Chapter 5: The Data Link Layer

Chapter 5: The Data Link Layer
Application Transport Network data link layer service Moving data between nearby network elements Move data between end-host and router Move data between end-hosts Move data between routers error detection, correction Encryption sharing a broadcast channel: multiple access link layer addressing and routing reliable data transfer, flow control Interact/act as a bridge between the network layer and the physical layer There are many types of physical layer Which services does the link layer provide that other layers also provide?

Link Layer 5.1 Introduction and services
5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-layer Addressing 5.5 Ethernet 5.6 Link-layer switches 5.7 PPP 5.8 Link virtualization: ATM, MPLS

Link Layer: Introduction
Some terminology: hosts and routers are nodes communication channels that connect adjacent nodes along communication path are links wired links wireless links LANs layer-2 packet is a frame, encapsulates datagram data-link layer has responsibility of transferring datagram from one node to adjacent node over a link - This is not entirely correct.

Link layer: context transportation analogy trip from Princeton to Lausanne limo: Princeton to JFK plane: JFK to Geneva train: Geneva to Lausanne tourist = datagram transport segment = communication link transportation mode = link layer protocol Note that a bus or plane trip might contain many changes of the bus or plane, but this seems like a single hop travel agent = routing algorithm datagram transferred by different link protocols over different links: e.g., Ethernet on first link, frame relay on intermediate links, on last link each link protocol provides different services e.g., may or may not provide reliability over link

Link Layer Services framing, link access: Routing
encapsulate datagram into frame, adding header, trailer channel access if shared medium “MAC” addresses used in frame headers to identify source, dest different from IP address! Routing reliable delivery between adjacent nodes we learned how to do this already (chapter 3)! seldom used on low bit-error link (fiber, some twisted pair) wireless links: high error rates Q: why both link-level and end-end reliability?

Link Layer Services (more)
flow control: pacing between adjacent sending and receiving nodes Encryption Some links can easily be tapped, so encryption is needed for privacy error detection: errors caused by signal attenuation, noise. receiver detects presence of errors: signals sender for retransmission or drops frame error correction: receiver identifies and corrects bit error(s) without resorting to retransmission half-duplex and full-duplex with half duplex, nodes at both ends of link can transmit, but not at same time

Where is the link layer implemented?
in each and every host in the network Which other layers are implemented in every host? link layer implemented in “adaptor” (aka network interface card NIC) Ethernet card, PCMCI card, card implements link, physical layer attaches into host’s system buses combination of hardware, software, firmware host schematic application transport network link physical cpu memory host bus (e.g., PCI) controller physical transmission network adapter card

Adaptors Communicating
datagram datagram controller controller sending host receiving host datagram frame receiving side looks for errors, rdt, flow control, etc extracts datagram passes to upper layer at receiving side Moves frame to another link sending side: encapsulates datagram in frame adds error checking bits, rdt, flow control, etc.

5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-layer Addressing 5.5 Ethernet 5.6 Link-layer switches 5.7 PPP 5.8 Link Virtualization: ATM. MPLS

Error Detection EDC= Error Detection and Correction bits (redundancy)
D = Data protected by error checking, may include header fields Error detection not 100% reliable! protocol may miss some errors, but rarely larger EDC field yields better detection and correction otherwise

Parity Checking Two Dimensional Bit Parity: Single Bit Parity:
Detect single bit errors Two Dimensional Bit Parity: Detect and correct single bit errors

Internet checksum (review)
Goal: detect “errors” (e.g., flipped bits) in transmitted packet (note: used at transport layer only) Receiver: compute checksum of received segment check if computed checksum equals checksum field value: NO - error detected YES - no error detected. But maybe errors nonetheless? Sender: treat segment contents as sequence of 16-bit integers checksum: addition (1’s complement sum) of segment contents sender puts checksum value into UDP checksum field

Checksumming: Cyclic Redundancy Check
view data bits, D, as a binary number choose r+1 bit pattern (generator), G goal: choose r CRC bits, R, such that <D,R> exactly divisible by G (modulo 2) receiver knows G, divides <D,R> by G. If non-zero remainder: error detected! can detect all burst errors less than r+1 bits widely used in practice (Ethernet, WiFi, ATM)

CRC Example Want: D.2r XOR R = nG equivalently: D.2r = nG XOR R
if we divide D.2r by G, want remainder R D.2r G R = remainder[ ]

5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-layer Addressing 5.5 Ethernet 5.6 Link-layer switches 5.7 PPP 5.8 Link Virtualization: ATM, MPLS

Multiple Access Links and Protocols
Two types of “links”: point-to-point PPP for dial-up access point-to-point link between Ethernet switch and host broadcast (shared wire or medium) old-fashioned Ethernet wireless LAN humans at a cocktail party (shared air, acoustical) shared wire (e.g., cabled Ethernet) shared RF (e.g., WiFi) shared RF (satellite)

Multiple Access Control (MAC) protocols
single shared broadcast channel two or more simultaneous transmissions by nodes: interference collision if node receives two or more signals at the same time multiple access protocol An algorithm that determines how nodes share channel, i.e., determine when node can transmit communication about channel sharing must use channel itself! out-of-band channel for coordination is difficult

Ideal Multiple Access Protocol
Broadcast channel of rate R bps 1. when one node wants to transmit, it can send at rate R. 2. when M nodes want to transmit, each can send at average rate R/M 3. fully decentralized: no special node to coordinate transmissions no synchronization of clocks, slots Generally, centralized MAC are much more efficient 4. simple

MAC Protocols: a taxonomy
Three broad classes: Channel Partitioning divide channel into smaller “pieces” (time slots, frequency, code) allocate piece to node for exclusive use this approach is difficult since we know that statistical multiplexing can support more users Random Access channel not divided, allow collisions Detect and recover from collisions Detection and recovery (e.g., retransmission) can be inefficient Predictable/guaranteed performance is difficult to achieve Centralized/taking turns

Channel Partitioning MAC protocols: TDMA
TDMA: time division multiple access access to channel in "rounds" each station gets fixed length slot (length = pkt trans time) in each round unused slots go idle GSM (some cell phones) uses TDMA Why? So service is predictable and calls can be rejected if there is not enough bandwidth example: 6-station LAN, 1,3,4 have pkt, slots 2,5,6 idle 6-slot frame 1 3 4 1 3 4

Channel Partitioning MAC protocols: FDMA
FDMA: frequency division multiple access channel spectrum divided into frequency bands each station assigned fixed frequency band unused transmission time in frequency bands go idle GSM also uses FDMA example: 6-station LAN, 1,3,4 have pkt, frequency bands 2,5,6 idle time frequency bands FDM cable

Random Access Protocols
When node has packet to send transmit at full channel data rate R. no a priori coordination among nodes Some approaches use limited coordination two or more transmitting nodes ➜ “collision”, random access MAC protocol specifies: how to detect collisions how to recover from collisions (e.g., via delayed retransmissions) Examples of random access MAC protocols: slotted ALOHA ALOHA CSMA, CSMA/CD, CSMA/CA

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. Aloha is the simplest of MAC protocols Aloha is old but still widely used As will be seen, many protocols have a period of time where nodes transmits when they want. During such periods of time, the MAC essentially Aloha

Collisions Invalid frames may be caused by channel noise or
Because other station(s) transmitted at the same time: collision. Collisions and other link layer losses must be detected and corrected Question 1. Where are all the places that losses can occur? Question 2: where can errors be detected and corrected Roughly speaking, a collision happens even when the last bit of a frame overlaps with the first bit of the next frame.

If another node transmits here, then there is a collision
ALOHA’s Performance 1 If another node transmits here, then there is a collision t0+t t0+3t t0 t0+2t Time vulnerable If another node starts to transmit during this vulnerable period, then a collision will occur

ALOHA’s Performance Assume that users try to send frames at random times (Poisson events). Let G be the average rate that users try to send frames per frame time G is the utilization The probability of trying to send k frames during the vulnerable period (which is TWO frame times long) is The probability zero other frames are sent is P(0)=e-2G. The throughput is the rate that frames are sent multiplied by the probability that the transmission is successful G e-2G

ALOHA’s Performance The best throughput occurs for what value of G?
What is this best throughput?

If a frame is transmitted here, then a collision occurs
Slotted Aloha – frames are only transmitted during slots, they cannot cross slot boundaries If a frame is transmitted here, then a collision occurs But this will only happen if a packet arrives at the MAC layer during this period Time t0+t t0+3t t0 t0+2t vulnerable If another node selects to transmit during this vulnerable period, then a collision will occur The vulnerable period is half the size of unslotted aloha

Slotted Aloha Vulnerable period is halved.
Doubles performance of ALOHA. Throughput=S = G e-G. S = Smax = 1/e = for G = 1. G=1 means typically a node tries to transmit each slot However, the throughput is well below 1; there any many collisions

Slotted Aloha Performance

How long does it take to send a frame?

How long does it take to send a frame? one success k-1 failures Expected number of transmissions

How long does it take to send a frame? one success k-1 failures Expected number of transmissions This analysis is funny because it does not account for the fact that if packets are not successfully transmitted, then the rate at which transmissions are attempted increases.

ALOHA and Slotted ALOHA
Pros single active node can continuously transmit at full rate of channel decentralized simple Cons Collisions wasting slots Inefficient idle slots nodes may be able to detect collision in less than time to transmit packet Slotted aloha requires clock synchronization Lose synchronization requires guard times, which reduces efficiency

CSMA (Carrier Sense Multiple Access)
CSMA: listen before transmit: If channel sensed idle: transmit entire frame If channel sensed busy, defer transmission human analogy: don’t interrupt others!

Question For 10 Mbps ethernet, the maximum cable length is 2000m
Why is the maximum length for 100Mbps 10 times shorter than 10Mbps?

CSMA collisions collisions can still occur: collision: note:
spatial layout of nodes collisions can still occur: propagation delay means two nodes may not hear each other’s transmission collision: entire packet transmission time wasted note: role of distance & propagation delay in determining collision probability

CSMA/CD collision detection
Transmitter 1 Transmitter 2 Propagation delay Position on wire Receiver 1 Receiver 1 receives garbled signal Transmission time Collision detected by transmitter 2 time Collision detected by transmitter 1. When is it detected?

CSMA/CD collision detection
Transmitter 1 Transmitter 2 Position on wire Receiver 1 Transmission time Propagation delay Receiver 1 receives garbled signal time Collision NOT detected by transmitter 1 Collision detected by transmitter 2 What are the requirements to ensure that collisions are detected? The transmitter must transmit for 2*Tpropagation + epsilon The transmit time is frame length / bit rate Therefore 2*CableLength/speed of propagation + epsilon < FrameLength/bit-rate

CSMA/CD What are the requirements to ensure that collisions are detected? The transmitter must transmit for 2*Tpropagation + epsilon The transmit time is frame length / bit rate Therefore 2*CableLength/speed of propagation + epsilon < FrameLength/bit-rate If frame length can be arbitrarily small, then the cable length must be very short Thus, frames cannot be arbitrarily small. Minimum frame length in Ethernet is 64B. The minimum frame length in Ethernet is independent of bit-rate. Why is the maximum cable length of a 10Mbps ethernet cable 10 times longer than the maximum cable length of a 100Mbps ethernet?

CSMA/CD (Collision Detection)
CSMA/CD: carrier sensing, deferral as in CSMA collisions detected within short time colliding transmissions aborted, reducing channel wastage collision detection: easy in wired LANs: measure signal strengths, compare transmitted, received signals Difficult/impossible in wireless LANs: received signal strength overwhelmed by local transmission strength human analogy: the polite conversationalist

persistent 1-persistent p-persistent Exponential Backoff
What to do when the link is found to be busy? 1-persistent If medium is idle, then transmit. If medium is not idle, then wait until it is and then transmit. In this case, all nodes that desire to transmit during the period when a node is transmitting will collide! p-persistent If medium is not idle, then wait until it is idle Once idle then transmit with probability p. And wait for the next slot with probability 1-p and repeat. Here slot does not have to be the time to send a full frame, but just enough time to let other hosts start sending. Exponential Backoff Next slide

Exponential Backoff Upon desiring to transmit a frame, set BackOff = BO (some starting value, 4 and 8 are common) If medium is idle, then transmit. If medium is not idle, then wait until it is idle Once idle, pick an integer, r, between 0 and BO-1 Wait r time slots A time slot is long enough so that if a node begins to trasnmit at the beginning of the time slot, then all nodes will hear the transmission before the time slot end Give an equation for the length of a time slot If no other transmission begins before the r time slots, then transmit If a collision is detected, Continue to transmit so that all nodes will know that a collision occurred, then stop Set BO = min( 2 * BO , BO_Max ) In ethernet BO_max = 1024 Go to step 4 Question: discuss the different ways in which backoff is used in network protocols

“Taking Turns” MAC protocols
channel partitioning MAC protocols: share channel efficiently and fairly at high load inefficient at low load: delay in channel access, 1/N bandwidth allocated even if only 1 active node! Random access MAC protocols efficient at low load: single node can fully utilize channel high load: collision overhead Be careful. Here we say that high load is when the number of users increases. If the number of users is fixed (and small), then the efficiency under high load is not as bad “taking turns” protocols look for best of both worlds! Use in mobile phones data access aka WiMax partly uses this approach specifies this capability, but it is not widely deployed YET

Polling: master node “invites” slave nodes to transmit in turn typically used with “dumb” slave devices concerns: polling overhead latency single point of failure (master) QoS guarantees can be made If a VoIP call requires 12bps. The master can determine if the call will receive the desire quality and ensure that it does. When congested, new calls are rejected, but existing call continue to receive good performance Consider the difference between the demands by VoIP and services provided by TCP Guarantees are worth much more money than non-guarantees data poll master data slaves

Token passing: control token passed from one node to next sequentially. token message concerns: token overhead Latency single point of failure (token) T (nothing to send) T data

Summary of MAC protocols
channel partitioning, by time, frequency or code Time Division, Frequency Division random access (dynamic), ALOHA, S-ALOHA, CSMA, CSMA/CD carrier sensing: easy in some technologies (wire), hard in others (wireless) CSMA/CD used in Ethernet CSMA/CA used in (We’ll study it when we talk about wireless) taking turns polling from central site, token passing Bluetooth, FDDI, IBM Token Ring

5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet 5.6 Link-layer switches 5.7 PPP 5.8 Link Virtualization: ATM, MPLS

MAC Addresses and ARP 32-bit IP address:
network-layer address used to get datagram to destination IP subnet MAC (or LAN or physical or Ethernet) address: function: get frame from one interface to another physically-connected interface (same network) The textbook is wrong about this. Today, hosts are almost never physically connected 48 bit MAC address (for most LANs) burned in NIC ROM, also sometimes software settable

LAN Addresses and ARP Each adapter on LAN has unique LAN address
1A-2F-BB AD Broadcast address = FF-FF-FF-FF-FF-FF LAN (wired or wireless) = adapter 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98

LAN Address (more) MAC address allocation administered by IEEE
manufacturer buys portion of MAC address space (to assure uniqueness) Check OUI lookup Google OUI lookup Enter MAC address See manufacture analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address MAC flat address ➜ portability can move LAN card from one LAN to another IP hierarchical address NOT portable address depends on IP subnet to which node is attached If a NIC is changed, then the MAC is changed Whereas, the IP address can stay the same

ARP: Address Resolution Protocol
Question: how to determine MAC address of B knowing B’s IP address? Each IP node (host, router) on LAN has ARP table At prompt, >> arp -a ARP table: IP/MAC address mappings for some LAN nodes < IP address; MAC address; TTL> TTL (Time To Live): time after which address mapping will be forgotten (typically 20 min) 1A-2F-BB AD LAN 71-65-F7-2B-08-53 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98

ARP protocol: Same LAN (network)
A wants to send datagram to C Check if C’s IP address is in the same subnet Use subnet mask and compare this nodes IP to C’s IP E.g., my IP= B’s IP= Subnet mask is => the first 8 bytes define the subnet So in this case, A and B are in different subnets Thus, the datagram is sent to the gateway, which must be in the same subnet. Suppose that the B is the gateway, but only the IP address of B is known Suppose a host wants to send to B and only B’s IP address is know and B is in the same subnet and B’s MAC address not in A’s ARP table. A broadcasts ARP query packet, containing B's IP address dest MAC address = FF-FF-FF-FF-FF-FF Ethernet frame type = ARP query Other types include datagram all machines on LAN receive ARP query B receives ARP packet, replies to A with its (B's) MAC address frame sent to A’s MAC address (unicast) A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) soft state: information that times out (goes away) unless refreshed ARP is “plug-and-play”: nodes create their ARP tables without intervention from net administrator

Addressing: routing to another LAN
walkthrough: send datagram from A to B via R assume A knows B’s IP address two ARP tables in router R, one for each IP network (LAN) R 1A-23-F9-CD-06-9B E6-E BB-4B CC-49-DE-D0-AB-7D A C-E8-FF-55 88-B2-2F-54-1A-0F B 49-BD-D2-C7-56-2A

R A creates IP datagram with source A, destination B
A uses ARP to get R’s MAC address for A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram A’s NIC sends frame R’s NIC receives frame R removes IP datagram from Ethernet frame, sees its destined to B R uses ARP to get B’s MAC address R creates frame containing A-to-B IP datagram sends to B This is a really important example – make sure you understand! R 1A-23-F9-CD-06-9B E6-E BB-4B CC-49-DE-D0-AB-7D A C-E8-FF-55 88-B2-2F-54-1A-0F B 49-BD-D2-C7-56-2A

ARP Watch wireshark without any connections
What happens if I set an entry in the ARP table with the IP address of my gateway, but my MAC address? E.g., take two machines A and B on the same LAN (what does this mean? How can you tell if two machines are on the same LAN). Let P be a nonexistent IP address in the LAN. On machine A ping P. Use wireshark on B to see no evidence of the ping. On A, set an arp entry on A with IP = P and MAC = B’s MAC Then ping P Watch ping messages appear in wireshark on B But still, no response.

ARP spoofing – man-in-the-middle attack
If the medium is shared, then a node can eavesdrop on transmissions Wireless uses link layer encryption These days, wired ethernet used a dedicate wires from the switch (link layer router) to each host But ARP attack still works Goal: intercept messages between the victim and anyone else I record the real MAC address of the victim When an ARP query request is made for the victim, I respond with my MAC Or better yet, I I w

Victim: MAC=00:12:12:12:12:12 IP: Who has IP address Who has IP address switch Some other host Who has IP address attacker: MAC=00:11:11:11:11:11 IP=

Victim: MAC=00:12:12:12:12:12 IP: MAC 00:12:12:12:12 has IP address switch MAC 00:12:12:12:12 has IP address MAC 00:12:12:12:12 has IP address Some other host Save MAC/IP mapping in cache for 20 minutes attacker: MAC=00:11:11:11:11:11 IP= Attacker knows the MAC of victim

Later (like the next day at 2AM), when all caches have been cleared Victim: MAC=00:12:12:12:12:12 IP: Confused… but ignores it MAC 00:11:11:11:11 has IP address MAC 00:11:11:11:11 has IP address switch Some other host MAC 00:11:11:11:11 has IP address attacker: MAC=00:11:11:11:11:11 IP= Save IP/ARP mapping in cache Attacker knows the MAC of victim

Later (like the next day at 2AM), when all caches have been cleared Victim: MAC=00:12:12:12:12:12 IP: Ahh, I got the secret plan I was expecting switch Some other host MAC 00:11:11:11:11: IP: : The secret plan is ….. attacker: MAC=00:11:11:11:11:11 IP= Changed MAC address to correct address MAC 00:12:12:12:12: IP: The secret plan is ….. Attacker knows the secret plan

Some new switches can protect against these attacks How can these attacks be detected and stopped? One way is to detect a attacker is to look at ARP tables and see is a single IP has two MACs Is real IP and the victims IP But if a machine has wired and wireless NICs and is running microsoft OS, the OS will sometimes send a frame with the wireless IP as source address over the wired LAN and hence with the wired MAC address Then tables will record the mapping between the MAC and IP, and there will be two IPs for a single MAC

5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet 5.6 Link-layer switches 5.7 PPP 5.8 Link Virtualization: ATM and MPLS

Ethernet “dominant” wired LAN technology: cheap $20 for NIC
first widely used LAN technology simpler, cheaper than token LANs and ATM kept up with speed race: 10 Mbps – 10 Gbps Metcalfe’s Ethernet sketch

Star topology bus topology popular through mid 90s star topology LAN
all nodes in same collision domain (can collide with each other) star topology active switch in center each “spoke” runs a (separate) Ethernet protocol (nodes do not collide with each other) LAN Multiple stars connected (we’ll see later) switch bus: coaxial cable star

Ethernet Frame Structure
Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: 7 bytes with pattern followed by one byte with pattern used to synchronize receiver, sender clock rates

Ethernet Frame Structure (more)
Addresses: 6 bytes if adapter receives frame with matching destination address, or with broadcast address (eg ARP packet), it passes data in frame to network layer protocol otherwise, adapter discards frame (unless in promiscuous modes) Type: ARP query/response LAN routing higher layer protocol (mostly IP but others possible, e.g., Novell IPX, AppleTalk) CRC: checked at receiver, if error is detected, frame is dropped

Ethernet: Unreliable, connectionless
connectionless: No handshaking between sending and receiving NICs unreliable: receiving NIC doesn’t send acks or nacks to sending NIC stream of datagrams passed to network layer can have gaps (missing datagrams) gaps will be filled if app is using TCP otherwise, app will see gaps Ethernet’s MAC protocol: unslotted CSMA/CD

Ethernet CSMA/CD algorithm
NIC receives datagram from network layer, creates frame If NIC senses channel idle, starts frame transmission If NIC senses channel busy, waits until channel idle, then transmits 1-persistant! If NIC transmits entire frame without detecting another transmission, NIC is done with frame ! 4. If NIC detects another transmission while transmitting, aborts and sends jam signal 5. After aborting, NIC enters exponential backoff: after mth collision, NIC chooses K at random from {0,1,2,…,2m-1}. NIC waits K slots where one slot is 512 bit times, returns to Step 2

Ethernet’s CSMA/CD (more)
Jam Signal: make sure all other transmitters are aware of collision; 48 bits Bit time: .1 microsec for 10 Mbps Ethernet ; for K=1023, wait time is about 50 msec Exponential Backoff: Goal: adapt retransmission attempts to estimated current load heavy load: random wait will be longer first collision: choose K from {0,1}; delay is K· 512 bit transmission times after second collision: choose K from {0,1,2,3}… after ten or more collisions, choose K from {0,1,2,3,4,…,1023}

CSMA/CD efficiency Tprop = max prop delay between 2 nodes in LAN
ttrans = time to transmit max-size frame efficiency goes to 1 as tprop goes to 0 as ttrans goes to infinity larger frame size is better, higher bit-rate is worst better performance than ALOHA: and simple, cheap, decentralized! Most ethernet is used with switches. So collision never occur

802.3 Ethernet Standards: Link & Physical Layers
many different Ethernet standards common MAC protocol and frame format different speeds: 2 Mbps, 10 Mbps, 100 Mbps, 1Gbps, 10G bps different physical layer media: fiber, cable Very large ethernets are possible QoS MPLS runs over ethernet (so traffic engineering is possible) MAC protocol and frame format application transport network link physical copper (twister pair) physical layer 100BASE-TX fiber physical layer 100BASE-T2 100BASE-FX 100BASE-T4 100BASE-SX 100BASE-BX

Manchester encoding used in 10BaseT each bit has a transition
allows clocks in sending and receiving nodes to synchronize to each other no need for a centralized, global clock among nodes! Hey, this is physical-layer stuff!

5.2 Error detection and correction 5.3 Multiple access protocols 5.4 Link-layer Addressing 5.5 Ethernet 5.6 Link-layer switches 5.7 PPP 5.8 Link Virtualization: ATM, MPLS

Hubs … physical-layer (“dumb”) repeaters:
bits coming in one link go out all other links at same rate all nodes connected to hub can collide with one another no frame buffering no CSMA/CD at hub: host NICs detect collisions twisted pair hub

Interconnecting with hubs
Backbone hub interconnects LAN segments Extends max distance between nodes But individual segment collision domains become one large collision domain Can’t interconnect 10BaseT & 100BaseT hub hub hub hub

Switch link-layer device: smarter than hubs, take active role
Store and forward Ethernet frames Question: do switches in circuit switching networks store and forward? examine incoming frame’s MAC address, selectively forward frame to one-or-more outgoing links when frame is to be forwarded on segment, uses CSMA/CD to access segment transparent hosts are unaware of presence of switches plug-and-play, self-learning switches do not need to be configured

Switch: allows multiple simultaneous transmissions
hosts have dedicated, direct connection to switch switches buffer packets Ethernet protocol used on each incoming link, but no collisions; full duplex each link is its own collision domain switching: A-to-A’ and B-to-B’ simultaneously, without collisions not possible with dumb hub C’ B 1 2 6 3 4 5 C B’ A’ switch with six interfaces (1,2,3,4,5,6)

switch with six interfaces
Switch Table A Q: how does switch know that A’ reachable via interface 4, B’ reachable via interface 5? A: each switch has a switch table, each entry: (MAC address of host, interface to reach host, time stamp) looks like a routing table! Q: how are entries created, maintained in switch table? something like a routing protocol? C’ B 1 2 6 3 4 5 C B’ A’ switch with six interfaces (1,2,3,4,5,6)

Switch: self-learning
Source: A Dest: A’ A A A’ switch learns which hosts can be reached through which interfaces Some interfaces are configured. But in other cases… when frame received, switch “learns” location of sender: incoming LAN segment records sender/location pair in switch table C’ B 1 2 6 3 4 5 C B’ A’ MAC addr interface TTL A 1 60 Switch table (initially empty)

Switch: frame filtering/forwarding
When frame received: 1. record link associated with sending host 2. index switch table using MAC dest address 3. if entry found for destination then { if dest on segment from which frame arrived then drop the frame else forward the frame on interface indicated } else flood 4. periodically, purge all old table entries forward on all but the interface on which the frame arrived

Self-Learning A 1 1 2 1 3 2 3 2 3 1 2 3 B MAC Interface MAC Interface

Self-Learning A 1 1 2 1 3 2 3 2 3 1 2 3 B MAC Interface MAC Interface
Dest=B; Source=A 1 1 2 3 1 2 2 3 3 1 MAC Interface 2 3 B

Self-Learning MAC Interface A 1 MAC Interface MAC Interface A Dest=B; Source=A Dest=B; Source=A 1 1 2 3 1 2 2 3 3 Make table entry for A No table entry for B, so flood 1 MAC Interface Note: if the switch has ports manually configured, then the frame is not flooded to a host. But they are flooded to other switches 2 3 B

Self-Learning A 1 1 2 1 3 2 3 2 3 1 2 3 B Make table entry for A
No table entry for B, so flood MAC Interface A 1 MAC Interface A 1 MAC Interface A 1 Dest=B; Source=A Dest=B; Source=A 1 2 3 1 2 2 3 3 1 MAC Interface 2 3 B

Self-Learning A 1 1 2 1 3 2 3 2 3 1 2 3 B Make table entry for A
No table entry for B, so flood MAC Interface A 1 MAC Interface A 1 MAC Interface A 2 A 1 1 2 Dest=B; Source=A Dest=B; Source=A 3 1 2 2 3 3 Dest=B; Source=A 1 Dest=B; Source=A MAC Interface A 1 2 3 B Make table entry for A No table entry for B, so flood

Self-Learning A 1 1 2 1 3 2 3 2 3 1 2 3 B MAC Interface A 1 MAC
Dest=A; Source=B B

Self-Learning A 1 1 2 1 3 2 3 2 3 1 2 3 B MAC Interface A 1 MAC
Dest=A; Source=B MAC Interface A 1 B 2 2 3 B Make table entry for B Have a table entry for A, so forward

Self-Learning A 1 1 2 1 3 2 3 2 3 1 2 3 B Make table entry for B
Have a table entry for A, so forward MAC Interface A 1 MAC Interface A 1 B 3 MAC Interface A 2 A 1 1 2 3 1 2 2 3 3 Dest=A; Source=B 1 MAC Interface A 1 B 2 2 3 B

Self-Learning A 1 1 2 1 3 2 3 2 3 1 2 3 B MAC Interface A 1 B 3 MAC
Make table entry for B Have a table entry for A, so forward A 1 1 2 3 1 2 2 3 3 Dest=A; Source=B 1 MAC Interface A 1 B 2 2 3 B

Self-Learning 20 minutes later, all table entries are deleted A 1 1 2
MAC Interface MAC Interface MAC Interface A 1 1 2 3 1 2 2 3 3 1 MAC Interface 2 3 B

Poorly Designed Institutional network. Why?
to external network router LAN - IP subnet mail server web server

Institutional network without a single point of failure
to external network router IP subnet mail server web server A Explain self learning on this network Suppose that A sends a frame to the mail server and all tables are empty? Due to the loops, the frames will loop and overwhelm the network. Loops provide robustness, but have to be eliminated.

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 vertices and bridges/switches are represented by edges. This is backwards from what you might expect, i.e., switches as vertices and LANs as edges If a LAN A’s next hop toward the root is LAN B, then the switch between LAN A and B uses the interfaces to A and B LAN A B3 LAN B B2

Loop Resolution Goal: remove “extra” paths by removing “extra” bridges. Approach: 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. This tree is formed by “disconnecting” switches from some LANs Note 1. here a LAN connects two switches. It is possible that end-host are also attached to the same LAN (CSMA/CD will work), but today, the “LAN” only has a switch on each end. Note 2. The switches are not physically disconnected. Instead, when “disconnected” from a LAN they simply never flood packets over to the LAN. Of course, the spanning tree is recomputed often and if something breaks, then the LAN might be “reconnected” to the switch

Institutional network without a single point of failure
mail server to external network web server router IP subnet A Edge in spanning tree “disconnected” interface, i.e., do not forward or flood frames through this interface

Spanning Tree Algorithm (1)
LANs are represented by vertices and bridges/switches are represented by edges. This is backwards from what you might expect, i.e., switches as vertices and LANs as edges When manufactured, each bridge is given a unique ID. The root is the node with the smallest ID. Approach: Compute paths to the node with smallest ID Paths indicate which of a bridge’s/switch’s interface leads to the switch with smallest ID If a LAN A’s next hop toward the root is LAN B, then the switch between LAN A and B uses the interfaces to A and B (today, a LAN is a wire between two switches. But in theory, multiple host could be attached to this wired and use CSMA/CD to deal with collisions.) If a LAN has more than one bridge, all the bridges on that LAN must decide which bridge can route frames to and from the LAN. Switches remain connected if they are part of the shortest path to switch with min ID If there is a tie, then the one with smaller ID is selected. B3 LAN A LAN B B2

Spanning Tree Algorithm (2)
Bridges exchange messages with the following information 1. The ID of the bridge that is sending the message. 2. The ID for what the sending bridge believes to be the root bridge. 3. The distance (hops) from the sending bridge to the root bridge.

Which interfaces to keep and which to ignore.
Pretend that the objective is to find shortest paths from each LAN to root switch (the one with smallest ID) and use least cost with minimum ID to break ties. By shortest path, we mean paths from a LAN to the route switch that visits the smallest number of switches A switch will keep an interface active if the interface is along a LAN’s shortest path to the root If a LAN has more than one shortest path, then switch with the smallest ID is used. Take a distance vector approach, so we only consider neighbors B A B3 B7 C B5 D F E B2 G B1 H Note, we find these paths not for forwarding, but only to decide which interfaces to “turn off.’” Of course, if a frame is headed to the root, then it will follow the shortest path. Unfortunately, the root might not be the gateway B6 B4 J I

Pretend that the objective is to find shortest paths from each LAN to root switch (the one with smallest ID) and use least cost with minimum ID to break ties. By shortest path, we mean paths from a LAN to the route switch that visits the smallest number of switches A switch will keep an interface active if the interface is along a LAN’s shortest path to the root If a LAN has more than one shortest path, then switch with the smallest ID is used. Take a distance vector approach, so we only consider neighbors B A B3 B7 2 1 C B5 1 D F E Each switch computes distance to root in terms of LAN hops. B2 1 G B1 H 1 B6 1 B4 J I

Pretend that the objective is to find shortest paths from each LAN to root switch (the one with smallest ID) and use least cost with minimum ID to break ties. By shortest path, we mean paths from a LAN to the route switch that visits the smallest number of switches A switch will keep an interface active if the interface is along a LAN’s shortest path to the root If a LAN has more than one shortest path, then switch with the smallest ID is used. Take a distance vector approach, so we only consider neighbors B A B3 B7 2 1 C B5 1 D F E Each of the roots interfaces is ON B2 1 G B1 H 1 B6 1 B4 J I

Pretend that the objective is to find shortest paths from each LAN to root switch (the one with smallest ID) and use least cost with minimum ID to break ties. By shortest path, we mean paths from a LAN to the route switch that visits the smallest number of switches A switch will keep an interface active if the interface is along a LAN’s shortest path to the root If a LAN has more than one shortest path, then switch with the smallest ID is used. Take a distance vector approach, so we only consider neighbors B A B3 B7 2 1 C B5 1 D F E LAN A’s next hop is LAN E. B2 1 G B1 H 1 B6 1 B4 J I

Pretend that the objective is to find shortest paths from each LAN to root switch (the one with smallest ID) and use least cost with minimum ID to break ties. By shortest path, we mean paths from a LAN to the route switch that visits the smallest number of switches A switch will keep an interface active if the interface is along a LAN’s shortest path to the root If a LAN has more than one shortest path, then switch with the smallest ID is used. Take a distance vector approach, so we only consider neighbors B A B3 B7 2 1 C B5 1 D F E LAN A’s next hop is LAN E. Turn on the two interfaces B2 1 G B1 H 1 B6 1 B4 J I

Pretend that the objective is to find shortest paths from each LAN to root switch (the one with smallest ID) and use least cost with minimum ID to break ties. By shortest path, we mean paths from a LAN to the route switch that visits the smallest number of switches A switch will keep an interface active if the interface is along a LAN’s shortest path to the root If a LAN has more than one shortest path, then switch with the smallest ID is used. Take a distance vector approach, so we only consider neighbors B A B3 B7 2 1 C B5 1 D F E LAN D’s next hop is LAN G. Turn on the two interfaces B2 1 G B1 H Note that B3 will not have any interfaces “on” 1 B6 1 B4 J I

Pretend that the objective is to find shortest paths from each LAN to root switch (the one with smallest ID) and use least cost with minimum ID to break ties. By shortest path, we mean paths from a LAN to the route switch that visits the smallest number of switches A switch will keep an interface active if the interface is along a LAN’s shortest path to the root If a LAN has more than one shortest path, then switch with the smallest ID is used. Take a distance vector approach, so we only consider neighbors B A B3 B7 2 1 C B5 1 D F E LAN B’s next hop is LAN E or F. But B5 has a lower ID than B7, so LAN E is used as the next hop. B2 1 G B1 H 1 B6 1 B4 J I

Pretend that the objective is to find shortest paths from each LAN to root switch (the one with smallest ID) and use least cost with minimum ID to break ties. By shortest path, we mean paths from a LAN to the route switch that visits the smallest number of switches A switch will keep an interface active if the interface is along a LAN’s shortest path to the root If a LAN has more than one shortest path, then switch with the smallest ID is used. Take a distance vector approach, so we only consider neighbors B A B3 B7 2 1 C B5 1 D F E LAN B’s next hop is LAN E or F. But B5 has a lower ID than B7, so LAN E is used as the next hop. Turn on the interface B2 1 G B1 H 1 B6 1 B4 J I

Pretend that the objective is to find shortest paths from each LAN to root switch (the one with smallest ID) and use least cost with minimum ID to break ties. By shortest path, we mean paths from a LAN to the route switch that visits the smallest number of switches A switch will keep an interface active if the interface is along a LAN’s shortest path to the root If a LAN has more than one shortest path, then switch with the smallest ID is used. Take a distance vector approach, so we only consider neighbors B A B3 B7 2 1 C B5 1 D F E LAN C’s next hop is LAN F. Turn on the interfaces B2 1 G B1 H 1 B6 1 B4 J I

Pretend that the objective is to find shortest paths from each LAN to root switch (the one with smallest ID) and use least cost with minimum ID to break ties. By shortest path, we mean paths from a LAN to the route switch that visits the smallest number of switches A switch will keep an interface active if the interface is along a LAN’s shortest path to the root If a LAN has more than one shortest path, then switch with the smallest ID is used. Take a distance vector approach, so we only consider neighbors B A B3 B7 2 1 C B5 1 D F E Which other interfaces are “on” B2 1 G B1 H 1 B6 1 B4 J I

Spanning Tree Routing Aka transparent bridges.
Bridge routing table is automatically maintained (set up and updated as topology changes). 3 mechanisms: Loop resolution Address learning Frame forwarding Typically ignore security such as ARP attacks, access control, etc. Loop resolution must happen before address learning. On the EECIS network, the link to the campus network would go down for ~50ms. This would trigger loop resolution During which time no packets were forwarded

Switches vs. Routers both store-and-forward devices
routers: network layer devices (examine network layer headers) switches are link layer devices routers maintain routing tables, implement routing algorithms switches maintain switch tables, implement filtering, learning algorithms

Summary comparison

5.2 Error detection and correction 5.3Multiple access protocols 5.4 Link-Layer Addressing 5.5 Ethernet 5.6 Hubs and switches VLAN

Typical LAN Grouped based on the hub (physically)
Use routers as LAN segmentation (broadcast) A single enterprise LAN is too large Each ARP request is broadcast over the entire LAN When self-learning (e.g., every 20 minutes), too much traffic is flooded This traffic is viewable by anyone in the LAN (not easy to provide firewalls between groups) Solution Create smaller LANs each with subnet. The subnet could represent a workgroup Shared drives, printers, etc LAN-based Firewall/access control However, 20% to 40% of work force moves every year Recabling / readdressing and reconfiguration Work group members might be in different locations e.g., dedicated switch for each work group in each floor or each building. Can’t we share switches with other work groups?

VLAN VLAN is a broadcast domain
Grouped based on logical function, department or application. Traffic can only be switched between VLANS with a router Like switching between regular LANs

VLAN VLANs can logically segment users into different subnets (broadcast domains) Broadcast frames are only switched on the same VLAN ID. Users can be logically group via software based on: Ethernet port/jack port number protocol being used application being used

LAN VS. VLAN

VLAN across backbone Backbone Inter-Domain communication
High-speed link (100 Mbps or more) Inter-connect to router VLAN traffic between switches (trunks) is tagged (802.1q) or encapsulated (ISL) to identify VLAN membership

Router’s Role Provides connection between different VLANs
For example, you have VLAN1 and VLAN2. Within the switch, users on separate VLANs cannot talk to each other (benefit of a VLAN!) However, users on VLAN1 can users on VLAN2 but they need a router to do it.

VLAN Techniques Two techniques
Frame Filtering--examines particular information about each frame (MAC address or layer 3 protocol type) Frame Tagging--places a unique identifier in the header of each frame as it is forwarded throughout the network backbone.

Frame Tagging IEEE 802.1q Assigns a VLAN ID to each frame
Switch understands the tag Places a tag in the frame Tags are removed by the switch

VLAN implementation Created by software running on Layer 2 switches
Three methods for implementing VLANs Port-Centric Static Dynamic

Port-Centric VLAN 3 Port-Centric VLANs
Same VLAN, same router interface Easy for management

Static VLAN Ports on a switch are administratively assigned to a VLAN
Benefits can be assigned by port, address, or protocol type secure, easy to configure and monitor works well in networks where moves are controlled

Dynamic VLAN Switch ports can automatically determine a user’s VLAN assignment based on either/or: MAC / logical address / protocol type When connected to an unassigned port, the switch dynamically configures the port with the correct VLAN

Virtualization of networks
Virtualization of resources: powerful abstraction in systems engineering: computing examples: virtual memory, virtual devices Virtual machines: e.g., java IBM VM os from 1960’s/70’s layering of abstractions: don’t sweat the details of the lower layer, only deal with lower layers abstractly

The Internet: virtualizing networks
1974: multiple unconnected nets ARPAnet data-over-cable networks packet satellite network (Aloha) packet radio network … differing in: addressing conventions packet formats error recovery routing ARPAnet satellite net "A Protocol for Packet Network Intercommunication", V. Cerf, R. Kahn, IEEE Transactions on Communications, May, 1974, pp

The Internet: virtualizing networks
Internetwork layer (IP): addressing: internetwork appears as single, uniform entity, despite underlying local network heterogeneity network of networks Gateway: “embed internetwork packets in local packet format or extract them” route (at internetwork level) to next gateway gateway ARPAnet satellite net

Cerf & Kahn’s Internetwork Architecture
What is virtualized? two layers of addressing: internetwork and local network new layer (IP) makes everything homogeneous at internetwork layer underlying local network technology cable satellite 56K telephone modem today: ATM, MPLS … “invisible” at internetwork layer. Looks like a link layer technology to IP!

ATM and MPLS ATM, MPLS separate networks in their own right
different service models, addressing, routing from Internet viewed by Internet as logical link connecting IP routers just like dialup link is really part of separate network (telephone network) ATM, MPLS: of technical interest in their own right

Asynchronous Transfer Mode: ATM
1990’s/00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architecture Goal: integrated, end-end transport of carry voice, video, data meeting timing/QoS requirements of voice, video (versus Internet best-effort model) “next generation” telephony: technical roots in telephone world packet-switching (fixed length packets, called “cells”) using virtual circuits

ATM architecture adaptation layer: only at edge of ATM network
physical ATM AAL end system switch adaptation layer: only at edge of ATM network data segmentation/reassembly roughly analagous to Internet transport layer ATM layer: “network” layer cell switching, routing physical layer

ATM: network or link layer?
Vision: end-to-end transport: “ATM from desktop to desktop” ATM is a network technology Reality: used to connect IP backbone routers “IP over ATM” ATM as switched link layer, connecting IP routers IP network ATM network

ATM Adaptation Layer (AAL)
ATM Adaptation Layer (AAL): “adapts” upper layers (IP or native ATM applications) to ATM layer below AAL present only in end systems, not in switches AAL layer segment (header/trailer fields, data) fragmented across multiple ATM cells analogy: TCP segment in many IP packets physical ATM AAL end system switch

ATM Adaptation Layer (AAL) [more]
Different versions of AAL layers, depending on ATM service class: AAL1: for CBR (Constant Bit Rate) services, e.g. circuit emulation AAL2: for VBR (Variable Bit Rate) services, e.g., MPEG video AAL5: for data (eg, IP datagrams) User data AAL PDU ATM cell

ATM Layer Service: transport cells across ATM network
analogous to IP network layer very different services than IP network layer Guarantees ? Network Architecture Internet ATM Service Model best effort CBR VBR ABR UBR Congestion feedback no (inferred via loss) no congestion yes Bandwidth none constant rate guaranteed minimum Loss no yes Order no yes Timing no yes

ATM Layer: Virtual Circuits
VC transport: cells carried on VC from source to dest call setup, teardown for each call before data can flow each packet carries VC identifier (not destination ID) every switch on source-dest path maintain “state” for each passing connection link,switch resources (bandwidth, buffers) may be allocated to VC: to get circuit-like perf. Permanent VCs (PVCs) long lasting connections typically: “permanent” route between to IP routers Switched VCs (SVC): dynamically set up on per-call basis

ATM VCs Advantages of ATM VC approach:
QoS performance guarantee for connection mapped to VC (bandwidth, delay, delay jitter) Drawbacks of ATM VC approach: Inefficient support of datagram traffic one PVC between each source/dest pair) does not scale (N*2 connections needed) SVC introduces call setup latency, processing overhead for short lived connections

ATM Layer: ATM cell 5-byte ATM cell header 48-byte payload
Why?: small payload -> short cell-creation delay for digitized voice halfway between 32 and 64 (compromise!) Cell header Cell format

ATM cell header VCI: virtual channel ID
will change from link to link thru net PT: Payload type (e.g. RM cell versus data cell) CLP: Cell Loss Priority bit CLP = 1 implies low priority cell, can be discarded if congestion HEC: Header Error Checksum cyclic redundancy check

ATM Physical Layer (more)
Two pieces (sublayers) of physical layer: Transmission Convergence Sublayer (TCS): adapts ATM layer above to PMD sublayer below Physical Medium Dependent: depends on physical medium being used TCS Functions: Header checksum generation: 8 bits CRC Cell delineation With “unstructured” PMD sublayer, transmission of idle cells when no data cells to send

ATM Physical Layer Physical Medium Dependent (PMD) sublayer
SONET/SDH: transmission frame structure (like a container carrying bits); bit synchronization; bandwidth partitions (TDM); several speeds: OC3 = Mbps; OC12 = Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps TI/T3: transmission frame structure (old telephone hierarchy): 1.5 Mbps/ 45 Mbps unstructured: just cells (busy/idle)

IP-Over-ATM IP over ATM
replace “network” (e.g., LAN segment) with ATM network ATM addresses, IP addresses Classic IP only 3 “networks” (e.g., LAN segments) MAC (802.3) and IP addresses ATM network Ethernet LANs Ethernet LANs

IP-Over-ATM AAL ATM phy Eth IP app transport

Datagram Journey in IP-over-ATM Network
at Source Host: IP layer maps between IP, ATM dest address (using ARP) passes datagram to AAL5 AAL5 encapsulates data, segments cells, passes to ATM layer ATM network: moves cell along VC to destination at Destination Host: AAL5 reassembles cells into original datagram if CRC OK, datagram is passed to IP

IP-Over-ATM Issues: IP datagrams into ATM AAL5 PDUs
from IP addresses to ATM addresses just like IP addresses to MAC addresses! ATM network Ethernet LANs

Multiprotocol label switching (MPLS)
initial goal: speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding borrowing ideas from Virtual Circuit (VC) approach but IP datagram still keeps IP address! PPP or Ethernet header MPLS header IP header remainder of link-layer frame label Exp S TTL 20 3 1 5

MPLS capable routers a.k.a. label-switched router
forwards packets to outgoing interface based only on label value (don’t inspect IP address) MPLS forwarding table distinct from IP forwarding tables signaling protocol needed to set up forwarding RSVP-TE forwarding possible along paths that IP alone would not allow (e.g., source-specific routing) !! use MPLS for traffic engineering must co-exist with IP-only routers

MPLS forwarding tables
in out out label label dest interface A in out out label label dest interface A D D A R6 D 1 1 R4 R3 R5 A R2 in out out label label dest interface A R1 in out out label label dest interface A

VLAN Recall that ARPs are flooded throughout the entire LAN
On a very large LAN, this flooding can be expensive VLANs can construct multiple LANs within a single LAN ARPs are only flooded in the VLAN

Chapter 5: Summary principles behind data link layer services:
error detection, correction sharing a broadcast channel: multiple access link layer addressing instantiation and implementation of various link layer technologies Ethernet switched LANS PPP virtualized networks as a link layer: ATM, MPLS

Chapter 5: let’s take a breath
journey down protocol stack complete (except PHY) solid understanding of networking principles, practice ….. could stop here …. but lots of interesting topics! wireless multimedia security network management

Chapter 5: The Data Link Layer

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