CN Unit VI - Lower level protocols and implementation

Category: Education

Presentation Description

HDLC, PPP protocols, internetworking devices like hubs, switches, routers, bridges. Link virtualization (ATM, MPLS)


Presentation Transcript

PowerPoint Presentation:

Unit IV: Internetworking Reference: Kurose, Ross “Computer Networking-a top down approach featuring the internet” Slides p repared by: Mr. Vaibhav Dabhade for TE Computer Engineering

The Data Link Layer:

5a- 2 The Data Link Layer Our goals: understand principles behind data link layer services: error detection, correction sharing a broadcast channel: multiple access link layer addressing reliable data transfer, flow control: done! instantiation and implementation of various link layer technologies


5a- 3 Outline 1 Introduction and services 2 Error detection and correction 3Multiple access protocols 4 Link-Layer Addressing 5 Ethernet 6 Hubs and switches 7 PPP 8 Link Virtualization: ATM and MPLS

Link Layer: Introduction:

5a- 4 Link Layer: Introduction Some terminology: hosts and routers are nodes (bridges and switches too) communication channels that connect adjacent nodes along communication path are links wired links wireless links LANs Link-layer PDU is a frame , encapsulates a network-layer datagram “ link” Link-layer protocol has the responsibility of transferring datagram from one node to adjacent node over a link

Link layer: context:

5a- 5 Link layer: context Datagram transferred by different link protocols over different links: e.g., Ethernet on first link, frame relay on intermediate links, 802.11 on last link Each link protocol provides different services e.g., may or may not provide reliable data transfer over link 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 travel agent = routing algorithm

Link Layer Services:

5a- 6 Link Layer Services Framing: encapsulate datagram into frame, adding header, trailer ‘physical addresses’ used in frame headers to identify source, destination different from IP address! Link access Media access control (MAC) protocol Coordinate the frame transmissions of many nodes if multiple nodes share a medium 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) Used on wireless links: high error rates Correct an error locally at link level

Link Layer Services (more):

5a- 7 Link Layer Services (more) Flow Control: pacing between adjacent sending and receiving nodes 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

Adaptors Communicating:

5a- 8 Adaptors Communicating link layer implemented in “adaptor” (aka NIC) Ethernet card, PCMCI card, 802.11 card sending side: encapsulates datagram in a frame adds error checking bits, rdt, flow control, etc. receiving side looks for errors, rdt, flow control, etc extracts datagram, passes to receiving node adapter is semi-autonomous link & physical layers sending node frame rcving node datagram frame Adapter card Adapter card link layer protocol


5a- 9 Outline 1 Introduction and services 2 Error detection and correction 3Multiple access protocols 4 Link-Layer Addressing 5 Ethernet 6 Hubs and switches 7 PPP 8 Link Virtualization: ATM

Error Detection:

5a- 10 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

Techniques for Error Detection:

5a- 11 Techniques for Error Detection Parity checks Checksumming methods Cyclic redundancy checks

Parity Checks:

5a- 12 Parity Checks Single Bit Parity: Detect single bit errors Even parity scheme: choose the value of the parity bit such that the total number of 1s in the d+1 bits is even Odd parity scheme: choose the value of the parity bit such that the total number of 1s in the d+1 bits is odd

Parity Checks (Cont.) :

5a- 13 Parity Checks (Cont.) Two Dimensional Bit Parity : Detect and correct single bit errors 0 0 (Even parity scheme)

Checksumming Methods :

5a- 14 Checksumming Methods Sender: treat segment contents as sequence of 16-bit integers checksum: addition (1’s complement sum) of segment contents sender puts checksum value into segment header 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? More later …. Goal: detect “errors” (e.g., flipped bits) in transmitted segment (note: used at transport layer only ) Internet checksum: Checksum is easy and fast to compute Typically used in software implemented protocols (e.g. ,TCP and UDP )

Cyclic Redundancy Check:

5a- 15 Cyclic Redundancy Check view data bits, D , as a binary number choose r+1 bit pattern (generator), G (both sender and receiver know G) sender chooses 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 (ATM, HDLC) Left shifts r bits

CRC Example:

5a- 16 CRC Example Want to find R such that: D . 2 r XOR R = nG XOR R to the right of both sides : D . 2 r = nG XOR R equivalently: if we divide D . 2 r by G, the remainder is R R = remainder[ ] D . 2 r G 0 0 0 0

Chapter 5 outline:

5a- 17 Chapter 5 outline 1 Introduction and services 2 Error detection and correction 3 Multiple access protocols 4 LAN addresses and ARP 5 Ethernet 6 Hubs, bridges, and switches 7 Wireless links and LANs 8 PPP 9 ATM 10 Frame Relay

Multiple Access Links and Protocols:

5a- 18 Multiple Access Links and Protocols Two types of “links”: point-to-point PPP (point-to-point protocol) for dial-up access point-to-point link between Ethernet switch and host broadcast (shared wire or medium) traditional Ethernet upstream HFC (Hybrid fiber coaxial cable) 802.11 wireless LAN

Multiple Access protocols:

5a- 19 Multiple Access protocols single shared broadcast channel two or more simultaneous transmissions by nodes: interference only one node can send successfully at a time multiple access protocol distributed algorithm that determines how nodes share channel, i.e., determine when node can transmit communication about channel sharing must use channel itself! no out-of-band channel for coordination

Ideal Mulitple Access Protocol:

5a- 20 Ideal Mulitple Access Protocol What to look for in multiple access protocols? 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 4. Simple

MAC Protocols: a taxonomy:

5a- 21 MAC Protocols: a taxonomy Three broad classes: Channel Partitioning protocols divide channel into smaller “pieces” (time slots, frequency, code) allocate piece to node for exclusive use Random Access protocols channel not divided, allow collisions “recover” from collisions Taking-turns protocols tightly coordinate shared access to avoid collisions

Channel Partitioning MAC protocols: TDMA:

5a- 22 Channel Partitioning MAC protocols: TDMA TDMA: time division multiple access channel divided into N time slots, one per user access to channel in "rounds" each station gets fixed length slot (length = packet trans time) in each round unused slots go idle inefficient with low duty cycle users and at light load example: 6-station LAN, 1,3,4 have packets, slots 2,5,6 idle

Channel Partitioning MAC protocols: FDMA:

5a- 23 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 example: 6-station LAN, 1,3,4 have packets, frequency bands 2,5,6 idle frequency bands time

LAN technologies:

5a- 24 LAN technologies Data link layer so far: services, error detection/correction, multiple access Next: LAN technologies addressing Ethernet hubs, switches PPP

PowerPoint Presentation:

5a- 25 Outline 1 Introduction and services 2 Error detection and correction 3Multiple access protocols 4 Link-Layer Addressing 5 Ethernet 6 Hubs and switches 7 PPP 8 Link Virtualization: ATM and MPLS

LAN Addresses and ARP:

5a- 26 LAN Addresses and ARP 32- bit IP address: network-layer address used to get datagram to destination IP network (recall IP network definition) LAN (or MAC or physical or Ethernet) address: used to get datagram from one interface to another physically-connected interface (same network) 48 bit MAC address (for most LANs) burned in the adapter ROM

LAN Addresses and ARP:

5a- 27 LAN Addresses and ARP Each adapter on LAN has unique LAN address Six bytes Expressed in hexadecimal notation 1A-2F-BB-76-09-AD 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 71-65-F7-2B-08-53 LAN (wired or wireless) Broadcast address = FF-FF-FF-FF-FF-FF = adapter

LAN Address (more):

5a- 28 LAN Address (more) MAC address allocation administered by IEEE manufacturer buys portion of MAC address space (to assure uniqueness) Analogy: (a) MAC address: like Social Security Number (b) IP address: like postal address MAC flat address => portability MAC address of an adapter card does not change when it is moved from one LAN to another IP hierarchical address NOT portable depends on IP network to which node is attached

Recall earlier routing discussion:

5a- 29 Recall earlier routing discussion A B E Starting at A, given IP datagram addressed to B: look up network address of B, find B on same network as A link layer send datagram to B inside link-layer frame B’s MAC addr A’s MAC addr A’s IP addr B’s IP addr IP payload datagram frame frame dest address datagram source, dest address frame source address

ARP: Address Resolution Protocol:

5a- 30 ARP: Address Resolution Protocol Each IP node (Host, Router) on LAN has an ARP table 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) Question: how to determine MAC address of B knowing B’s IP address? 1A-2F-BB-76-09-AD 58-23-D7-FA-20-B0 0C-C4-11-6F-E3-98 71-65-F7-2B-08-53 LAN

ARP protocol: Same LAN (network):

5a- 31 ARP protocol: Same LAN (network) A wants to send datagram to B, 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 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

Routing to another LAN:

5a- 32 Routing to another LAN walkthrough: send datagram from A to B via R assume A know’s B IP address Two ARP tables in router R, one for each IP network (LAN) In routing table at source Host, find router In ARP table at source, find MAC address E6-E9-00- 17-BB-4B, etc A R B

PowerPoint Presentation:

5a- 33 A creates 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 destination, frame contains A-to-B IP datagram A’s data link layer sends frame R’s data link layer receives frame R removes IP datagram from Ethernet frame, sees its destined to B R uses ARP to get B’s physical layer address R creates frame containing A-to-B IP datagram sends to B A R B

PowerPoint Presentation:

5a- 34 Outline 1 Introduction and services 2 Error detection and correction 3Multiple access protocols 4 Link-Layer Addressing 5 Ethernet 6 Hubs and switches 7 PPP 8 Link Virtualization: ATM and MPLS


5a- 35 Ethernet “ dominant” wired LAN technology: cheap $20 for 100Mbs! first widely used LAN technology Simpler, cheaper than token LANs and ATM Kept up with speed race: 10, 100, 1000 Mbps Metcalfe’s Ethernet sketch

PowerPoint Presentation:

5a- 36 Star topology Bus topology popular through mid 90s Now star topology prevails Connection choices: hub or switch (more later) hub or switch

Ethernet Frame Structure:

5a- 37 Ethernet Frame Structure Sending adapter encapsulates IP datagram (or other network layer protocol packet) in Ethernet frame Preamble: 7 bytes with pattern 10101010 followed by one byte with pattern 10101011 used to synchronize receiver, sender clock rates

Ethernet Frame Structure (more):

5a- 38 Ethernet Frame Structure (more) Data: 46 to 1500 bytes Addresses: 6 bytes if adapter receives frame with matching destination address, or with broadcast address (eg ARP packet), it passes data in frame to net-layer protocol otherwise, adapter discards frame Type: indicates the higher layer protocol (mostly IP but others may be supported such as Novell IPX and AppleTalk) CRC: checked at receiver, if error is detected, the frame is simply dropped

Unreliable, connectionless service:

5a- 39 Unreliable, connectionless service Connectionless: No handshaking between sending and receiving adapter. Unreliable: receiving adapter doesn’t send acks or nacks to sending adapter stream of datagrams passed to network layer can have data gaps due to discarded fames if the application is using UDP data gaps will be filled by retransmissions if application is using TCP otherwise, application will see the gaps

Ethernet uses CSMA/CD:

5a- 40 Ethernet uses CSMA/CD adapter may begin to transmit at anytime, i.e., no slots are used adapter doesn’t transmit if it senses that some other adapter is transmitting, that is, carrier sense transmitting adapter aborts when it senses that another adapter is also transmitting, that is, collision detection Before attempting a retransmission, adapter waits a random time, that is, random access

Ethernet CSMA/CD algorithm:

5a- 41 Ethernet CSMA/CD algorithm 1. Adaptor receives datagram from network layer and creates frame 2. If adapter senses channel idle , it starts to transmit frame. If it senses channel busy , waits until channel idle and then transmits 3. If adapter transmits entire frame without detecting another transmission, the adapter is done with frame ! 4. If adapter detects another transmission while transmitting, aborts and sends jam signal 5. After aborting, adapter enters exponential backoff : after the n th collision , adapter chooses a K at random from {0,1,2,…,2 m -1} where m = min(n, 10). Adapter waits K*512 bit times and returns to Step 2

Ethernet’s CSMA/CD (more):

5a- 42 Ethernet’s CSMA/CD (more) Jam Signal: make sure all other transmitters are aware of collision; 48 bits; Bit time: 0.1 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 x 512 bit transmission times after second collision: choose K from {0,1,2,3}… after ten collisions, choose K from {0,1,2,3,4,…,1023} See/interact with Java applet on AWL Web site: highly recommended !

CSMA/CD efficiency:

5a- 43 CSMA/CD efficiency T prop = max propagation delay between 2 nodes in LAN t trans = time to transmit max-size frame Efficiency: the long-run fraction of time during which frames are being transmitted on the channel without collisions when there are a large number of active nodes Efficiency goes to 1 as t prop goes to 0 Goes to 1 as t trans goes to infinity Much better than ALOHA, but still decentralized, simple, and cheap [Lam 1980, Bertsekas 1991]

Ethernet Technologies: 10Base2:

5a- 44 Ethernet Technologies: 10Base2 10: 10 Mbps; 2: under 200 meters max cable length thin coaxial cable in a bus topology repeaters used to connect up to multiple segments repeater repeats bits it hears on one interface to its other interfaces: physical layer device only! has become a legacy technology

10BaseT and 100BaseT:

5a- 45 10 BaseT and 100BaseT 10/100 Mbps rate; latter called “fast ethernet” T stands for Twisted Pair Nodes connect to a hub: “star topology”; 100 m max distance between nodes and hub twisted pair hub


5a- 46 Hubs Hubs are essentially physical-layer repeaters : bits coming from one link go out all other links at the same rate no frame buffering no CSMA/CD at hub: adapters detect collisions provides net management functionality twisted pair hub

Manchester encoding:

5a- 47 Manchester encoding Used in 10BaseT, 10Base2 Each bit has a transition – 1: up to down, 0: down to up 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!

Gbit Ethernet:

5a- 48 Gbit Ethernet use standard Ethernet frame format allows for point-to-point links as well as shared broadcast channels Point-to-point links use switches Shared broadcast channels use hubs called “Buffered Distributors” in shared broadcast channels, CSMA/CD is used; short distances between nodes to be efficient 10 Gbps now !

Interconnecting with hubs:

5a- 49 Interconnecting with hubs Backbone hub interconnects LAN segments Extends max distance between nodes Limitations: But individual segment collision domains become one large collision domain – all hosts share 10Mbps if a node in CS and a node EE transmit at same time: collision Can’t interconnect 10BaseT & 100BaseT A collision domain has restrictions on the maximum allowable number of nodes, the maximum distance between two hosts, the maximum number of tiers in a multi-tier design


5a- 50 Switch Link layer device stores and forwards Ethernet frames examines frame header and selectively forwards frame based on MAC dest address 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


5a- 51 Forwarding How do switches determine to which LAN segment to forward frame? Looks like a routing problem... hub hub hub switch

Self learning:

5a- 52 Self learning A switch has a switch table entry in switch table: (MAC Address of a node, Switch Interface, Time Stamp) stale entries in table dropped (TTL can be 60 min) Switch learns which hosts can be reached through which interfaces when frame received, switch “learns” location of sender: incoming interface records sender/interface pair in switch table


5a- 53 Filtering/Forwarding When switch receives a frame: index switch table using MAC destination address if entry found for destination then { if destination on interface from which frame arrived then drop the frame else forward the frame on interface indicated } else flood forward on all but the interface on which the frame arrived

Switch example:

5a- 54 Switch example Suppose C sends frame to D and D replies back with frame to C. Switch receives frame from C records in switch table that C is on interface 1 because D is not in table, switch forwards frame into interfaces 2 and 3 frame received by D hub hub hub switch A B C D E F G H I address interface A B E G 1 1 2 3 1 2 3

Switch example:

5a- 55 Switch example Suppose D replies back with frame to C. Switch receives frame from from D records in switch table that D is on interface 2 because C is in table, switch forwards frame only to interface 1 frame received by C hub hub hub switch A B C D E F G H I address interface A B E G C 1 1 2 3 1

Switch: traffic isolation:

5a- 56 Switch: traffic isolation switch installation breaks subnet into LAN segments switch filters packets: same-LAN-segment frames not usually forwarded onto other LAN segments segments become separate collision domains hub hub hub switch collision domain collision domain collision domain

Switches: dedicated access:

5a- 57 Switches: dedicated access Switch with many interfaces Hosts have direct connection to switch No collisions; full duplex Switching: A-to-A’ and B-to-B’ simultaneously, no collisions switch A A’ B B’ C C’

More on Switches:

5a- 58 More on Switches cut-through switching: when the output buffer is empty, a frame forwarded from input to output port without first collecting entire frame slight reduction in latency combinations of shared/dedicated, 10/100/1000 Mbps interfaces

Institutional network:

5a- 59 Institutional network hub hub hub switch to external network router IP subnet mail server web server

Switches vs. Routers:

5a- 60 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 Switch

Summary comparison:

5a- 61 Summary comparison

Link Layer:

5a- 62 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 Hubs and switches 5.7 PPP 5.8 Link Virtualization: ATM

Point to Point Data Link Control:

5a- 63 Point to Point Data Link Control one sender, one receiver, one link: easier than broadcast link: no Media Access Control no need for explicit MAC addressing e.g., dialup link, ISDN line popular point-to-point Data Link Control (DLC) protocols: PPP (point-to-point protocol) HDLC: High level data link control (Data link used to be considered “high layer” in protocol stack!)

PPP Design Requirements [RFC 1557]:

5a- 64 PPP Design Requirements [RFC 1557] packet framing: encapsulation of network-layer datagram in data link frame carry network layer data of any network layer protocol (not just IP) at same time ability to demultiplex upwards bit transparency: must carry any bit pattern in the data field error detection (no correction) connection liveness: detect a link failure, signal link failure to network layer network layer address negotiation: endpoint can learn/configure each other’s network address

PPP non-requirements:

5a- 65 PPP non-requirements no error correction/recovery no flow control out of order delivery OK no need to support multipoint links (e.g., polling) Error recovery, flow control, data re-ordering all relegated to higher layers!

PPP Data Frame:

5a- 66 PPP Data Frame Flag: delimiter (framing) Address: does nothing (only one option) Control: does nothing; in the future possible multiple control fields Protocol: upper layer protocol to which frame delivered (eg, PPP-LCP, IP, IPCP, etc)

PPP Data Frame:

5a- 67 PPP Data Frame info: upper layer data being carried, default maximum length = 1500 bytes check: cyclic redundancy check for error detection

Byte Stuffing:

5a- 68 Byte Stuffing “ data transparency” requirement: data field must be allowed to include flag pattern <01111110> Q: is received <01111110> data or flag? Sender: adds (“stuffs”) extra < 01111101> byte before each < 01111110> data byte adds (“stuffs”) extra < 01111101> byte before each < 01111101> data byte Receiver: single 01111101 byte: discard 01111101 two 01111101 bytes in a row: discard first byte, continue data reception single 01111110: flag byte

Byte Stuffing:

5a- 69 Byte Stuffing flag byte pattern in data to send flag byte pattern plus stuffed byte in transmitted data

PPP Control Protocol:

5a- 70 PPP Control Protocol Begins and ends in the dead state Enters link establishment state when the physical layer is present and ready to be used In the link establishment state, PPP link-control protocol (LCP) is used to negotiate link configuration options such as maximum frame size, authentication protocol (if any) to be used, etc.

PowerPoint Presentation:

5a- 71 PPP Control Protocol (Cont.) Then, the end points enter the network layer configuration state to learn/configure network layer information using a network-control protocol The network-control protocol to be used depends on the specific network layer protocol for IP: IP Control Protocol (IPCP) (protocol field: 8021) is used to configure/learn IP address Once the network layer has been configured, PPP enters the open state and may begin sending network layer datagrams

PowerPoint Presentation:

5a- 72 PPP Control Protocol (Cont.) The LCP echo-request frame and echo reply frame can be exchanged between Two PPP endpoints in order to check the status of the link To terminate the link, one end of the PPP link sends a terminate-request LCP frame and the other end replies with a terminate-ack LCP frame The link enter the dead state

Link Layer:

5a- 73 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 Hubs and switches 5.7 PPP 5.8 Link Virtualization: ATM and MPLS

Virtualization of networks:

5a- 74 Virtualization of networks Virtualization of resources: a 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:

5a- 75 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. 637-648.

The Internet: virtualizing networks:

5a- 76 The Internet: virtualizing networks ARPAnet satellite net gateway Internetwork layer (IP): addressing: internetwork appears as a 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

Cerf & Kahn’s Internetwork Architecture:

5a- 77 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!


5a- 78 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, MPSL: of technical interest in their own right

Asynchronous Transfer Mode: ATM:

5a- 79 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-to-end transport for carrying 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 :

5a- 80 ATM architecture The ATM protocol stack consists of three layers: adaptation layer: only at edge of ATM network data segmentation/reassembly roughly analagous to Internet transport layer Several different types of AALs to support different types of services ATM layer: the core of the ATM standard cell switching, routing physical layer

ATM: network or link layer?:

5a- 81 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

ATM Adaptation Layer (AAL):

5a- 82 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) is fragmented across multiple ATM cells analogy: TCP segment is fragmented in many IP packets

ATM Adaptation Layer (AAL) [more]:

5a- 83 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)

PowerPoint Presentation:

5a- 84 ATM Adaptation Layer (AAL) [more] AAL has two sublayers : Convergence sublayer : higher-layer data are encapsulated in a common part convergence sublayer (CPCS) Segmentation and reassembly (SAR) sublayer : segments the CPCS-PDU and adds AAL header and trailer bits to form the payloads of the ATM AAL PDU ATM cell User data

ATM Layer:

5a- 85 ATM Layer Service: transport cells across ATM network analogous to IP network layer very different services than IP network layer Network Architecture Internet ATM ATM ATM ATM Service Model best effort CBR VBR ABR UBR Bandwidth none constant rate guaranteed rate guaranteed minimum none Loss no yes yes no no Order no yes yes yes yes Timing no yes yes no no Congestion feedback no (inferred via loss) no congestion no congestion yes no Guarantees ?

ATM Layer: Virtual Channels:

5a- 86 ATM Layer: Virtual Channels VC transport: cells carried on VC from source to dest call setup for each call before data can flow each packet carries a virtual channel identifier (VCI) 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 performance Two types of VCs Permanent VCs (PVCs) long lasting connections typically: “permanent” route between IP routers Switched VCs (SVC): dynamically set up on per-call basis


5a- 87 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/destination pair does not scale (N*2 connections needed) SVC introduces call setup latency, processing overhead for short lived connections

ATM Layer: ATM cell:

5a- 88 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:

5a- 89 ATM cell header VCI: virtual channel ID will change from link to link through 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:

5a- 90 ATM Physical Layer Two classes of physical layer: Structured: have a transmission frame structure (TDM like frame) Unstructured: do not have frame structure Two sublayers of physical layer: Transmission Convergence Sublayer (TCS): Accept ATM cells from the ATM layer and prepare them for transmission Group bits arriving from the physical medium into cells and pass the cells to the ATM layer Physical Medium Dependent (PMD) Sublayer: depends on physical medium being used Generates and delineating bits

PowerPoint Presentation:

5a- 91 ATM Physical Layer (more) Transmission Convergence Sublayer (TCS) At the transmit side: generates header checksum (HEC) byte -- 8 bits CRC If the Physical Medium Dependent (PMD) sublayer is cell-based with no frames, TCS sends idle cells when ATM layer has not provided data cells to send At the receive side, uses the HEC byte to correct all one-bit errors and some multiple-bit errors in the header At the receive side, delineates cells by running the HEC on all contiguous sets of 40 bits (When a match occurs, a cell is delineated)

ATM Physical Layer (more):

5a- 92 ATM Physical Layer (more) Physical Medium Dependent (PMD) sublayer Some possible PMD sublayers: SONET/SDH (synchronous optical network/synchronous digital hierarchy) : have transmission frame structure (like a container carrying bits); bit synchronization; Generates and delineates frames bandwidth partitions (TDM); several speeds: OC3 = 155.52 Mbps; OC12 = 622.08 Mbps; OC48 = 2.45 Gbps, OC192 = 9.6 Gbps T1/T3: have transmission frame structure (old telephone hierarchy): T1 = 1.5Mbps/ T3 = 45Mbps Cell-based with no frames : just cells (busy/idle cells)


5a- 93 IP-Over-ATM replace “network” with ATM network ATM addresses, IP addresses


5a- 94 IP-Over-ATM AAL ATM phy phy Eth IP AAL ATM phy phy Eth IP

Datagram Journey in IP-over-ATM Network :

5a- 95 Datagram Journey in IP-over-ATM Network at entry router: maps between IP destination address and ATM destination 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 exit router: AAL5 reassembles cells into original datagram if CRC OK, datagram is passed to IP

Multiprotocol label switching (MPLS):

5a- 96 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 IP header remainder of link-layer frame MPLS header label Exp S TTL 20 3 1 5

MPLS capable routers:

5a- 97 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 table RSVP-TE (RFC 3209) 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:

5a- 98 MPLS forwarding tables R1 R2 D R3 R4 R5 0 1 0 0 A R6 in out out label label dest interface 6 - A 0 in out out label label dest interface 10 6 A 1 12 9 D 0 in out out label label dest interface 10 A 0 12 D 0 1 in out out label label dest interface 8 6 A 0 0 8 A 1 R1


5a- 99 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

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