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CS 640 Introduction to Computer Networks Fall 1999 Professor Landweber Copyright 1999, Lawrence H. Landweber Copyright 1996, Larry Peterson (slides marked.

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Presentation on theme: "CS 640 Introduction to Computer Networks Fall 1999 Professor Landweber Copyright 1999, Lawrence H. Landweber Copyright 1996, Larry Peterson (slides marked."— Presentation transcript:

1 CS 640 Introduction to Computer Networks Fall 1999 Professor Landweber Copyright 1999, Lawrence H. Landweber Copyright 1996, Larry Peterson (slides marked with @ in lower left corner and those with white background with the exception of the OSPF and TCP dialog slides which are from IETF RFCs)

2 Computer Communications Network (network, computer network) Distributed system with: Transmission media (coaxial cable, twisted pair (cat3, cat5), glass fibre, wireless-ether) Switching elements (switch, router, node, hub, bridge, gateway …) Hosts (computer, server, workstation end- system, node) Protocols (physical, link, network, transport, session, presentation, application)

3 Communications Medium Connects switching elements to each other and to hosts Transports digital data stream Link –point-to-point - connects single host to switching elt or switching elt to another switching elt –multipoint or broadcast - connects one or more hosts and switching elts to each other

4 Units - Orders of Magnitude kilo (thousand), mega (million), giga (billion), tera (trillion… thousand billion), peta (10**15), exa (10**18) Till early 1980s high bandwidth meant 56Kbps - today Gbps is high speed A commercial fibre can transmit 800Gbps (80 data streams at 10Gbps - DWDM - Dense Wave Division Multiplexing) Research state of the art is 3.2Tbps (80 data streams at 40Gbps) for a single fibre

5 Switching Elements Responsible for the transport of data across a network - routing May support congestion handling or quality of service mechanisms Examples –routers - IP –switches - ATM or local area networks (e.g., Ethernet-802.3, token ring) –bridges, hubs - local area networks

6 Network Types - Direct Connectivity point-to-point mulitpoint /broadcast Ethernet Token Ring Also satellite/radio

7 Network Types - Indirect Connectivity switched networks internetworks backbone link host switching element

8 Protocols A protocol is a set of conventions that specifies the rules or parameters for communication between two entities (hosts, processes, switching elts, devices, etc).

9 Protocols specify conventions for: The type, semantics, and format of information to be conveyed between entities Establishing/closing connections or sessions Routing across a network or internetwork Flow control: speed matching between entities Reliability: error detection, handling, correction

10 Protocols also specify Data and object types and their representation Session characteristics (e.g., checkpointing, encryption-security, compression) Congestion detection and control Quality of service parameters and handling

11 The Basic Idea: Packet Switching Data networks use packet switching instead of circuit switching, the method traditionally used in the telephone system. Packet switching was first invented in the 1960s as a method for building a survivable communications system in a battlefield environment.

12 Circuit Switching - Telephone System Model Dedicated bandwidth (capacity) reserved along fixed path from caller to called for duration of conversation (signalling -setup) Several conversations can share a link…but each is allocated fixed fraction of link bw Bandwidth not shared so during periods of silence, it is wasted If dedicated bandwidth not available for a call, then the call not completed (blocking)

13 Circuit Switching



16 Path is fixed - all data follows this path Bandwidth / capacity reserved on all links Data flows continuously - NO PACKETS BW unavailable --> BLOCKING

17 Packet Switching - Data Network Model PacketHeaderPayload 1234 5 Each data stream divided into chunks, called packets; each packet sent separately into the network; packets may follow diff paths from source to dest Each packet has a header and possibly a trailer that contain information as to its source and destination

18 Packet Switching - Data Network Model Bandwidth not reserved but is shared between data streams Complete packet arrives at sw elt before it is forwarded to next sw elt; store / forward Store and forward systems must deal with allocation of buffers in sw elts and with congestion that may result from too many packets arriving in too short a time If a buffer not available when a packet arrives at sw elt, a packet is discarded (may not be one that just arrived)

19 Packet Switching - Datagrams and Virtual Circuits Examples of packet switching ( i.e., S&F, buffers, congestion, no link bw reserved) Virtual Circuit –must set up path - vc (conn estab - signalling) –path fixed for all packets of a data stream –after setup, header need only specify vc/path Datagram –packets for data stream may travel on diff paths –headers must include full address of dest

20 Multiplexing Share network resources (sw elt and links) among multiple users TDM - time div multiplexing (Alohanet, satellite) FDM - frequency div multiplexing (CATV) @

21 TDM Slots can be statically or dynamically assigned; assignment decision can be centralized or distributed Circuit switching can use TDM - e.g., T1 Statistical TDM - packet switching –packets from different sources interleaved –buffer contending packets –order of service may be FIFO or other A B C A A C B

22 TDM - Circuit Sw. - T1 24 calls/conversations in parallel 125 microseconds - each with 8 bits (digital) for 8 bits 7 data 1 parity Each call gets 8 * 8000 bits per second or 64Kbps Total BW is 64Kbps * 24 + some admin bits or about 1.53Mbps T3 or DS3 = 28 T1 or ~45Mbps E1= 2Mbps

23 What Can Go Wrong? Signal errors - noise - leads to garbled bit(s) –CRC, checksum Packet loss due to congestion –sequence numbers, acks, timers, retransmission (voice vs. file transfer) Packets delayed, garbled, duplicated Link or sw elt failure Depending on application, protocols may have to compensate for or correct the above @

24 Performance - Bandwidth Amount of data that can be transmitted per unit time (Kbps, Gbps, etc) Related to bit width / coding BW of link vs. end-end throughput Throughput determined by – bw of links on path – sw elts proc speed –congestion-queuing time –transmission rate of sender @

25 Performance - Latency Delay Time to send message or single bit from A to B Components –propagation - signal speed in medium - length of links –transmission / processing time at sender and sw elts –queuing time at sw elts and at entrance to network Speed of light –3.0 * (10**8) meters / second in a vacuum or ~1 foot per nanosecond (billionth of a second) –2.0 to 2.7 * (10**8) meters per second in various media –latency in LAN vs. terrestrial WAN vs. satellite @

26 Latency vs. Bandwidth Independent Bandwidth - Delay Product –amount of data that the network can store between source and destination –if RTT used for delay, then amount that can be sent before sender can receive an ack –example: 100ms RTT and 45Mbps: bw-delay product is 560Kbytes Jitter - difference in time between arrival of successive packets in a data stream

27 Protocol Architecture - Layering Use layers to localize protocol functionality Protocol that implements a layer has interfaces to: –higher level protocol and lower layer protocol services to / from (service interface) encapsulation / decapsulation *** connection oriented or connectionless –peer protocol messages exchanged with peer protocol entity control information in header / trailer

28 Interfaces service interface service interface peer-to-peer interface Protocol Entity Protocol Entity

29 Connection-oriented Service Interface (CO) Users of peer protocol entities can assume there a direct physical connection or wire between them (connection setup required) Data sent by one user is received by the peer user in same order as it was sent by first user Definition of CO Service Interface is indep of its implementation in network (vc/dg) May or may not be reliable (TCP vs. ATM)

30 Connectionless Service Interface (CL) No notion of a connection between users of peer protocol entities CL service interface promises best effort delivery of data from one user to the other, data may or may not reach the peer user and may or may not be delivered in order sent AGAIN NOTE that this says nothing about the implementation method within the network

31 More on Service Interfaces CO, CL refer to service interfaces NOT to the protocols used to implement the service The spec of a service interface does not imply anything about how the interface is implemented by a protocol It also does not imply anything about how data is transported across the network THIS DISTINCTION IS OFTEN BLURRED

32 OSI Open Systems Interconnection Model 7 Layers application presentation session transport network link physical end system sw elts in network

33 OSI Model Model of protocol functionality (CCITT and ISO - Int. Org. for Standardization) Just a model … does not specify details of protocols and interfaces or their implementation Different protocols may conform to the model but not interoperate Useful as a way of thinking about protocols - a taxonomy of protocol functionality

34 Layers Physical - what happens on wire - coding of bits, electrical or optical characteristics, config of pins, connectors and other hardware components (Data) Link - handles transfer of data between two or more entities connected by a single link or hop. Service interface may be CO or CL. May or may not be reliable. (X.25 Level 2, IEEE 802.x)

35 Layers Network - Transfer of data across a network. Includes switching (dg, vc), routing, congestion avoidance and control, network wide addressing. (IP addresses, IP/ICMP, X.25 Level 3, ATM, BGP, OSPF) Transport - Process to process transfer of data. Connectionless (UDP) or connection- oriented service (TCP) interface.

36 Layers Session - conventions for sessions. E.g., checkpointing, half or full duplex, security. Presentation - syntactic conventions for communication between apps, encoding rules and file formats. Application - includes session and presentation in the Internet protocol architecture

37 FTP TCP IP UDP TelnetSMTPHTTPOSPF RTP X.25 802.2 802.5 802.3 Internet Protocol Architecture (IETF)

38 Link Layer Two nodes (hosts, sw elts) connected by a single link Framing and packet format CO or CL service interface Reliability Flow control

39 Link Layer - Framing Link layer packet = frame Problem: how to recognize beginning and end of frame? Three methods –byte counting (DDCMP) –byte stuffing/oriented (BISYNCH, IMP-IMP) –bit stuffing/oriented (X.25 Level 2, 802.x)

40 Byte Counting Include count field giving number of bytes in frame Problem: what if byte field is corrupted? Might be able to catch with CRC… but only if max length assumed SYN…Count Header Body CRC @

41 Byte Stuffing/Oriented Special characters used for control Must distinguish special characters when used for control from when in data Use special data link escape character, DLE E.g., STX = start of text… use DLE STX when STX is in data (stuff DLE before STX if it appears in data (also for SOH, ETX,...) If DLE in data, send DLE DLE

42 Byte Stuffing Problems Dependence on fixed character set Must examine every byte of data on sending and receiving (insert / remove DLE) Was used widely in IBM bisynch (at 9600bps)

43 Bit Stuffing/Oriented Frame beginning and end marked by special bit string …. usually 01111110 If 5 1’s in data to be sent, sender inserts 0 If receiver sees 5 1’s check next bit(s) –if 0, remove it (stuffed bit) –if 10, end of frame marker (01111110) –if 11, error (7 1’s cannot be in data) Can be done fast in hardware FLAG Header Body … CRC FLAG

44 Error Detection Cyclic Redundancy Check (CRC) Bitwise binary arithmetic 0 + 0 = 1 + 1 = 0 0 + 1 = 1 + 0 = 1 Addition and subtraction are same operation Example 11011000 +/- 10001001 01010001

45 CRC Represent n bit message as an n-1 degree polynomial Message M = 101101101 M(x) = x**8 + x**6 + x**5 + x**3 + x**2 + 1 Special generator polynomial, C(x), degree k C(x) chosen to have special properties - this is key to the method working well

46 CRC - Method Pad M with k 0s yielding M’ (k = deg C(x)) Represent M’ as M’(x) R(x) = rem ( M’(x) / C(x)) P = M’ + R (P(x) = M’(x) + R(x)) Transmit P (P(x)) At dest, receive P(x) + E(x) Compute rem (P(x) + E(x)) / C(x) –If 0, either message rec ok or C(x) divides E(x) –If not 0, then error in transmission

47 Shift Register Consider C(x) = x**3 + x**2 + 1 M(x) = X**7 + x**4 + x**3 + x M’ = 10011010000 1001 ++ X**3X**2X**11 1010000 The shift register performs long division using bitwise operations - M’ / C

48 Shift Register Consider C(x) = x**6 + x**5 + x**3 + x +1 M’ = 10110110101000000 10 + 11 + X**6X**5X**4X**3 0101000000 The shift register performs long division using bitwise operations - M’ / C 01 + X**2x 1 1 +

49 Properties of C(x): Should not divide E(x) for common errors Single bit errors –non-zero x**k, x**0 terms Double (2 consecutive) bit errors –C(x) has factor with at least 3 terms Odd number errors: contains factor (x+1) –consider E(x) with odd no. terms - so E(1) = 1 –but if (x+1) is factor of E, then E(1) = 0 – so if odd number of errors, E cannot have (x+1) factor

50 More on C(x) Burst error - at least first and last bits of burst are in error –if length of burst is <= k, the degree of C(x), then burst error is detected –C(x) with degree k cannot divide E(x) with degree less than k High probability of detecting burst errors of length greater than k… but not 100%

51 Reliable Transmission When CRC detects error, frame discarded Corrected by link layer retransmission - ARQ (Automatic Repeat reQuest) –mechanisms include: frame sequence numbers, acknowledgements (acks), negative acks (naks), selective acks (sacks), timers / timeout –often implemented by sliding window Also used by transport layer - but link impl simpler - wire connects protocol entities

52 Sliding Window - Functions Put data in order for delivery to user of service interface Recovery from losses –retransmission Flow control - prevent sender from overrunning receiver’s buffers –link - SWS usually fixed, RWS often = 1 –credits - signal from receiver to sender - used with transport protocols

53 Timer / Retransmission Strategies Timers for link layer –1 timer for each frame –1 timer for link Retransmission on timeout –retransmit frame whose timer goes off –retransmit earliest unacked frame –retransmit all frames from earliest unacked frame - go back n Retransmit on nak or sack

54 Sliding Window Examples Principle SWS + RWS <= MaxSeqNum (size seq # space) MaxSeqNum = 8, SWS = 4, RWS = 4 SenderReceiver 0,…,3 0,…,3 accepted ack 0,…,3, NFE = 4lost resend old 0,…,3 all rejected-no overlap with expected frames Problem occurs if resent frame overlaps with expected new frame. Not possible if above principle observed

55 Sliding Window Examples SenderReceiver 0,…,4 0,…,3 accepted, disc 4 ack 0,…,3, NFE = 4lost resend old 0,…,4acc 4, disc 0,1,2,3 Problem occurs because resent frame overlaps with expected new frame. ack 4, will acc 5,6,7,0, NFE = 4 lost resend old 0,…,4acc 0, as new but is retrans

56 Bit Oriented Data Link Protocols ANSI Standard - ADCCP, Advanced Data Communications Control Procedures Choices of functionality and operation Designed to operate –over point-point or multi-point links –in 2-way alternate or 2-way simultaneous mode –over terrestrial and satellite links –when communication is between logical equals or non-equals

57 Station Types Primary –flow control –error recovery (other than retransmission) –initialize and terminate link Secondary –simpler than primary primary secondary commands responses

58 Data Transfer Modes NRM - Normal Response Mode –one primary, multiple secondary stations –polled multipoint, secondary often a device ARM - Asynchronous Response Mode –one primary, one secondary station –secondary can send at will as responses –point-point - not good for multipoint ABM - Asynchronous Balanced Mode –two combined stations (both issue commands and responses)

59 Frame Format F A C Info FCS F F = 01111110 (flag sequence) A = address of receiver if command sender if response all 1s = broadcast C = 8 or 16 control bits - type of frame + functions FCS = CRC

60 Control Fields All can be commands or responses Information transfer –0 N(S) P/F N(R) Supervisory (4) –10 SS P/F N(R) Unnumbered (32) –11 MM P/F MMM N(S) and N(R) are 3 or 7 bits; P/F is 1 bit

61 P/F Bit NRM - P=1, poll secondary, secondary sets F=1 in last response frame ARM/ABM - P=1 is sent by primary asking secondary to set F=1 in next response P/F bit used for synchronization

62 The Sliding Window N(R), N(S) N(S) is the sequence number of the information frame that contains it N(R) acks all info frames with seq numbers < N(R); i.e., N(R) = NFE Sequence number space is 7 (3 bits) or 127 (7 bits) With X.25 L2, receive window is always 1 so frames only accepted in order

63 Miscellaneous To abort a frame (so it will be recognized as not correct) –append incorrect FCS or –transmit 7 1s 15 ones transmitted means that station is giving up control of the link

64 Supervisory Frames (4) Contains N(R), ack to N(R)-1 RR - receive ready - ready to rec I frame –clear busy condition (RNR) RNR - receive not ready - busy condition –received frames may be discarded REJ - reject - ask retrans from N(R) –clear busy condition –cleared when frame with N(S) = N(R) of rejected frame is received SREJ - selective reject - ask retrans of N(R)

65 Selected Unnumbered Frames SABM - set asynchronous balanced mode - link establishment (also SARM, SNRM) DISC - disconnect UA - unnumbered ack, ack unnumbered command DM - disconnect mode - request set mode command, reject set mode FRMR - frame reject - bad control field or bad I frame length or invalid N(R)

66 Link Initialization SABM UA (yes) P=! F=1 Or DM (no) DM F=0 SABM DTEDCE

67 Timers / Constants T1 = frame retransmission timer T2 = may delay timer T3 = idle channel timer - inform protocol user N1 = Max I frame size with headers N2 = Max # retransmission attempts K = max number of outstanding frames (<= 7 or <= 127) - pipelining

68 CommandsResponses PrimarySecondary I I RR RR RNR RNR SREJ* SREJ * SNRM UA DISC DM FRMR UI UI Unbalanced Normal Class (NRM) UNC 3,4 *Receive window can be > 1

69 B, SNRM,P B,UA,F B,RR0,P B,RR0,F B,I00 B,I10 B,I20,P B,I03 B,I13 B,I23,F B,RNR3 B,I33,P B,I34 B,I44,F B,RNR5,P B,DISC,P B,RR4,F B,UA,F PrimarySecondary

70 Balanced Asynchronous Class (ABM) BAC 2,8 CommandsResponses PrimarySecondary I RR RR RNR RNR REJ * REJ * SABME UA DISC DM FRMR *Receive window = 1, only receive in order, REJ requests resend of ALL sent, beginning with lost one

71 Local Area Networks Standardized by IEEE as 802.x A variety of Media Access Control (MAC) protocols and associated topologies –802.3, Ethernet, CSMA/CD (3u, 3z) –802.4, Token Bus –802.5, Token Ring –802.11, Wireless – DQDB, etc With 802.2, comprises link layer

72 802.x Model LLC Logical Link Control - 802.2 MAC Media Access Control - 802.3, 802.4,... PHYPhysical Layer - 802.3,802.4,... medium Link Layer IP or other network protocol

73 Token Token Ring FDDI Ethernet Hub Computer Collision Domain

74 802.2 - Logical Link Control PDU - Protocol Data Unit DSAP SSAP Control Information Addresses are 8 bits - all 1s = broadcast DSAP = Destination Service Access Point, address leftmost bit for Group or Individual SSAP = Source Address leftmost bit for Command or Response (a la X.25 L2) Control = 16 bits if seq no included, else 8 bits

75 LLC Types / Classes There are two types of operation –Type 1 - provides connectionless service no flow control or error control no connection setup –Type 2 - provides a connection-oriented service similar to X.25 L2 (BAC 2,8) I frames can be responses There are also 2 classes of procedure –Class 1 - includes Type 1 only –Class 2 - includes Type 1 and Type 2

76 Commands/Responses CommandsResponses UI XID XID TEST TEST I I RR RR RNR RNR REJ REJ SABME UA DISC DM FRMR Type 1 Type 2

77 Details - Type 1 Every LLC on a LAN supports Type 1 UI frames used for data transfer Does not use XID TEST used by one entity to see if other still is “alive”

78 Details - Type 2 I frames used for data transfer Seq number space is 7 bits Receive window is1, frames only accepted in order Exchange Information (XID) is used by receiver to tell sender the max amount, k, that the sender can send beyond the most most recently received ack (N(R)). This bounds pipelining.

79 Ethernet - 802.3 Ethernet developed by Xerox in mid 1970s Basic ideas from AlohaNet packet radio project Ethernet standardized by Xerox, DEC, Intel in 1978 IEEE later standardized as 802.3 - at MAC layer differs in one header field from Ethernet 10, 100, 1000 Mbps

80 CSMA/CD Carrier sense - stations sense whether there is traffic on the wire Multiple access - more than one station can simultaneously be connected and contend for the right to send Collision detection - individual stations detect when there has been a collision (transmissions from two stations interfere with each other) and react appropriately

81 Classical Ethernet (Thick-Net) Standardized as 802.3, 10Base5 Coaxial cable for medium Parameters - max – segment length = 500m, 2 repeaters on path –length = 1500m, 1024 nodes Also –10Base2 (thin-net, 200m) –10BaseT (twisted-pair, star config, 100m) –10BaseF (fiber, 1-2KM, FP,FB, FL)

82 Repeater Multi-Port Bridge Repeater=Hub Bridge=Switch CD=Collision Domain CD 1CD 2 CD 3 CD 4 Each CD includes a MP Bridge Port

83 802.3 Frame Format Preamble-7 StartDel-1 DstAddr-6 SrcAddr-6 Length-2 Payload FCS-4 StartDelimiter = 10101011 Dst and Src Addr = 6 bytes from flat address space first bit is individual/group Length = length of LLC packet in payload max is 1500 bytes Ethernet uses this field to specify upper layer protocol, values are over 1500 *802.3 uses 802.2 to select upper level protocol Payload is LLC packet plus PAD (min payload is 46 bytes) so min packet length is 64 bytes (without preamble, and start delimiter

84 TransDataEncapsRecDataDecaps Framing Addressing (SRC, DST) Error detection (FCS) TransMediaAccessRecMediaAccess Management Management Medium allocation (collision avoidance) Contention Resolution (collision handling) TransDataEncodingRecDataDecoding Bit transmission/reception Collision detection Carrier sense Preamble generation/detection PLSPLS MACMAC LLC

85 Transmission Without Contention Client layer (LLC) sends transmission request to MAC Construct frame, compute FCS Monitor carrier sense signal –if channel clear, initiate frane transmission after interframe delay (9.6 us) - send bits to PLS Transmit encoded preamble, encode and transmit bits Listen for collision

86 Reception Without Contention Synchronize with preamble, convert signal to binary Set carrier sense while receiving bits Check address - if for this station or station is in promiscuous mode, unpack frame, pass data to client Compute FCS and send status code to client

87 Collision Handling on Sending If collision detect signal turned on while transmitting Stop sending frame and transmit jam signal (32 bits, special pattern) to insure collision is detected by all stations Schedule retransmission after back-off time

88 Collision Handling on Reciving Smaller than minimum size packet is detected and discarded (jam detected) Terminated transmitted frame plus jam must be smaller than minimum size packet

89 Retransmission When station detects collision on transmitting, backoff/retransmission is scheduled as follows For n th retransmission, randomly choose r in the range – 0 < = r < 2**k, where k=min(10,n) –wait r slot times before retransmission where a slot time is 512 bit times (64 * 8) or 51.2us for 10mbps Truncated exponential backoff

90 To insure collision detection, enough bits must be sent so that if other sender is maximally distant, its first byte arrives while first sender is still sending Consider 10Base5 –max length = 1500m –sender must continue sending for number of bit times necessary for data to travel 1500 * 2 = 3000m –at 10**7 bits/sec, assuming 2/3 speed of light in medium, each bit occupies 20m so 150 bits must be sent to deal with propagation time, plus additional bits to deal with other delays, but min packet size =512 bits –the jam is a special 32 bit pattern that allows all stations to learn that that there has been a collision Collision Detection

91 Higher Bandwidth 802.3 802.3u, 802.3z standardize 100, 1000Mbps Bridges (switches) can be connected to collision domains with different bandwidths To insure collision detection must maintain relationship between –802.3 bandwidth –minimum frame size –length of combined media –e.g., higher bandwidth, same min frame size, must have smaller length

92 Parameters Parameters10Mbps100Mbps1000Mbps slotTime512 bT512 bT4096 bT interFrameGap9.6us9.6us.096us jamSize323232 maxFrameSize1518 B1518 B1518 B minFrameSize64 B64 B64 B extendSize0 B0 B448 B

93 Repeater Multi-Port Bridge CD 1(shared 100Mbps) CD 3 CD 4 (shared 10Mbps) CD2 (dedicated 100Mbps)

94 Multi-Port Bridge Multi-Port Bridge Multi-Port Bridge 1Gbps (dedicated) CD2 1Gbps (dedicated) CD1 Repeater 100Mbps (shared) CD3 Repeater 100Mbps (shared) CD4

95 DSL - Digital Subscriber Line Digital copper loop transmission technology and associated equipment Connection between subscriber and telco central office or concentrator - local loop access network Achieves broadband speed (up to Mbps) over twisted pair

96 Terminology ATU - ADSL (Asynchronous Digital Subscriber Line) Transceiver Unit CSU - Channel Service Unit DSU - Digital Service Unit CPE - Customer Premise Equipment

97 Current Telco Infrastructure Terminology –ILEC - Incumbent Local Exchange Carrier –CLEC - Competitive Local Exchange Carrier –PTO - Public Telephone Operator –CO - Central Office –DLC/RT - Digital Loop Carrier/Remote Term –MDF - Main Distribution Frame - where local loops terminate –DSLAM - DSL Access Multiplexer

98 The Access Network Typically, cable bundles with up to thousands of twisted wire pairs each of which terminates at a customer premise Long loops –signals dissipate/weaken - lower signal to noise –solutions for loops > 18K feet loading coils - every 6K feet, amplify signal, not compatible with DSL DLC - concentrator, terminate twisted pair and backhaul to CO with T1/E1, fiber or copper, terminate DSL which needs end-end twisted pair

99 Transmission Frequency Higher frequency enables greater BW POTS - 0 to 3600 Hertz (cycles per sec) –can also support modem up to 56Kbps (analog) DSL uses higher frequencies Problems –attenuation - dissipation of signal as it traverses copper wire –crosstalk - interference between signals on different copper wires

100 Attenuation Higher frequencies attenuate energy faster - so shorter loop distance possible Solution –use lower resistance wire - thicker gauge –but telcos have used thinnest possible wire because thicker wire more expensive to buy/install

101 Crosstalk Electrical energy transmitted on wire radiates energy to adjacent wires in bundle Interferes more with similar frequencies Less of a problem for lower frequencies, e.g., those used with POTS Solutions for DSL –two pair instead of one pair so lower freq can be used to get desired BW –use different frequencies upstream/downstream, lower upstream because more crosstalk at CO end - many lines converge

102 Some DSL Types HDSL - High Bit Rate DSL, 2 pair, T1/E1, symmetric SDSL - Symmetric DSL, 1 pair, multiple data rates to T1/E1 ADSL - Asymmetric DSL, 1 pair, up to 6Mbps RADSL - Rate Adaptive DSL, 1 pair, up to 12Mbps, asymmetric or symmetric

103 Issues Frequencies to use for up/down stream Desired max distance - subscriber/DLC, about 18K feet is usual upper bound Coding of bits NON ISSUE - compatibility with other DSL systems - DSL is point-point and single organization controls both ends

104 Network Layer A variety of different types of protocols Access / service interface –connection-oriented (X.25 Level 3, ATM) or connectionless (IP) Within network –switching method - DG (IP), VC (ATM) Routing - OSPF / BGP (IP) Congestion detection / handling - RED (IP) QoS - TM4.0 (ATM), RSVP-DiffServ (IP)

105 Routing Problem: Find path from a source to a destination through a network (internet) Possible path criteria –correctness - packets sent to correct destination –simplicity –stability - converge to equilibrium) –robustness - operates in presence of failures –fairness - serves all users - can prioritize –optimality - minimize cost of path –QoS - paths depend on app requirements

106 Cost of Link Capacity / bandwidth Type - satellite / land Error rate Delay Security Cost of path = sum of costs of links on path

107 Some Issues for Routing Protocols Who decides route –centralized - Routing or Network Control Center (RCC or NCC) –distributed - individual sw elts When are routes changed –fixed or static or non-adaptive - at system startup time by administrator –adaptive or dynamic - with each DG, with each VC, weekly, ??

108 More Issues for Routing Protocols Changes based on –measurements or estimate of traffic over some period –network topology - links, sw elts up/down, added/removed Algorithm used –distance vector (centralized or distributed) –link state (distributed)

109 Process for Changing Routes Determine relevant parameters - topology, delay, queue lengths, traffic intensity Send information to place(s) making decisions (sw elts or RCC/NCC) Place receiving info computes routing tables Routing tables distributed to sw elts (if NCC)

110 Distance Vector Routing Each sw elt maintains set of triples –(Desination, Cost, NextHop) Each sw elt sends updates to (receives updates from) directly connected neighbors –periodically (on the order of several seconds) –when its table changes (triggered update) Update is list of pairs - (Destination, Cost) Update local table if receive “better” route Refresh existing routes; delete if time out

111 Database at ABCDEF A--3A--1A---- B3B--8B1B---- C--8C----4C-- D1D1D------2D E----4E----1E F------2F1F-- Destination FEC B A D 1 1 1 2 3 4 8 Distributed algorithm, each column at different sw elt Centralized algorithm, entire table at RCC

112 Database at ABCDEF A--2D--1A---- B3B--8B1B12C-- C--8C----4C-- D1D1D----3F2D E--12C4E----1E F--3D--2F1F-- Destination FEC B A D 1 1 1 2 3 4 8 Changes in B, E tables after one exchange of routing infomation

113 Example A E C B F G D Routing table at node B Dest Cost NextHop A1A C1C D2C E2A F2A G3A @

114 A E C B F G D F detects that link to G has failed F sets distance to G to inf, sends update to A A sets distance to G to inf since it uses F to reach G A receives update from C with 2 hop path to G A sets distance to G to 3 and sends update to F F decides it can reach G in 4 hops via A Example - Recovery from Link Failure @

115 A E C B F G D Link from A to E fails A advertises distance of infinity to E B and C advertise distance of 2 to E B decides it can reach E in3 hops, advertises this to A A decides it can reach E in 4 hops, advertises this toC C decides it can reach E in 5 hops A, B, C form a routing loop Looping Problem @

116 1. Destination D 2. Cost from A3B (cost to D from A is 3, first hop is B) B2C C1D 3. C-D fails (second column of 4) 4. Successive table entries for destination D from node A entries 3B3B3B5B5B7B7B… B entries2C2C4A4A6A6A8A… C entries1Dinf3B5B5B7B7B… Eventually, algorithm converges but only after “count to infinity” Problem: path to D from B uses B but B is not aware of this ABCD 11 1

117 Solution - Split Horizon If sw elt W learned route to X from Y, W does not tell Y about its route to X Modification: split horizon with poisoned reverse - use metric infinity in update for routes that have ceased to exist –advantage - convergence can be faster Can also speed up convergence by triggering update to neighbors when a sw elt’s routing database changes

118 With Split Horizon A3B3B3Binf B2C2Cinfinf C1Dinfinfinf ABCD Cost From Split Horizon: do not report route to destination to the neighbor from which the route was learned

119 Consider 1. Destination E 2. Cost from A3B (cost to D is 3, first hop is B) B2D C3B 3. B-D fails 4. Successive table entries for destination E from node A entries3B3B4C5C6C… B entries2Dinf4C5C6C… C entries3B3B4A5A6A… Eventually, algorithm converges - after “count to infinity” Split horizon does not help! 1 1 1 ABDE C

120 ABDE C Split horizon does not help Successive table entries for destination E from nodes A3B3B4Cinf6B B2Dinfinf5Ainf C3B3B4Ainfinf

121 Solution - Hold Down When a route to a destination is lost by a sw elt, it does not accept a new route to that destination for a certain amount of time - time based on max possible path loop Difficulty is determining hold down period. Too short may lead to (incorrect) acceptance of new routes. Too long may lead to loss of data enroute to desination

122 ABDE C With sufficient hold down Successive table entries for destination E from nodes A3B3Binf B2Dinfinf C3B3Binf

123 Link State Each sw elt sends information on directly connected links to ALL sw elts in the AD Contrast with distance vector - entire routing table sent just to neighbors Link state packet - packet used to carry info Reliable flooding - hop-hop reliable (optional) Shortest Path First Algorithm (Dijkstra) used to compute routes

124 Link State Packet - LSP Routing protocol packet - on Internet encapsulated in UDP DG or TCP segment Includes –ID of sw elt that created LSP –cost of link to each directly connected neighbor –sequence number (SEQNO) –time-to-live (TTL) for LSP - max hop count or max number of time units, as defined by protocol @

125 LSP Handling Each sw elt periodically generates new LSP with increased SEQNO (set to 0 on reboot) LSP forwarded on all links except one on which it arrived Each sw elt stores newest LSP from each source sw elt TTL decremented before being forwarded TTL of stored LSP may also be periodically decremented @

126 Dijkstra’s Shortest Path First Algorithm Goal - find shortest path from source node s to each node in a graph N = set of nodes in the graph l(i, j)= cost (weight) for edge (i, j) (>= 0) s  N = the source node M = set of nodes incorporated so far C(n) = cost of path from s to n @

127 M = {s} for each n in N - {s} C(n) = l(s,n) (inf if s not dir conn to n) while (N   -  union {w}: where C(w) is the minimum for all w in (N - M) -for each n directly conn to a w C(n) = MIN (C(n), C(w) + l(w,n)) Shortest Path First Algorithm @

128 Assume for all nodes in M, the min distance and correct first hop from S have been found - consider node X in N-M having min dist from S among all nodes in N-M. Is there shorter path to X? NO, WHY? M w added s Min dist from S X Consider shorter path to X. If node before X is not in M, then it would have been chosen instead of X. So X is first node outside of M on this shorter path. But then dist to X would already be this shorter value...

129 Route Table Calculation Each sw elt contains two lists: Tentative and Confirmed Each list contains a set of triples (Destination, Cost, NextHop) Note that “NextHop” is the first sw elt on the path from the source S to Destination @

130 D SC A B 15 3 10 11 2 3 Confirmed Tentative 1. (S,0,-) 2. (S,0.-) (A,15,A) (C,3,C) (D,11,D) 3. (S,0,-) (A.15,A) (C,3,C) (D,11,D) 4. (S,0,-) (A.13,C) (C,3,C) (D,5,C) 5. (S,0,-) (A,13,C) (C,3,C) (D,5,C) 6. (S,0,-) (A,6,C) (C,3,C) (D,,5,C) 1 7. (S,0,-) (C,3,C) (D,,5,C) (A,6,C) 8. (S,0,-) (B,9,C) (C,3,C) (D,,5,C) (A,6,C)

131 Distance Vector vs. Link State Distance vector –does not reveal net topology to sw elt –looping behavior with ad hoc solutions –slow to converge Link state –scales well –can secure by authenticating origin of LSPs –each sw elt knows topology of entire network

132 The Transport Layer Implemented in end-systems (hosts) not sw elts Uses network layer services Issues –addressing –CO vs. CL, reliable vs. unreliable –connection establishment –flow control –connection termination

133 CO vs. CL Transport layer services may be CO or CL, reliable or unreliable Consider CO transport service over CL network service Consider reliable transport service over unreliable network service

134 Connection Establishment 2-way Handshake Initiator sends connection request (w. id) Responder sends accept or reject (w. id) Responder assumes connection exists when it sends the ack Initiator assumes connection exists when it receives the ack OK if network service reliable and CO If not? (consider lost, old conn reqs/ack)

135 Deal with Old Connection Requests Keep track of obsolete connection ids –problem: lose state info on crash Limit packet lifetime with max hop count –problem: queuing times If can bound time in network for request and ack, then can use initial seq number based on clock to distinguish requests –done in TCP, problem: queuing times 3-way handshake

136 3-way Handshake Initiator sends request with ID (ISN in TCP) Responder sends ack of initiator’s ID (ISN) plus its own ID (ISN) Initiator sends ack of responder’s ID (ISN) and includes its ID Solves old connection request problem because only establishes connection when both sides ack the other’s ID

137 Flow Control Similar mechanism (e.g., sliding window) to link layer flow control Link layer receive window is usually fixed Transport protocols may include a “credit” parameter so receiver can inform sender of its current receive window (variable receive window), i.e., how much it will accept beyond what is being acked Consider credit of zero!!

138 Connection Termination Abort User indicates it will not send any more data nor will it accept any more data Transport protocol immediately closes connection Application must insure that all data sent has been received before issuing abort

139 Connection Termination Graceful Close User indicate it will send no more data but will continue to accept data Transport protocol does not close connection until both users have issued graceful close Application can count on transport protocol to insure all data sent is delivered












151 When is a segment accepted by a receiver? Segment Receive Test Length Window -------- -------- --------------------------------------------------- 0 0SEG.SEQ = = RCV.NXT 0>0RCV.NXT <= SEG.SEQ < RCV.NXT+RCV.WND >0 0not acceptable >0>0RCV.NXT <= SEG.SEQ < RCV.NXT+RCV.WND or RCV.NXT <= SEG.SEQ+SEG.LEN-1 < RCV.NXT+RCV.WND




155 Half-Open Connection Discovery TCP A TCP B SEQ ACK CTL 1. (Crash) (Send 300, receive 100) 2. CLOSED ESTABLISHED 3. SYN-SENT   (??) 4. (!!)   ESTABLISHED 5. SYN-SENT   (Abort!!) 6. SYN-SENT CLOSED 7. SYN-SENT  

156 Old Duplicate SYN Initiates a Reset on Two Passive Sockets TCP A TCP B SEQ ACK CTL 1. LISTEN LISTEN 2....  SYN-RECEIVED 3. (??)   SYN-RECEIVED 4.   (return to LISTEN) 5. LISTEN LISTEN


158 Simultaneous Close Sequence TCP A TCP B SEQ ACK CTL 1. ESTABLISHED ESTABLISHED 2. (Close) (Close) FIN-WAIT-1 ... FIN-WAIT-1  ...  3. CLOSING ... CLOSING  ...  4. TIME-WAIT TIME-WAIT (2 MSL) (2 MSL) CLOSED CLOSED

159 Service Interface (to IP) Net Send –identifier –source, destination IP address –protocol (TCP) –data, data length –type of service (precision, reliability, delay, throughput) –don’t fragment –options (IP source routing, time stamp, etc)

160 Service Interface (from IP) Net Deliver –source, destination IP address –protocol (TCP) –data, data length –type of service –options

161 Service Interface (from application) Open –unspecified passive - source port –fully specified passive - +dest port, address –active, active w. data - +data length, data, psh, urg+ Send - lcn, data length, data, psh, urg+ Allocate (accept) - lcn, data length, data, urg+ Close (graceful) - lcn Abort - lcn Status - lcn

162 Service Interface (to application) Open ID - local connection name, source port, dest port and address (if known) Open failure - lcn Open success - lcn Deliver - lcn, data, data length, urg+ Closing - lcn Terminate - lcn Status response and error

163 When Bad Segments Arrive Send RST (stay in same state) –closed state and anything arrives except RST –listen, syn-sent, syn-received states and incoming acks something not sent Send ACK - established and other synchronized states Use seq, ack numbers that other side will believe (e.g., from incoming ack or data)




167 TCP Throughput - Error Free Environment Max TCP window/credit = 64KB Max TCP throughput –cross US - 100ms RTT - 5.2Mbps –metropolitan - 1ms RTT - 520Mbps –local -.1ms RTT - 5.2Gbps Problem is the max credit of 64KB

168 TP4 Throughput Max TP4 window/credit –(8KB per TPDU) * (32K TPDUs) –256MB = 2Gb Max TP4 Throughput –cross US - 100ms RTT - 20Gbps –metropolitan - 1ms RTT - 2Tbps –local -.1ms RTT - 20Tbps

169 TCP Solution - Window Scaling Add window scale option Both ends of TCP connection must agree at connection setup time to use scale factors Each end tells the other what scale factor it will accept (0 if willing to use scaling for other end but not accept scaled window) ActualReceiveWindow = SegmentReceiveWindow * 2**shift-cnt

170 Window Scaling Max shift-cnt = 14 Max window/credit = 64KB * 2**14 = ~8Gb Max TCP througput across US = 80Gbps

171 TCP vs. TP4 - Common Features Connection-oriented reliable service interface for client processes Sliding windows with cumulative acks and credits Timers to trigger retransmission Checksum to insure data integrity Three-way handshake for connection establishment

172 TCP vs. TP4 - Differences Sequence number space –TCP - 32 bits indexing BYTES –TP4 - 31 bits indexing TPDUs Maximum packet size –TCP - 64KB –TP4 - 8KB Credit - maximum receive window size –64KB –32K TPDUs, each with max.size 8KB

173 TCP vs. TP4 - Differences Packet types –TCP - one type with flags (segment) SYN, FIN, ACK, RST, PSH, URG –TP4 - multiple packet types (TPDU) CR, CC, DT, AK, DR, DC, ED, EA Out of band data –TCP - URG, PSH –TP4 expedited data Termination –TCP - graceful close or abort –TP4 - abort

174 TCP vs. TP4 - Differences Checksum –TCP - one addition for 16 bytes of data –TP4 - two additions for 1 byte of data Opening connections –TCP - passive or active open, listen –TP4 - active open, no listen Connections –TCP - one between address/port pairs –TP4 - multiple between TSAPs with diff ref #s










184 The Internet Protocol - IP Version 4

185 Internet Model R R R R R Hub R R TCP IP TCP IP R @

186 Packet Delivery Model Connectionless service interface –datagrams may not be delivered to destination user in the same order as sent by source user Potentially unreliable –datagrams sent may never be delivered Datagrams are used within the network –unbounded delays, reordering, loss, duplication within network are possible

187 IP Header Version (4) - currently version 4 HLEN (4) - number of 32 bit words in header TOS (8) - Type of Service (not widely used) Length (16) - number of bytes in this DG (max 64K) Ident (16) - used by fragmentation/reassemlby Flags/Offset (16) - used by frag/reass (3 +13) TTL (8) - Time to Live - number of hops before discard this DG Protocol (8) - TCP = 6, UDP = 17 Checksum (16) - of header only Destination / Source IP address (32 bits each) @

188 Fragmentation and Reassembly Each network has some MTU Strategy –fragment when necessary (MTU < DG size) –avoid fragmentation at source host –regragmentation is possible –fragments are self contained datagrams –delay reassembly until destination host –do not recover from lost fragments @

189 Indent = x Offset=0 512 1024 512 bytes 376 bytes 0 1 1 Flag = more fragments Indent = x Offset=0 1400 bytes 0 @

190 Global Addresses Properties – globally unique –hierarchical - network + host –each network interface has an IP address Four classes - A, B, C, D (multicast-111…) 0 10host 110network net 16 8 247 14 21 @

191 Datagram Forwarding Strategy –every DG contains destination’s address –if directly connected destination network, then forward to host –otherwise, forward to some router –forwarding table maps network number into next hop –each host has a default router –each router maintains a forwarding table @

192 Address Translation Map IP addresses into physical addresses (e.g., 48 bit IEEE) Address Resolution Protocol (ARP) –table of IP to physical address bindings –sender broadcasts request if destination IP address not in its table –target machine responds with its physical address @

193 How do Hosts/Routers Build/Maintain ARP Caches Table entries discarded if not refreshed (~10 minutes) Update your table with source when you are the target Update your table if you already have an entry and see info pass by on net Do not add new table entry based on info that passes on net

194 ICMP Internet Control Message Protocol Uses IP but is a separate protocol in the network layer ICMP HEADER IP HEADER PROTOCOL = 1 TYPE CODE CHECKSUM REMAINDER OF ICMP MESSAGE (FORMAT IS TYPE SPECIFIC) IP HEADER IP DATA

195 ICMP Types Echo - Echo Reply (ping) Redirect (use other router) Destination Unreachable (prot, port, host) Time Exceeded Source Quench Checksum Failed Reassembly Failed Cannot Fragment @

196 Echo and Echo Reply TYPE CODE CHECKSUM IDENTIFIER SEQUENCE # DATA …. TYPE = 8 = ECHO; 0 = ECHO REPLY CODE = 0 IDENTIFIER An identifier to aid in matching echoes and replies SEQUENCE # Same use as for IDENTIFIER UNIX “ping” uses echo/echo reply

197 Redirect TYPE CODE CHECKSUM NEW ROUTER ADDRESS IP HEADER + 64 bits data from original DG TYPE = 5 CODE = 0 = Network redirect 1 = Host redirect 2 = Network redirect for specific TOS 3 = Host redirect for specific TOS

198 Destination Unreachable TYPE CODE CHECKSUM UNUSED IP HEADER + 64 bits data from original DG TYPE = 3 CODE 0 = Net unreachable 1 = Host unreachable 2= Protocol unreachable 3 = Port unreachable 4 = Fragmentation needed but DF set 5 = Source route failed

199 Time Exceeded TYPE CODE CHECKSUM UNUSED IP HEADER + 64 bits data from original DG TYPE = 11 CODE 0 = Time to live exceeded in transit 1 = Fragment reassembly time exceeded

200 Source Quench TYPE CODE CHECKSUM UNUSED IP HEADER + 64 bits data from original DG TYPE = 4; CODE = 0 Indicates that a router has dropped the original DG or may indicate that a router is approaching its capacity limit. Correct behavior for source host is not defined.

201 Traceroute UNIX utility - displays router used to get to a specified Internet Host Operation –router sends ICMP Time Exceeded message to source if TTL is decremented to 0 –if TTL starts at 5, source host will receive Time Exceeded message from router that is 5 hopes away Traceroute sends a series of probes with different TTL values… and records the source address of the ICMP Time Exceeded message for each Probes are formatted to that the destination host will send an ICMP Port Unreachable message

202 Scaling Issues Problems –Internet addresses have only 32 bits and only 2 levels of hierarchy –original addresses only allowed 256 networks –routing protocols do not scale IPv6 solves both of these problems Classes (A, B, C) provided for more networks Subnetting adds a third level of hierarchy CIDR makes routing more efficient

203 Subnetting Add another level to address/routing hierarchy: subnet Subnet masks define variable partition of host part of Class A and B addresses Subnets are visible only within the local site Network Number SubnetID HostID Subnetted Address Class B Address Subnet Mask ( 00000000 111111111111111111111111

204 H3 R2 H2 H1 R1 128,96.33.1 1228.96.34.130 Subnet Mask: Subnet number: Subnet Mask: Subnet number: Subnet Mask: Subnet number: Subnet Subnet Next Number Mask Hop int0 int1 R2 @

205 Forwarding Algorithm D = destination IP address for each entry (SubnetNum, SubnetMask, NextHop) D1 = SubnetMask & D if D1 = SubnetNum if NextHop is an interface deliver datagram directly to destination else deliver datagram to NextHop (a router) @

206 Notes Would use a default router if nothing matches Not necessary for all ones in subnet mask to be contiguous Can put multiple subnets on one physical network Subnets not visible from rest of the Internet @

207 Supernetting Assign block of contiguous network numbers to near-by networks Called CIDR: Classless Inter-Domain Routing Represent blocks with a single pair –(first _network_address, count) Restrict block sizes to powers of 2 Use bit mask (CIDR mask) to identify block size All routers must understand CIDR addressing @

208 Route Propagation Idea: Impose a second hierarchy on the network that limits what routers talk to each other. (The first hierarchy is the address hierarchy that governs packet forwarding.) Autonomous Systems (AS) –corresponds to administrative domain –example: company backbone, ISP backbone –assign each AS a 16 bit number

209 Routing Protocol Types Two-level route propagation hierarchy Interior gateway protocol –inside an AS –each AS selects its own –routing decisions based on metric Exterior gateway protocol –between ASs –Internet-wide standard) –routing decisions based on administrative issues

210 Popular Interior Gateway Protocols RIP: Route Information Protocol –developed for XNS, distributed with UNIX –distance vector algorithm, metric is hop count OSPF: Open Shortest Path First –recent Internet standard – link state algorithm –routes changed when topology changes –supports load balancing –2-level internal hierarchy

211 OSPF

212 OSPF - Open Shortest Path First Intra-AS Routing Protocol Used within a single AS Calculates routes to destination IP network address Link state protocol - SPF/flooding Route changes based on topology not traffic

213 OSPF Supports different routes depending on TOS required Supports multiple equal cost paths Exchange of routing updates is reliable between adjacent routers Link state advertisement (LSA) contains info on state of router’s interfaces and adjacencies, LSAs are flooded

214 Areas Two level routing hierarchy Divide AS into areas SPF is used to determine routes within each area and on the backbone that routes between the areas The backbone is also an area Area 1 Area 2 Area 3 Area 4

215 Packet Types Hello - acquire neighbors Database description Link state request Link state update, Link state ack –reliable hop-hop transfer of link state advertisements (LSA) –may aggregate LSAs for several sources

216 Link State Advertisements Router link - originated by all routers, flooded in area Network link - originated by designated router, flooded in area Summary link - originated by area border routers, flooded into area, describes route to destination outside the area but in the AS AS external link - originated by an AS border router, flooded throughout AS, describes route to dest in another AS

217 Types of OSPF Routers Internal router - within an area Area border router –attached to multiple areas –runs a copy of SPF for each attached area –relays topological info on attached areas to backbone Backbone router - includes area border routers AS boundary routers











228 Popular Exterior Gateway Protocols EGP - Exterior Gateway Protocol –original protocol in this class –based on reachability tree –no metric BGP4 - Border Gateway Protocol\ –recent internet standard

229 Routing Hierarchy Summary AS to destination AS (BGP4) Within AS (OSPF) –2 levels – to destination network Within network –to subnet, host



232 Border Routers Each AS has –one or more border routers –one or more border routers that serve as the speakers for the AS –border routers forward datagrams between ASs Speakers communicate with border routers in adjacent ASs, using TCP Speakers advertise and receive routing info and determine which routes to use Message types: OPEN, KEEPALIVE, UPDATE, NOTIFICATION (errors)

233 UPDATE MESSAGES Routes are advertised and withdrawn in UPDATE messages A route includes a sequence or set of ASs on the path to the destination IP networks unfeasible routes length (2B) withdrawn routes (destination IP addresses) total path attribute length (2B) path attributes (routes) network layer reachability info (destination IP addresses)

234 Routes When a BGP speaker advertises a route, it adds itself to the route A BGP speaker only advertises routes that it uses itself Policy considerations are used to determine route; routes are not optimal An advertised route (sequence or set of ASs) may change -- downstream ASs may not discover in a timely fashion

235 Update Handling Receive an update with routes Decision process –D (attributes of route) ------> non neg integer –D specifies the degree of preference for a route –route to dest with highest preference used Note that D does not depend on the existence or non-existence of other routes D is based on policy considerations, not necessarily on the cost of routes














































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