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Lecture 14MPLS

Intro to Transport, Reliability, and TCP

15-441 Networking, Spring 2007

As usual, adapted from Srini Seshan and David Anderson, 15-441, Fall ‘06

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Multi Protocol Label Switching - MPLS

Selective combination of VCs + IP» Today: MPLS useful for traffic engineering, reducing core

complexity, and VPNs

Core idea: Layer 2 carries VC label» Could be ATM (which has its own tag)

» Could be a “shim” on top of Ethernet/etc.:

» Existing routers could act as MPLS switches just by examining that shim -- no radical re-design. Gets flexibility benefits, though not cell switching advantages

Layer 2 header

Layer 3 (IP) header

Layer 2 header

Layer 3 (IP) header

MPLS label

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MPLS + IP

Map packet onto Forward Equivalence Class (FEC)» 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

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MPLS core, IP interface

A

B

R2

R1

R3

R4

C1

2

3

4

3

3

3

1

1

1

2

2

4

4

4

2

D

IP IP

MPLS tag assigned

IP

IP

MPLS forwarding in core

MPLS tag stripped

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MPLS use case #1: VPNs

A

B

R2

R1

R3

R4

C1

2

3

4

3

3

3

1

1

1

2

2

4

4

4

2

D

10.1.0.0/24

10.1.0.0/24

10.1.0.0/24

10.1.0.0/24

MPLS tags can differentiate green VPN from orange VPN.

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MPLS use case #2: Reduced State Core

A R2

R1

R3

R4

C

.

EBGP EBGP

A R2

R1

R3

R4

C1

2

3

4

3

3

3

1

1

1

2

2

4

4

4

2

EBGP

IP Core

MPLS Core

A-> C pkt

Internal routers must know all C destinations

R1 uses MPLS tunnel to R4.

R1 and R4 know routes, but R2 and R3 don’t.

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MPLS use case #3: Traffic Engineering

As discussed earlier -- can pick routes based upon more than just destination

Used in practice by many ISPs, though certainly not all.

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MPLS Mechanisms

MPLS packet forwarding: implementation of the label is technology specific.

» Could be ATM VCI or a short extra “MPLS” header

Supports stacked labels.» Operations can be “swap” (normal label swapping),

“push” and “pop” labels.

– VERY flexible! Like creating tunnels, but much simpler -- only adds a small label.

Label CoS S TTL

20 3 1 8

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MPLS Discussion

Original motivation.» Fast packet forwarding:

– Use of ATM hardware

– Avoid complex “longest prefix” route lookup

– Limitations of routing table sizes

» Quality of service

Currently mostly used for traffic engineering and network management.

» LSPs can be thought of as “programmable links” that can be set up under software control

» on top of a simple, static hardware infrastructure

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Further reading - MPLS

MPLS isn’t in the book - sorry. Juniper has a few good presentations at NANOG (the North American Network Operators Group; a big collection of ISPs):» http://www.nanog.org/mtg-0310/minei.html» http://www.nanog.org/mtg-0402/minei.html» Practical and realistic view of what people are

doing _today_ with MPLS.

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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)

muxmux

muxmux

muxmuxOC-48

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Take Home Points

Costs/benefits/goals of virtual circuits Cell switching (ATM)

» Fixed-size pkts: Fast hardware» Packet size picked for low voice jitter. Understand trade-

offs.» Beware packet shredder effect (drop entire pkt)

Tag/label swapping» Basis for most VCs. » Makes label assignment link-local. Understand

mechanism. MPLS - IP meets virtual circuits

» MPLS tunnels used for VPNs, traffic engineering, reduced core routing table sizes

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Transport Layer

Transport introduction

Error recovery & flow control

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Transport Protocols

Lowest level end-to-end protocol.

» Header generated by sender is interpreted only by the destination

» Routers view transport header as part of the payload

77

66

55

77

66

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TransportTransport

IPIP

DatalinkDatalink

PhysicalPhysical

TransportTransport

IPIP

DatalinkDatalink

PhysicalPhysical

IPIP

router

22 22

11 11

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Functionality Split

Network provides best-effort delivery End-systems implement many functions

» Reliability

» In-order delivery

» Demultiplexing

» Message boundaries

» Connection abstraction

» Congestion control

» …

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Transport Protocols

UDP provides just integrity and demux TCP adds…

» Connection-oriented

» Reliable

» Ordered

» Point-to-point

» Byte-stream

» Full duplex

» Flow and congestion controlled

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UDP: User Datagram Protocol [RFC 768]

“No frills,” “bare bones” Internet transport protocol

“Best effort” service, UDP segments may be:

» Lost

» Delivered out of order to app

Connectionless:» No handshaking between

UDP sender, receiver

» Each UDP segment handled independently of others

Why is there a UDP? No connection establishment

(which can add delay) Simple: no connection state

at sender, receiver Small header No congestion control: UDP

can blast away as fast as desired

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UDP, cont. Often used for

streaming multimedia apps» Loss tolerant» Rate sensitive

Other UDP uses (why?):» DNS, SNMP

Reliable transfer over UDP» Must be at application

layer» Application-specific error

recovery

Source port # Dest port #

32 bits

Applicationdata

(message)

UDP segment format

Length ChecksumLength, in

bytes of UDPsegment,includingheader

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UDP Checksum

Sender: Treat segment contents as

sequence of 16-bit integers Checksum: addition (1’s

complement sum) of segment contents

Sender puts checksum value into UDP checksum field

Receiver: Compute checksum of

received segment Check if computed

checksum equals checksum field value:

» NO - error detected

» YES - no error detected

But maybe errors nonethless?

Goal: detect “errors” (e.g., flipped bits) in transmitted segment – optional use!

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High-Level TCP Characteristics

Protocol implemented entirely at the ends» Fate sharing

Protocol has evolved over time and will continue to do so

» Nearly impossible to change the header

» Use options to add information to the header

» Change processing at endpoints

» Backward compatibility is what makes it TCP

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TCP Header

Source port Destination port

Sequence number

Acknowledgement

Advertised windowHdrLen Flags0

Checksum Urgent pointer

Options (variable)

Data

Flags: SYNFINRESETPUSHURGACK

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Evolution of TCP

1975 1980 1985 1990

1982TCP & IP

RFC 793 & 791

1974TCP described by

Vint Cerf and Bob KahnIn IEEE Trans Comm

1983BSD Unix 4.2

supports TCP/IP

1984Nagel’s algorithmto reduce overhead

of small packets;predicts congestion

collapse

1987Karn’s algorithmto better estimate

round-trip time

1986Congestion

collapseobserved

1988Van Jacobson’s

algorithmscongestion avoidance and congestion control(most implemented in

4.3BSD Tahoe)

19904.3BSD Renofast retransmitdelayed ACK’s

1975Three-way handshake

Raymond TomlinsonIn SIGCOMM 75

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TCP Through the 1990s

1993 1994 1996

1994ECN

(Floyd)Explicit

CongestionNotification

1993TCP Vegas

(Brakmo et al)delay-based

congestion avoidance

1994T/TCP

(Braden)Transaction

TCP

1996SACK TCP(Floyd et al)

Selective Acknowledgement

1996Hoe

NewReno startup and loss recovery

1996FACK TCP

(Mathis et al)extension to SACK

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Outline

Transport introduction

Error recovery & flow control

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Stop and Wait

Time

Packet

ACKTim

eou

t

ARQ» Receiver sends acknowledgement

(ACK) when it receives packet

» Sender waits for ACK and timeouts if it does not arrive within some time period

Simplest ARQ protocol Send a packet, stop and

wait until ACK arrives

Sender Receiver

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Recovering from Error

Packet

ACK

Tim

eou

t

Packet

ACK

Tim

eou

t

Packet

Tim

eou

t

Packet

ACKT

ime

out

Time

Packet

ACK

Tim

eou

t

Packet

ACK

Tim

eou

t

ACK lost Packet lost Early timeoutDUPLICATEPACKETS!!!

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How to recognize a duplicate Performance

» Can only send one packet per round trip

Problems with Stop and Wait

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How to Recognize Resends?

Use sequence numbers» both packets and acks

Sequence # in packet is finite How big should it be? » For stop and wait?

One bit – won’t send seq #1 until received ACK for seq #0

Pkt 0

ACK 0

Pkt 0

ACK 1

Pkt 1ACK 0

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How to Keep the Pipe Full?

Send multiple packets without waiting for first to be acked

» Number of pkts in flight = window

Reliable, unordered delivery» Several parallel stop & waits

» Send new packet after each ack

» Sender keeps list of unack’ed packets; resends after timeout

» Receiver same as stop & wait

How large a window is needed?» Suppose 10Mbps link, 4ms delay,

500byte pkts

– 1? 10? 20?

» Round trip delay * bandwidth = capacity of pipe

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Sliding Window

Reliable, ordered delivery Receiver has to hold onto a packet until all

prior packets have arrived» Why might this be difficult for just parallel stop & wait?

» Sender must prevent buffer overflow at receiver

Circular buffer at sender and receiver» Packets in transit buffer size

» Advance when sender and receiver agree packets at beginning have been received

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ReceiverReceiverSenderSender

Sender/Receiver State

… …

Sent & Acked Sent Not Acked

OK to Send Not Usable

… …

Max acceptable

Receiver window

Max ACK received Next seqnum

Received & Acked Acceptable Packet

Not Usable

Sender window

Next expected

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Sequence Numbers

How large do sequence numbers need to be?» Must be able to detect wrap-around

» Depends on sender/receiver window size

E.g.» Max seq = 7, send win=recv win=7

» If pkts 0..6 are sent succesfully and all acks lost

– Receiver expects 7,0..5, sender retransmits old 0..6!!!

Max sequence must be send window + recv window

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Window Sliding – Common Case

On reception of new ACK (i.e. ACK for something that was not acked earlier)

» Increase sequence of max ACK received

» Send next packet

On reception of new in-order data packet (next expected)» Hand packet to application

» Send cumulative ACK – acknowledges reception of all packets up to sequence number

» Increase sequence of max acceptable packet

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Loss Recovery

On reception of out-of-order packet» Send nothing (wait for source to timeout)

» Cumulative ACK (helps source identify loss)

Timeout (Go-Back-N recovery)» Set timer upon transmission of packet

» Retransmit all unacknowledged packets

Performance during loss recovery» No longer have an entire window in transit

» Can have much more clever loss recovery

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Go-Back-N in Action

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Selective Repeat Receiver individually acknowledges all correctly received

pkts» Buffers packets, as needed, for eventual in-order delivery to upper

layer

Sender only resends packets for which ACK not received» Sender timer for each unACKed packet

Sender window» N consecutive seq #’s

» Again limits seq #s of sent, unACKed packets

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Selective Repeat: Sender, Receiver

Windows

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Important Lessons

Transport service» UDP mostly just IP service

» TCP congestion controlled, reliable, byte stream

Types of ARQ protocols» Stop-and-wait slow, simple

» Go-back-n can keep link utilized (except w/ losses)

» Selective repeat efficient loss recovery

Sliding window flow control» Addresses buffering issues and keeps link utilized

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Next Lecture

Congestion control

TCP Reliability