ECE544: Communication Networks-II Spring 2009

Hang LiuLecture 8

Includes teaching materials from D. Raychaudhuri, S. Gopal, L. Peterson,

Today’s Lecture

• Introduction to transport protocols• UDP• TCP• RTP

Protocol Stack

R1

ETH FDDI

IPIP

ETH

TCP/UDP

R2

FDDI PPP

IPR3

PPP ETH

IP

Host1

IP

ETH

TCP/UDP

Host8

Appl. Appl.

• Transport protocol– Enable communication between 2 or more

processes (which may be on different hosts in different networks)

– The Transport Layer is the lowest Layer in the network stack that is an end-to-end protocol

Transport Protocols• Different applications have different requirements for

transport protocols.– Guarantee message delivery

• network may drop messages– Deliver messages in the same order they are sent

• Messages may be reordered in networks and incurs a long delay

– Delivers at most one copy of each message• Messages may duplicate in networks

– Support arbitrarily large message• Networks may limit message size

– Support synchronization between sender and receiver– Allows the receiver to apply flow control to the sender– Support multiple application processes on each host– ……

• Design just a few transport protocols to meet most of the current and future application requirements– Each satisfies the requirements for a class of appls– Many applications=>few transport protocols

Most Popular Transport Protocols• User Datagram Protocol (UDP):

– Support multiple applications processes on each host– Option to check messages for correctness with CRC check

• Transmission Control Protocol (TCP):– Ensures reliable delivery of packets between source and

destination processes– Ensures in-order delivery of packets to destination process– Other options

• Real Time Protocol (RTP):– Serves real-time multimedia applications– Header contains sequence number, timestamp, marker bit

etc– Runs over UDP

• TCP, UDP and RTP satisfy needs of the most common applications– Applications requiring other functionality usually use UDP for

transport protocol, and implement additional features as part of the application

6

User Datagram Protocol (UDP)

• De-multiplex the applications/processes on a host– Port: an identification for a communication process– Each process-to-process communication is identified by 4-Tuple

Connection Identifier<SrcPort, SrcIPAddr, DestPort, DestIPAddr >– Well-known port: Unix talk: port 517

SrcPort DesPort

Length Checksum

Data

0 16 31Appl process

UDP

IP

Appl process

Appl process

TCP

Appl process

Appl process

Port

Checksum: <UDP header, data, pseudoheader (protocol ID, src IP addr, dest IP addr in IP header + UDP length)>

Length: UDP header + data bytes

UserSpace

Kernel

Transmission Control Protocol (TCP)

• First proposed by Vinton Cerf and Robert Kahn, 1974– TCP/IP enabled computers of all sizes,

from different vendors, different OSs, to communicate with each other.

– Used by 80% of all traffic on the Internet

• Reliable, in-order delivery, connection-oriented, bye-stream service

A Simple File Transfer Application

• Server: – passive open and wait for connection

• Client:– Active open and initialize connection

establishment• After connection establishment

– Reliable data transport• Terminate connection

TCP Operation

• Sender application process only needs to provide a byte stream to the kernel

• Kernels on sending and receiving hosts operate TCP processes• Receiver application process only needs to read received

bytes from the assigned TCP buffers

4-Tuple Connection Identifier:•SrcPort, •SrcIPAddr, •DestPort, •DestIPAddr

TCP Operation Sequence

• TCP protocol completely implemented at the end hosts

• Sequence numbers maintained in bytes (remember, TCP serves a byte stream)

• Start of operation:– Connection Establishment by a Three-Way

Handshake algorithm– Consensus on Initial Sequence Number (ISN)

• Data Transfer:– Sends the data in packets, reliably and as fast as the

network/receiver permits• Finish:

– Both sides independently close their half of the connection

TCP Header Format

• Flags:– SYN– FIN– RESET– PUSH– URG– ACK

Connection Establishment

•Three-Way Handshake Algorithm– SYN and ACK flags in the header used

–Initial Sequence numbers x and y selected at random•Required to avoid

same number for previous incarnation on the same connection

Active participant(client)

Passive participant(server)

SYN, Seq#=x

SYN+ACK, Seq#=y

Ack#=x+1

ACK, Ack#=y+1Data+ACK

Data+ACK

Connection Establishment

Data transport

Termination

Connection tear-down

• Any side can terminate the connection• Each side closes its half of the connection independently

FIN

FIN-ACK

FIN-ACK

FIN

ACK

DATA Data write

Data ACK

TCP State-Transition

• Max segment lifetime (MSL): recommendation 120 sec

CLOSED

LISTEN

ESTABLISHED

SYN_RCVD SYN_SENT

FIN_WAIT_1

FIN_WAIT_2

CLOSE_WAIT

LAST_ACK

CLOSEDTIME_WAIT

CLOSING

Passive open Close

SYN/SYN+ACK

Close

Active open/SYN

SYN/SYN+ACK

ACK SYN+ACK/ACK

Send/SYN

Close/FIN

Close/FIN FIN/ACK

Event/Action

ACKACK

FIN/ACK

ACK+FIN/ACK Timeout after 2 x MSL

FIN/ACK

ACKClose/FIN

TCP Functions• Goal of TCP: Deliver data reliably and in order as fast as

possible (Throughput = bytes delivered/ time taken)

• Flow Control:– avoid that the sender sends data too fast so that the TCP

receiver cannot reliably receive and process it. • Error Control and Congestion Control:

– When packets are lost, it implies that the one or more queues in intermediate routers have overflowed.

– Retransmit lost packets– Scale back flow rate to reduce congestion– Congestion control and error control are intertwined using

the congestion window (cwnd)• TCP increases the sending rate to use the network (the

route) to full capacity or receiver capability– But scale back if congestion occurs or if receiver is flooded.

Flow Control• TCP uses a sliding window flow control protocol

– the receiver specifies the amount of data (in bytes) willing to buffer in the AdvertisedWindow field of each segment

– The sender can send only up to that amount of data before it must wait for an ack and window update from the receiver.

Sending Appl Receiving Appl

LastByteAcked LastByteSent

LastByteWritten

NextByteExpected LastByteRcvd

LastByteRead

LastByteRead < NextByteExp <= LastByteRcvd+1LastByteAcked <= LastByteSent <= LastByteWritten

AdvertisedWindow = MaxRcvBuffer- ((NextByteExp-1)-LastByteRead)

LastByteSent – LastByteAcked <= AdvertisedWindowEffWin = AdvertisedWin- (LastByteSent-LastByteAcked)

LastByteRcvd-LastByteRead<=MaxRcvBuffer

LastByteWritten – LastByteAcked <= MaxSendBuffer

TCP TCP

Sequence Number

• Protect against SequenceNum wraparound– Sliding window

• Seq # space >= 2 x WinSize• For TCP: 232 >> 2 x 216

– Seq # should not wraparound within a MSL (120 sec) period of time

– For OC-48 (2.5 Gbps), time until wraparound: 14 sec

• TCP extension to the sequence # space for protecting against seq # wrapping around– Add 32-bit timestamp as optional header

Keep the pipe full

• AdvertisedWindow: 216=>64 KB– Big enough to allow the sender to keep the

pipe full (assume that the receiver has enough buffer to handle the data)

– If RTT = 100 ms, • Delay x Bandwidth = 122 KB for 10 Mbps Ethernet• Delay x Bandwidth = 1.2 MB for 100 Mbps

Ethernet (AdvertisedWindow is not large enough)

• TCP Extension:– Scaling factor option for AdvertisedWindow,

• e.g. use 16-byte units of data

Triggering Transmission

• When to transmit a segment: – small segments subject to large overhead

• Reach max segment size (MSS): the size of the largest segment TCP can send without causing the local IP to fragment– MSS = local MTU – IP & TCP header

• The sending process explicitly ask the TCP to transmit, “push”

TCP Deadlock

• TCP Deadlock– receiver advertises a window size of 0, the

sender stops sending data – the window size update from the receiver is

lost

• To solve it:– the sender starts the persist timer when

AdvertisedWindow = 0– When the persist timer expires, the sender

sends a small packet

TCP Silly Window Syndrome• TCP Silly Window Syndrome

– Sender has MSS bytes of data to send, but window is closed

– ACK arrives with a small window– Sender sends a small segment (high overhead)– Receiver advertise a small window– Sender sends a small receive segment– Repeat the above

• To solve: Nagle’s Algorithm– When the application have data to send

• If both available data and the window >= MSS– Send a full segment

• Else– If there is unACKed data in flight

» Buffer the new data until an ACK arrives– Else

» Send all the new data now

TCP Error Control

• Cumulative retransmission: ack the expected seq # in Ack field– Extension: selective ack (SACK), ack

additional blocks of received data in TCP optional header

• Adaptive retransmission– Adapt the retran timer to RTT

TCP Timeout• Original Algorithm:

– EstimatedRTT = x EstimatedRTT + (1-) x SampleRTT

– Timeout = 2 x EstimatedRTT– Issue: does not distinguish whether the ACK is for

original transmission or retransmission

• Karn/Partridge Algorithm– Whenever TCP retransmits a segment, it stops taking

samples of the RTT• Only measure SampleRTT for segments that have only

have been send once– Each time TCP retransmits, set the next timeout to

be twice the last timeout• Relieve congestion

10

TCP Timeout (Cont)• TCP Timeout

– If timeout too soon, unnecessarily retransmit and add load to network

– If timeout too late, increase latency

• Jacobson/Karels Algorithm: better RTT estimation by considering the variance

Difference = SampleRTT - EstimatedRTTEstimatedRTT = EstimatedRTT + ( x Difference)Deviation = Deviation + (|Difference|- Deviation)Timeout = x EstimatedRTT + x Deviation(default: set = 1 and = 4, ) 10

Congestion

• TCP assumes packet loss as congestion

Source1

Source2

Source3

Dest2

Dest1

TCP Congestion Control

• TCP sends packets into network without reservation– Try to use network resource (bandwidth,

buffer) as much as it can

• As congestion occurs, scales back• Strategy:

– Conservatively increases packet sending rate if no congestion

– Quickly reduce sending rate as congestion detected (timeout)

Additive increase/multiplicative decrease

(AIMD) • Maintain a CongestionWindow

– MaxWindow = MIN(CongestionWindow, AdvertisedWindow)

– EffectiveWin = MaxWindow – (LastByteSent – LastByteAcked)

• Decrease congestion window aggressively and increase it conservatively– A simple algorithm: Additive increase/multiplicative

decrease (AIMD) – Each time a timeout occurs, congestion window size

(cwnd)• cwnd = cwnd/2

– cwnd = Max(Cwnd, MSS)– Each time an ACK received

• Increment = MSS x (MSS/Cwnd)– Cwnd += Increment

• (CongestionWindow increase by one packet or MSS after all packets sent out during last RTT have been acked)

Additive increase/multiplicative decrease (Cont)

• TCP sawtooth pattern

Time

Con

gest

ionW

indo

w

Siz

e

• Issues with additive increase: – takes too long to ramp up a

connection from the beginning

– The entire advertised window may be reopened when a lost packet retransmitted and a single cumulative ACK is received by the sender

TCP “Slow Start”• When timeout

– Slow Start Threshold (SSThresh) = cwnd/2– Cwnd = MSS

• when receive an ack – If cwnd <= SSThresh (Slow start phase) incr = MSS (exponential growth, double cwnd

every RTT)– Else (congestion avoidance mode) incr = MSS x MSS/cwnd (linear growth, add 1 MSS

per RTT)Cwnd = min(cwnd+incr, TCP_MAXWIN)

TCP Slow Start (Cont)

• Slow startTime

Con

gest

ionW

indo

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Siz

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Slow-start and congestion avoidance

• A closer look

Fast Retransmit•TCP timeout issue:

– may be a long time periods of time during which the connection went dead while waiting for a timer expire

•Solution: – Add fast retransmit (not

replace regular timeout): •Everytime receiver

receives a packet (out-of-order), send a duplicate ACK

•Sender retransmit the missing packet after it receives some number of duplicate ACKs (e.g. 3 duplicate ACKs)

ACK 1

ACK 2

ACK 2

ACK 2

ACK 2

ACK 6

PKT 1PKT 2

PKT 4

PKT 5PKT 6

PKT 3Retran

PKT 3

Fast Recovery

• during congestion avoidance mode, when packets (detected through 3 duplicate ACKs) are not received, the congestion window size is reduced to the slow-start threshold, rather than the small initial value 1 MSS.

– Cwnd = SSThresh = cwnd/2• (escape slow start phase when fast

retransmit detects a lost packet and additive increase begins)

TCP Sender Operation• TCP operation is paced by its ACKs

Wait for ACK

(operate in slow-start or congestion avoidance)•Measure RTT if applicable•Set cwnd_start = purge_acked_pkts()•Set cwnd = cwnd + increment_value()•Send new packets•Retart timer if applicable

•If 3rd duplicate ACK•Fast_retransmit()•Fast_recovery()

•Set ssthresh = cwnd/2, cwnd = 1•Retransmit cwnd_start packet

ACKs indicate lossless operation

ACKs indicate possible losses

Duplicate ACK

No

AC

Ks

Tim

eout

New ACK

TCP Reno• Most popular TCP flavor; implemented in most

operating systems• Includes Fast Retransmit and Fast Recovery

Real-time Traffic• Quality of Service (QoS) factors: Reliability, Delay and Jitter

– Late arrival = loss– Because of possibly unbounded retransmissions in TCP, large

delay and jitter may ensue.• Real-time applications prefer UDP instead.• Jitter can be smoothed out by a playback buffer, but initial

buffering introduces a longer delay and reduce the interactivity.– Depends on application delay tolerance

•Real-time transport protocol (RTP) operates over UDP, – RTP modules run in user-

space. RTP libraries included in the application.

– RTP provides no error recovery and congestion control; Applications handle all aspects themselves.

Application

RTP

UDP

IP

Subnet

RTP

• RTP standard: RFC 3550

V=2 P X CC MPayload Type

Sequence number

TimestampSync. source (SSRC) identifierContributing source (CSRC) identifier (optional)

..........

Extension header

RTP payload

Padding length

RTCP

• Real-time transport control protocol (RTCP)– provides out-of-band statistics and control

information for an RTP flow– partners RTP in the delivery and packaging

of multimedia data, but does not transport any media streams itself.

– RTP uses even port number, whereas RTCP uses the next higher odd port number.

RTCP Functions• RTCP Functions

– Gather statistics on quality aspects of the media distribution

• can be used by the source for adaptive media encoding (codec) and detection of transmission faults

– Convey the canonical end-point identifiers (CNAME) to all session participants

• SSRC may change during a session, but CNAME represents the unique identity of a sender

– Correlate and synchronize different media streams• RTCP messages

– Source reports– Receiver reports– Source descriptions– Application-specific control packets

Homework

• 5.13• 5.16• 5.28• 5.34• 5.39

Due 4/17