQ=y,ACK=x+1) (SEQ=x+1, ACK=y+1)

TCP Connection Release 1
Abrupt release: Send RESET. May cause data loss.

Graceful release: Each side of the connection released independently. Either side send TCP segment with FIN=1. When FIN acknowledged, that direction is shut down for data. Connection released when both sides shut down. 4 segments: 1 FIN and 1 ACK for each direction; 1st. ACK+2nd. FIN combined.

Timers to avoid 2-army problem. If response to FIN not received within 2*MSL, FIN sender releases connection. After connection released, TCP waits for 2*MSL (e.g., 120 sec) to ensure all old segments have aged.

TCP Transmission 1 Sender process initiates connection.
Once connection established, TCP can start sending data. Sender writes bytes to TCP stream. TCP sender breaks byte stream into segments. Each byte assigned sequence number. Segment sent and timer started.

TCP Transmission 2 If timer expires, retransmit segment.
After retransmitting segment for maximum number of times, assumes connection is dead and closes it. If user aborts connection, sending TCP flushes its buffers and sends RESET segment. Receiving TCP decides when to pass received data to upper layer.

TCP Flow Control Sliding window. Receiver’s advertised window.
Size of advertised window related to receiver’s buffer space. Sender can send data up to receiver’s advertised window.

TCP Flow Control: Example
App. writes 2K of data 4K 2K;SEQ=0 2K ACK=2048; WIN=2048 App. does 3K write 2K; SEQ=2048 Sender blocked App. reads 2K of data ACK=4096; WIN=0 ACK=4096; WIN=2048 2K 1K; SEQ=4096 Sender may send up to 2K 1K

TCP Flow Control: Observations
TCP sender not required to transmit data as soon as it comes in form application. Example: when first 2KB of data comes in, could wait for more data since window is 4KB. Receiver not required to send ACKs as soon as possible. Wait for data so ACK is piggybacked.

Delayed ACKs Tries to optimize ACK transmission.
Delay ACKs and window update (500msec) hoping to piggyback on data segment. Example: telnet to interactive editor: Send 1 character at a time: 20-byte TCP header+ 1-byte data+20-byte IP header. Receiver ACKs immediately: 40-byte ACK. When editor reads character, window update: 40-byte datagram. Then echoes character back: 41-byte datagram.

Nagle’s Algorithm Tries to optimize sending of small data chunks.
Example: telnet to interactive editor). Send first byte and buffer the rest until outstanding byte is ACKed; then send all buffered data in one segment; buffer until next ACK. Disabled in some cases (e.g., window application: mouse movements).

Silly Window Syndrome Caused by receiver sending window updates of very small values. Example: Receiver application reads 1 byte at a time and receiver TCP sends 1-byte window update. Sender TCP has large blocks to send but can only send 1 byte at a time. Solution: [Clark] prevent receiver from generating small window advertisements; also, sender can wait.

Congestion Control Why do it at the transport layer?
Real fix to congestion is to slow down sender. Use law of “conservation of packets”. Keep number of packets in the network constant. Don’t inject new packet until old one leaves. Congestion indicator: packet loss.

TCP Congestion Control 1
Like, flow control, also window based. Sender keeps congestion window (cwin). Each sender keeps 2 windows: receiver’s advertised window and congestion window. Number of bytes that may be sent is min(advertised window, cwin).

Slow start [Jacobson 1988]: Connection’s congestion window starts at 1 segment. If segment ACKed before time out, cwin=cwin+1. As ACKs come in, current cwin is increased by 1. Exponential increase.

Congestion Avoidance: Third parameter: threshold. Initially set to 64KB. If timeout, threshold=cwin/2 and cwin=1. Re-enters slow-start until cwin=threshold. Then, cwin grows linearly until it reaches receiver’s advertised window.

TCP Congestion Control: Example

TCP Retransmission Timer
When segment sent, retransmission timer starts. If segment ACKed, timer stops. If time out, segment retransmitted and timer starts again.

How to set timer? Based on round-trip time: time between a segment is sent and ACK comes back. If timer is too short, unnecessary retransmissions. If timer is too long, long retransmission delay.

Jacobson’s Algorithm 1 Determining the round-trip time:
TCP keeps RTT variable. When segment sent, TCP measures how long it takes to get ACK back (M). RTT = alpha*RTT + (1-alpha)M. alpha: smoothing factor; determines weight given to previous estimate. Typically, alpha=7/8.

Jacobson’s Algorithm 2 Determining timeout value:
Measure RTT variation, or |RTT-M|. Keeps smoothed value of cumulative variation D=alpha*D+(1-alpha)|RTT-M|. Alpha may or may not be the same as value used to smooth RTT. Timeout = RTT+4*D.

Karn’s Algorithm How to compute ACKs for retransmitted segments?
Count it for first or second transmission? Karn proposed not to update RTT on any retransmitted segment. Instead RTT is doubled on each failure until segments get through.

Persistence Timer Prevents deadlock if an window update packet is lost and advertised window = 0. When persistence timer goes off, sender probes receiver; receiver replies with its current advertised window. If 0, persistence timer is set again.

Keepalive Timer Goes off when a connection is idle for a long time.
Causes one side to check whether the other side is still alive. If no answer, connection terminated.

TIME_WAIT 2*MSL. Makes sure all segments die after connection is closed.

Wireless TCP 1 According to layered system design principles, transport protocol should be independent of underlying technology. However, wireless networks invalidate this principle. Ignoring properties of wireless medium can lead to poor TCP performance. Problem: TCP’s congestion control.

Wireless TCP 2 Problem: packet loss as congestion indicator.
When retransmission timer times out, sender slows down. Wireless links are lossy! Dealing with losses in this case should be re-sending lost segments asap.

Indirect TCP (I-TCP) [Bakne and Badrinath, 1995].
Split TCP connection in 2: one from sender to base station and the other from base station to receiver. Base station serves as “repeater”: copies segments between connections in both directions. Connections are homogeneous; timeouts on 1st. connection, slow down sender. Problem: violates TCP’s e2e’ness. Example: ACKs to sender mean base station received segments, not necessarily receiver.

Snoop TCP [Balakrishnan et al., 1995]. Does not break connection.
Modifications to base station’s network layer code. Snooping agent on base station observes and caches TCP segments sent to mobile host and ACKs coming back. If it doesn’t see an ACK for a segment or sees duplicate ACKs, it times out and retransmits. But source may time out anyway.

End-To-End Argument Design principle to help guide placement of functionality in distributed systems. Rationale for moving functions upward closer to application.

Where to place distributed systems functions?
Layered system design: Different levels of abstraction for simplicity. Lower layer provides service to upper layer. Very well defined interfaces. Some functions can be implemented at different layers or even at multiple layers.

E2E Argument Statement “The function in question can completely and correctly be implemented only with the knowledge and help of the application at the endpoints. Therefore providing that function in the communication system itself is not possible. Sometimes an incomplete version of the function provided by the communication system may be useful as performance enhancement.”

Functions Closer to Application
E2E argument paper argues that functions should be moved closer to the application that uses them. Rationale: Some functions can only be completely and correctly implemented with app’s knowledge. Example: file transfer. If error occurs in the network, network reliability can fix it. Otherwise, only application can.

Another perspective: Cost
Why pay for something you don’t need. Example 1: the Internet. Example 2: trend in kernel design - take away from kernel as much functionality as possible. Applications that don’t need certain functions should not have to pay for them.

E2E Counter Argument Performance!
Example: File transfer Reliability checks at lower layers detect problems earlier. Abort transfer and re-try without having to wait till whole file is transmitted. “Spread out” functionality across layers.

Domain Name System (DNS)
Basic function: translation of names (ASCII strings) to network (IP) addresses and vice-versa. Example: zephyr.isi.edu <->

History Original approach (ARPANET, 1970’s):
File hosts.txt listed all hosts and their IP addresses. Every night every host fetches file from central repository. OK for a few hundred hosts. Scalability? File size. Centrally managed.

DNS Hierarchical name space. Distributed database. RFCs 1034 and 1035.

How is it used? Client-server model.
Client DNS (running on client hosts), or resolver. Application calls resolver with name. Resolver contacts local DNS server (using UDP) passing the name. Server returns corresponding IP address.

DNS Name Space Tree-based hierarchy. us int com edu gov mil org net
ca … usc ibm cs ee eng sales

Name Space Structure Top-level domains: Leaf domains: no sub-domains.
Generic. Countries. Leaf domains: no sub-domains. In practice all US organizations are under a generic domain, while everything outside the US is under the corresponding country domain.

DNS Names Domain names:
Concatenation of all domain names starting from its own all the way to the root separated by “.”. Refers to a tree node and all names under it. Case insensitive. Components up to 63 characters. Full name less than 255 characters.

Name Space Management Domains are autonomous. Delegation:
Organizational boundaries. Each domain manages its own name space independently of other domains. Delegation: When creating new domain: register with parent domain. For name uniqueness. For name resolution.

Resource Records Entry in the DNS database.
Several types of entries or RRs. Example: RR “A” contains IP address. Name <-> several resource records. RR format: five-tuple. Name. TTL (in seconds). Class (usually “IN” for Internet info). Type: type of RR. Value.

RR Types 1 SOA: start of authority. A: address. MX: mail exchange.
Marks beginning of zone’s database. Provides general info about the zone: address of admin, default TTL, etc. A: address. Contains 32-bit IP address. Single name <-> several A RRs. MX: mail exchange. Name of mail server for this domain.

RR Types 2 NS: name server. CNAME: canonical name.
Name of name server for this domain. CNAME: canonical name. Alias. HINFO: host description. Provides information about host, e.g., CPU type, OS, etc. TXT: arbitrary string of characters. Generic description of the domain, where it is located, etc.

Name Servers Entire database in a single name server.
Practical? Why? DNS database is partitioned into zones. Each zone contains part of the DNS tree. Zone <-> name server. Each zone may be served by more than 1 server. A server may serve multiple zones. Primary and secondary name servers.

Name Resolution 1 Application wants to resolve name.
Resolver sends query to local name server. Resolver configured with list of local name servers. Select servers in round-robin fashion. If name is local, local name server returns matching authoritative RRs. Authoritative RR comes from authority managing the RR and is always correct. Cached RRs may be out of date.

Name Resolution 2 If information not available locally (not even cached), local NS will have to ask someone else. It asks the server of the top-level domain of the name requested.

Recursive Resolution Recursive query: Alternative:
Each server that doesn’t have info forwards it to someone else. Response finds its way back. Alternative: Name server not able to resolve query, sends back the name of the next server to try. Some servers use this method. More control for clients.

Example Suppose resolver on flits.cs.vu.nl wants to resolve linda.cs.yale.edu. Local NS, cs.vu.nl, gets queried but cannot resolve it. It then contacts .edu server. .edu server forwards query to yale.edu server. yale.edu contacts cs.yale.edu, which has the authoritative RR. Response finds its way back to originator. cs.vu.nl caches this info. Not authoritative (since may be out-of-date). RR TTL determines how long RR should be cached.

Review 1 Network-layer congestion control. What is it? CC versus FC.
Taxonomy: closed versus open loop. Open loop: Token and leaky bucket. Closed loop: Choke packets. Fair and weighted fair queuing. Load shedding.

Review 2 Internetworking. Gateways.
Connectionless versus connection-oriented. Tunneling. Fragmentation. Transparent. Non-transparent.

Review 3 IP. Companion protocols. IP header. Addressing.
Address formats. Subnetting. Companion protocols. ICMP, ARP, RARP, BOOTP.

Review 4 Internet Routing. CIDR. IPv6. IGPs versus EGPs.
RIP, OSPF, BGP. Internet multicast. Mobile IP. CIDR. IPv6.

Review 5 ATM network layer. Transport layer.
Types of transport services. Transport service primitives. Berkeley sockets. TPDUs. Connection management. Setting up and releasing. Avoiding duplicates. 3-way handshake. 2-army problem.

Review 6 UDP. TCP. Type of service. Header.
Connection setup and release. Flow control.

Review 7 TCP (cont’d). Wireless TCP. E2E argument. The Web and HTTP.
Delayed ACKs. Nagle’s algorithm. Silly window syndrome. Congestion control. Wireless TCP. E2E argument. The Web and HTTP.

Review 8 Network security. Reliable multicast. DNS.

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