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15-441 Computer Networking. ATM and Label Switching. Outline. ATM. IP over ATM. Label switching. History. Telephone companies supported voice telephony: 4 kHz analog, 64 kbps digital. They already provided lines for data networking. ISDN: 64 + 64 + 16 kbps T1 (1.544 Mbps)

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15-441 Computer Networking

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15 441 computer networking l.jpg

15-441 Computer Networking

ATM and Label Switching


Outline l.jpg

Outline

  • ATM.

  • IP over ATM.

  • Label switching.

Lecture #19: 11-08-01


History l.jpg

History

  • Telephone companies supported voice telephony: 4 kHz analog, 64 kbps digital.

  • They already provided lines for data networking.

    • ISDN: 64 + 64 + 16 kbps

    • T1 (1.544 Mbps)

    • T3 (44.736 Mbps)

  • They wanted to become the primary service provider for data networking services.

    • file transfer: bursty, many Mbps peak

    • database access: bursty, low latency

    • Multimedia: synchronized

    • Video: 6 MHz analog, 1.2-200 Mbps digital

  • How?

Lecture #19: 11-08-01


One bisdn stm l.jpg

H4

H3

H3

H2

H2

H2

H1

H1

H1

H1

H0

H0

H0

H0

H0

H0

B

B

B

B

B

B

B

B

B

B

B

D

One BISDN: STM

(Broadband Integrated Services Digital Network)

  • Synchronous Transfer Mode

  • Provide multirate frame structure:

    iH4 + jH3 + kH2 + lH1 + mH0 + nB + D

e.g.

  • Problems

    • complex channel assignment/subdivision

    • poor support for bursty connections

Lecture #19: 11-08-01


More flexible solution atm l.jpg

More Flexible Solution: ATM

  • Asynchronous Transfer Mode

  • Instead of predefined TDM slots, tag each slot with a virtual connection ID.

    • Bandwidth can change dynamically

  • Small packets allow good real time behavior.

  • Fixed sized packets (cells) support fast switching

VCI

data

Lecture #19: 11-08-01


Atm features l.jpg

ATM Features

  • Fixed size cells (53 bytes).

  • Virtual circuit technology using hierarchical virtual circuits (VP,VC).

  • PHY (physical layer) processing delineates cells by frame structure, cell header error check.

  • Support for multiple traffic classes by adaptation layer.

    • E.g. voice channels, data traffic

  • Elaborate signaling stack.

    • Backwards compatible with respect to the telephone standards

  • Standards defined by ATM Forum.

    • Organization of manufacturers, providers, users

Lecture #19: 11-08-01


Atm standard protocol layers l.jpg

ATM Standard Protocol Layers

Upper Layer Protocols

CS

AAL

Convergence sublayer

ATM adaptation

layer

SAR

Segmentation and reassembly

ATM

PMD

PHY

Physical medium dependent

TC

Transmission convergence

Lecture #19: 11-08-01


The atm cell uni l.jpg

GFC

VPI

VPI

VCI

VCI

VCI

PT

CLP

HEC

payload

The ATM Cell (UNI)

hdr

5 bytes

pld

48 bytes

(proportional)

Lecture #19: 11-08-01


Why 53 bytes l.jpg

Why 53 Bytes?

  • Small cells favored by voice applications

    • delays of more than about 10 ms require echo cancellation

    • each payload byte consumes 125 s (8000 samples/sec)

  • Large cells favored by data applications

    • Five bytes of each cell are overhead

  • France favored 32 bytes

    • 32 bytes = 4 ms

    • France is 3 ms wide

  • USA, Australia favored 64 bytes

    • 64 bytes = 8 ms

    • USA is 16 ms wide

  • Compromise

Lecture #19: 11-08-01


Virtual circuit switching l.jpg

Virtual Circuit Switching

  • Signaling establishes mapping from (Portin, VCIin) to (Portout, VCIout) at each switch on path.

    • VCI remapping

  • Cells in a VC arrive in order.

sw

P3

P7

sw

P1

P5

VCI=5

VCI=3

VCI=27

VCI=3

VCI=16

P0

P12

sw

P6

P12

sw

Lecture #19: 11-08-01


Virtual paths l.jpg

Virtual Paths

  • Virtual path is a bundle of virtual circuits.

    • VCs in a virtual path follow the same route

  • Benefits:

    • route and rerouting at the virtual path level

    • fast connection set up

    • bandwidth management

sw

P3

P7

sw

P1

P5

VPI=5

VCI=7

VPI=3

VCI=7

VPI=27

VCI=7

VPI=3

VCI=7

VPI=16

VCI=7

P0

P12

sw

P6

P12

sw

Lecture #19: 11-08-01


Virtual path trunking l.jpg

Virtual Path Trunking

  • Allows aggregated resource management and fault recovery.

sw

sw

sw

sw

sw

Lecture #19: 11-08-01


Atm adaptation layers l.jpg

1

2

3

4

5

synchronous

asynchronous

constant

variable bit rate

connection-oriented

connectionless

ATM Adaptation Layers

  • AAL 1: audio, uncompressed video

  • AAL 2: compressed video

  • AAL 3: long term connections

  • AAL 4/5: data traffic

Lecture #19: 11-08-01


Aal3 4 adaptation layer telco l.jpg

AAL3/4 Adaptation Layer (Telco)

includes

length prediction

header

data

trailer

. . .

ATM

header

SAR

header

payload

(44 bytes)

SAR

trailer

(SAR: segment and reassembly)

type, seq#, MID (message identifier)

length, CRC

Lecture #19: 11-08-01


Seal aal5 adaptation layer computer mfr l.jpg

SEAL (AAL5) Adaptation Layer (computer mfr.)

data

pad

ctl

len

CRC

. . .

ATM

header

payload

(48 bytes)

includes EOF flag

Lecture #19: 11-08-01


Aal relative merits l.jpg

AAL Relative Merits

  • AAL3/4

    • cell by cell data integrity promotes pipelined processing

    • packet multiplexing within VC supported

    • length prediction makes smart buffer allocation possible

  • AAL5

    • 48 byte cell makes better use of bursts on host buses, e.g. 32+16 vs. 32+8+4

    • cell processing simpler

    • CRC32 more robust (?)

    • lost cell means lost packet – very significant

Lecture #19: 11-08-01


Atm traffic classes l.jpg

ATM Traffic Classes

  • Constant Bit Rate (CBR) and Variable Bit Rate (VBR).

    • Guaranteed traffic classes for different traffic types.

  • Unspecified Bit Rate (UBR).

    • Pure best effort with no help from the network

  • Available Bit Rate (ABR).

    • Best effort, but network provides support for congestion control and fairness

    • Congestion control is based on explicit congestion notification

      • Binary or multi-valued feedback

    • Fairness is based on Max-Min Fair Sharing.

      (small demands are satisfied, unsatisfied demands share equally)

Lecture #19: 11-08-01


Ubr challenges l.jpg

UBR Challenges

  • Cell loss results in packet loss.

    • Cell from middle of packet: lost packet

    • EOF cell: lost two packets

  • Even low cell loss rate can result in high packet loss rate.

    • E.g. 0.2% cell loss -> 2 % packet loss

    • Disaster for TCP

  • Solution: drop remainder of the packet, i.e. until EOF cell.

    • Helps a lot: dropping useless cells reduces bandwidth and lowers the chance of later cell drops

    • Slight violation of layers

Lecture #19: 11-08-01


Abr max min fair sharing l.jpg

ABR: Max-Min Fair Sharing

  • Flows are divided in two groups.

    • Flows that are bottlenecked elsewhere

    • Flows that are bottlenecked here

  • The max-min fair share rate Rfair of a network link is defined such that

    • Flows bottlenecked at the link have rate r = Rfair

    • Flows bottlenecked elsewhere have rate r, where

      • r < Rfair

      • r is the max-main fair share rate of the bottleneck link

  • Two implementations:

    • Multi-valued feedback: switch returns rate

    • Single bit feedback: use congestion bit in the header

Lecture #19: 11-08-01


Max min fair sharing example l.jpg

Max-Min Fair Sharing Example

Lecture #19: 11-08-01


Connections and signaling l.jpg

Connections and Signaling

  • Permanent vs. switched virtual connections

    • static vs. dynamic

    • services often start with PVCs (Permanent Virtual Circuits)

  • Call = bundle of connections, e.g. voice + video + data

  • Topology

    • point to point

    • point to multipoint

    • multipoint to multipoint

  • Signaling VCs

    • dedicated

    • metasignaled, i.e. dynamically allocated

Lecture #19: 11-08-01


Connection setup l.jpg

Connection Setup

calling

party

network

called

party

SETUP

SETUP

CONNECT

CONNECT

CONNECT

ACK

CONNECT

ACK

Lecture #19: 11-08-01


Q saal signaling atm adaptation layer l.jpg

Q.SAAL: Signaling ATM Adaptation Layer

SAAL Service

Access Point

Service Specific

Coordination Function

SSCF UNI

SSCF NNI

Service Specific

Connection Oriented Protocol

SSCOP

CPCS

Common Part Convergence Sublayer

AAL5 common part

SAR

ATM Service

Access Point

Lecture #19: 11-08-01


Ip over atm and sonet l.jpg

IP over ATM and SONET

  • Many options!

  • IP over ATM, with signaling support.

  • IP over ATM, using statically configured ATM pipes.

  • IP over SONET (Packets over SONET).

  • Differences in efficiency and flexibility in bandwidth management.

Lecture #19: 11-08-01


Ip over atm l.jpg

IP over ATM

  • When sending IP packets over an ATM network, set up a VC to destination.

    • ATM network can be end to end, or just a partial path

    • ATM is just another link layer

  • Virtual connections can be cached.

    • After a packet has been sent, the VC is maintained so that later packets can be forwarded immediately

    • VCs eventually times out

  • Properties.

    • Overhead of setting up VCs (delay for first packet)

    • Complexity of managing a pool of VCs

    • Flexible bandwidth management

    • Can use ATM QoS support for individual connections (with appropriate signaling support)

Lecture #19: 11-08-01


Lan emulation l.jpg

LAN Emulation

  • Motivation:

    • support many protocols

    • reuse software interfaces

  • Chosen: IEEE 802.x, (specifically Ethernet, token ring)

  • Issues

    • MAC - ATM mapping

    • multicast and broadcast

    • VC lifetime

    • bridging

    • ARP

Lecture #19: 11-08-01


Atm arp l.jpg

ATM ARP

  • ARP server with well-known address (or PVC) - one per logical subnet.

    • Hosts communicate with ARP server directly instead of using broadcasting

  • IP hosts register.

  • Requests for IP-ATM bindings are sent to server.

  • IP-ATM bindings are time out.

  • “Classical IP”

Lecture #19: 11-08-01


Ip over atm 2 l.jpg

IP over ATM (2)

  • Establish a set of “ATM pipes” that defines connectivity between routers.

  • Routers simply forward packets through the pipes.

    • Each statically configured VC looks like a link

  • Properties.

    • Some ATM benefits are lost (per flow QoS)

    • Flexible but static bandwidth management

    • No set up overheads

Lecture #19: 11-08-01


Packets over sonet l.jpg

Packets over SONET

  • Same as statically configured ATM pipes, but pipes are SONET channels.

  • Properties.

    • Bandwidth management is much less flexible

    • Much lower transmission overhead (no ATM headers)

mux

OC-48

mux

mux

Lecture #19: 11-08-01


Atm discussion l.jpg

ATM Discussion

  • At one point, ATM was viewed as a replacement for IP.

    • Could carry both traditional telephone traffic (CBR circuits) and other traffic (data, VBR)

    • Better than IP, since it supports QoS

  • Complex technology.

    • Switching core is fairly simple, but

    • Support for different traffic classes

    • Signaling software is very complex

    • Technology did not match people’s experience with IP

      • deploying ATM in LAN is complex (e.g. broadcast)

      • supporting connection-less service model on connection-based technology

    • With IP over ATM, a lot of functionality is replicated

  • Currently used as a datalink layer supporting IP.

Lecture #19: 11-08-01


Ip switching l.jpg

IP Switching

  • How to use ATM hardware without the software.

    • ATM switches are very fast data switches

    • software adds overhead, cost

  • The idea is to identify flows at the IP level and to create specific VCs to support these flows.

    • flows are identified on the fly by monitoring traffic

    • flow classification can use addresses, protocol types, ...

    • can distinguish based on destination, protocol, QoS

  • Once established, data belonging to the flow bypasses level 3 routing.

    • never leaves the ATM switches

  • Interoperates fine with “regular” IP routers.

    • detects and collaborates with neighboring IP switches

Lecture #19: 11-08-01


Ip switching example l.jpg

IP Switching Example

IP

IP

IP

ATM

ATM

ATM

Lecture #19: 11-08-01


Ip switching example33 l.jpg

IP Switching Example

IP

IP

IP

ATM

ATM

ATM

Lecture #19: 11-08-01


Ip switching example34 l.jpg

IP Switching Example

IP

IP

IP

ATM

ATM

ATM

Lecture #19: 11-08-01


Ip switching discussion l.jpg

IP SwitchingDiscussion

  • IP switching selectively optimizes the forwarding of specific flows.

    • Offloads work from the IP router, so for a given size router, a less powerful forwarding engine can be used

    • Each data unit carries two addresses: IP and fast path

    • Can fall back on traditional IP forwarding if there are failures

  • IP switching couples a router with an ATM switching using the GSMP protocol.

    • General Switch Management Protocol

  • IP switching can be used for flows with different granularity.

    • Flows belonging to an application .. Organization

    • Controlled by the classifier

  • Introduces a notion of flows/connections in IP.

Lecture #19: 11-08-01


An alternative tag switching l.jpg

An Alternative:Tag Switching

  • Instead of monitoring traffic to identify flows to optimize, use routing information to guide the creation of “switched” paths.

    • Switched paths are set up as a side effect of filling in forwarding tables

  • Generalize to other types of hardware.

  • Also introduced stackable tags.

    • Made it possible to temporarily merge flows and to demultiplex them without doing an IP route lookup

    • Requires variable size field for tag

A

C

A

A

B

B

B

C

Lecture #19: 11-08-01


Ip switching versus tag switching l.jpg

IP Switchingversus Tag Switching

  • Flows versus routes.

    • tags explicitly cover groups of routes

    • tag bindings set up as part of route establishment

    • flows in IP switching are driven by traffic and detected by “filters”

      • Supports both fine grain application flows and coarser grain flow groups

  • Stackable tags.

    • provides more flexibility

  • Generality

    • IP switching focuses on ATM

    • not clear that this is a fundamental difference

Lecture #19: 11-08-01


Multi protocol label switching mpls l.jpg

Multi-Protocol Label SwitchingMPLS

  • Map packet onto Forward Equivalence Class (FEC) based on its header.

    • Simple case: longest prefix match of destination address

    • More complex if QoS of policy routing is used

  • In MPLS, a label is associated with the packet when it enters the network and forwarding is based on the label in the network core.

    • Label is swapped (as ATM VCIs)

  • Potential advantages.

    • Packet forwarding can be faster

    • Routing can be based on ingress router and port

    • Can use more complex routing decisions

    • Can force packets to followed a pinned route

Lecture #19: 11-08-01


Mpls mechanisms l.jpg

MPLS Mechanisms

  • Implementation of the label is technology specific.

    • Could be ATM VCI or an extra header

  • Label Distribution Protocols distributes information on label/FEC bindings.

    • Extensions of existing protocols (routing, RSVP) or stand-alone protocols

    • Can be upstream or downstream

  • Supports stacked labels.

Lecture #19: 11-08-01


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