The Data Link Layer Our goals: Previous understand principles - - PDF document

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12 October 2001 Link 3 1 Data Communications, Jonny Pettersson, UmU

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

Previous

❍ link layer services ❍ error detection, correction ❍ multiple access protocols and

LANs

❍ link layer addressing, ARP ❍ Ethernet ❍ Token Ring ❍ hubs, bridges, switches ❍ PPP

Today

❒ ATM ❒ X.25 and Frame Relay

12 October 2001 Link 3 2 Data Communications, Jonny Pettersson, UmU

Asynchronous Transfer Mode: ATM

❒ 1980s/1990s: standard for high-speed

Broadband Integrated Service Digital Network architecture

❒ Goal: integrated, end-end transport of

voice, video, data

❍meeting timing/QoS requirements of

voice, video

❍“next generation” telephony ❍packet-switching using virtual circuits

12 October 2001 Link 3 3 Data Communications, Jonny Pettersson, UmU

ATM architecture

❒ adaptation layer: only at edge of ATM network

❍ data segmentation/reassembly ❍ roughly analogous to Internet transport layer

❒ ATM layer: “network” layer

❍ cell switching, routing

❒ physical layer

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12 October 2001 Link 3 4 Data Communications, Jonny Pettersson, UmU

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

12 October 2001 Link 3 5 Data Communications, Jonny Pettersson, UmU

ATM Physical Layer

Two pieces (sublayers) of physical layer:

❒ Transmission Convergence Sublayer (TCS)

❍ adapts ATM layer above to PMD sublayer below ❍ Header checksum generation: 8 bits CRC ❍ Cell delineation ❍ With “unstructured” PMD sublayer, transmission of idle

cells when no data cells to send ❒ Physical Medium Dependent

❍ depends on physical medium being used ❍ with or without transmission frame structure ❍ bit generation and delineation 12 October 2001 Link 3 6 Data Communications, Jonny Pettersson, UmU

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 ?

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12 October 2001 Link 3 7 Data Communications, Jonny Pettersson, UmU

ATM Layer: Virtual Circuits

❒ VC transport: cells carried on VC from source to dest

❍ a VC must exist 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 ❒ Permanent VCs (PVCs)

❍ long lasting connections ❍ typically: “permanent” route between IP routers

❒ Switched VCs (SVC):

❍ dynamically set up on per-call basis 12 October 2001 Link 3 8 Data Communications, Jonny Pettersson, UmU

ATM VCs

❒ Advantages of ATM VC approach:

❍QoS performance guarantees

❒ Drawbacks of ATM VC approach:

❍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

12 October 2001 Link 3 9 Data Communications, Jonny Pettersson, UmU

ATM Layer: ATM cell

❒ 5-byte ATM cell header ❒ 48-byte payload

Cell header Cell format

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12 October 2001 Link 3 10 Data Communications, Jonny Pettersson, UmU

ATM Adaptation Layer (AAL)

❒ ATM Adaptation Layer (AAL): “adapts”

upper layers to ATM layer below

❒ AAL present only in end systems ❒ AAL layer segment (header/trailer fields,

data) fragmented across multiple ATM cells

❍ analogy: TCP segment in many IP packets

12 October 2001 Link 3 11 Data Communications, Jonny Pettersson, UmU

ATM Adaption Layer [more]

Different versions of AAL layers, depending on ATM service class:

❒ AAL1: for CBR services, e.g. circuit emulation ❒ AAL2: for VBR services, e.g., MPEG video ❒ AAL5: for data (eg, IP datagrams)

AAL PDU ATM cell User data

12 October 2001 Link 3 12 Data Communications, Jonny Pettersson, UmU

AAL5 - Simple And Efficient

❒ AAL5: low overhead AAL used to carry IP

datagrams

❍ 4 byte cyclic redundancy check ❍ PAD ensures payload multiple of 48 bytes ❍ large AAL5 data unit to be fragmented into 48-

byte ATM cells

❍ no CPCS header, no AAL header or trailer

• low OH

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12 October 2001 Link 3 13 Data Communications, Jonny Pettersson, UmU

IP-Over-ATM

Classic IP only

❒ 3 “networks” (e.g.,

LAN segments)

❒ MAC (802.3) and IP

addresses IP over ATM

❒ replace “network”

with ATM network

❒ ATM addresses, IP

addresses

ATM network Ethernet LANs Ethernet LANs

12 October 2001 Link 3 14 Data Communications, Jonny Pettersson, UmU

Datagram Journey in IP-over- ATM Network

ATM network Ethernet LANs

A B

12 October 2001 Link 3 15 Data Communications, Jonny Pettersson, UmU

ARP in ATM Nets

❒ ATM network needs destination ATM

address

❒ IP/ATM address translation done by ATM

ARP (Address Resolution Protocol)

❍hosts register their own ATM addresses

with the ARP server

❍hosts asks the ARP server for other

hosts ATM address

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12 October 2001 Link 3 16 Data Communications, Jonny Pettersson, UmU

ATM ABR congestion control

ABR: available bit rate:

❒ “elastic service” ❒ if sender’s path

“underloaded”:

❍ sender should use

available bandwidth

❒ if sender’s path

congested:

❍ sender throttled to

minimum guaranteed rate

RM (resource management) cells:

❒ sent by sender, interspersed

with data cells

❒ bits in RM cell set by switches

(“network-assisted”)

❍ NI bit: no increase in rate

(mild congestion)

❍ CI bit: congestion

indication

❒ RM cells returned to sender by

receiver, with bits intact

12 October 2001 Link 3 17 Data Communications, Jonny Pettersson, UmU

ATM ABR congestion control

❒ two-byte ER (explicit rate) field in RM cell

❍ congested switch may lower ER value in cell

❒ EFCI bit in data cells: set to 1 in congested switch

❍ if data cell preceding RM cell has EFCI set, sender sets CI

bit in returned RM cell

12 October 2001 Link 3 18 Data Communications, Jonny Pettersson, UmU

X.25 and Frame Relay

Like ATM:

❒ wide area network technologies ❒ virtual circuit oriented ❒ origins in telephony world ❒ can be used to carry IP datagrams

❍can thus be viewed as Link Layers by IP

protocol

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12 October 2001 Link 3 19 Data Communications, Jonny Pettersson, UmU

X.25

❒ X.25 builds VC between source and destination for

each user connection

❒ Per-hop control along path

❍ error control (with retransmissions) on each

hop using LAP-B

• variant of the HDLC protocol

❍ per-hop flow control using credits

• congestion arising at intermediate node

propagates to previous node on path

• back to source via back pressure

12 October 2001 Link 3 20 Data Communications, Jonny Pettersson, UmU

IP versus X.25

❒ X.25: reliable in-sequence end-end

delivery from end-to-end

❍“intelligence in the network”

❒ IP: unreliable, out-of-sequence end-

end delivery

❍“intelligence in the endpoints”

❒ gigabit routers: limited processing

possible

❒ 2000: IP wins

12 October 2001 Link 3 21 Data Communications, Jonny Pettersson, UmU

Frame Relay

❒ Designed in late ‘80s, widely deployed in

the ‘90s

❒ Frame relay service:

❍no error control ❍end-to-end congestion control

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12 October 2001 Link 3 22 Data Communications, Jonny Pettersson, UmU

Frame Relay (more)

❒ Designed to interconnect corporate customer

LANs

❍ typically permanent VC’s: “pipe” carrying aggregate

traffic between two routers

❍ switched VC’s: as in ATM

❒ corporate customer leases FR service from

public Frame Relay network

12 October 2001 Link 3 23 Data Communications, Jonny Pettersson, UmU

Link Layer: Summary

❒ principles behind data link layer

services:

❍ error detection, correction ❍ sharing a broadcast channel: multiple

access

❍ link layer addressing, ARP

❒ various link layer technologies

❍ Ethernet ❍ hubs, bridges, switches ❍ PPP ❍ ATM ❍ X.25, Frame Relay

❒ journey down the protocol stack

now OVER!

❍ Next stops: multimedia, security,

network management

Application UDP TCP IP Link Physical