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Modern Computer NetworksAn Open Source Approach

Chapter 4: End-to-End Protocols

Ying-Dar Lin


Content4.1 Issues

Port-Multiplexing, Reliability, Flow/Congestion Control

4.2 UDP - Unreliable Connectionless Transfer 4.3 TCP - Reliable Connection-Oriented Transfer

Connection ManagementReliabilityFlow ControlPerformance

4.4 Programming Interface: Socket4.5 Real-time Transport (RTP & RTCP) 4.6 Book roadmapPitfalls and misleadingFurther readingExercises


4.1 Issues


4.1 IssuesEnd-to-End Communication Channel: Port-Multiplexing

Port: communication end point

Reliability: Per-Link vs. Per-Packet + Per-FlowNote: per-link reliability such as Ethernet

Collision: can be detected and be retransmittedCRC/alignment error: can only rely on upper-layer protocols

Flow/Congestion Control: Per-Link vs. Per-Flow

Multi-Access Channel

MACMAC IP Networks TCPTCP

AP1 AP2 AP1 AP2IP IP

Condense delay distribution Loose delay distribution

Node-to-Node Channel End-to-End Channel

LAN host 1 LAN host 2 IP host 1 IP host 2


4.2 UDP – Unreliable Connectionless Transfers


4.2 UDP – For Unreliable Connectionless Transfers

ObjectivesPort-Multiplexing

Per-Packet Reliability: Checksum

Header Format…

Carrying Unicast/Multicast Real-Time TrafficRetransmission is Meaningless: No Per-Flow Reliability NeededBit-rate is Determined by Codec Used: No Flow Control Needed

IP Networks TCPTCP

AP1 AP2 AP1 AP2

IP host 1 IP host 2


Elementary Socket: UDP Client/Server


4.3 TCP – Reliable Connection-Oriented Transfers


4.3 TCP – For Reliable Connection-Oriented Transfers

ObjectivesPort-Multiplexing: Same as UDPPer-Flow ReliabilityPer-Flow Flow Control

Connection ManagementConnection Establishment & State Transitions

Per-Flow ReliabilityPer-Packet Checksum & Per-Flow ACKs

Per-Flow Flow/Congestion ControlPerformance

Interactive vs. Bulk-Data Transfers

Stateful (Ch1) !! Requires connection management


Elementary Socket: TCP Client/Server


TCP Connection Management

Establishment/Termination – 3-Way Handshake Protocol

SYN

ACK of SYNSYN

ACK

FIN

ACK of FIN

ACK

FIN

Establishment Termination


TCP State Transition Diagram


Reliability of Data Transfers

Per-Packet Reliability: ChecksumIn Linux:

Per-Flow Reliability: Sequence Number & ACKACK every successfully received data packetProtection Against Wrapped Sequence Numbers (PAWS)

Retransmitting Lost PacketsWhen to Retransmit Which?

IP Header TCP Header Application Data

return 0

csum=csum_partial(D,lenD,0)

csum=csum_partial(T,lenT,csum)

ip_send_check(iph)


Retransmitting Lost Packets

Retransmit Which Packet?Fast RetransmitTowards Better Accuracy: TCP SACK Option

Further Refinement: FACK (based on SACK)

When to Retransmit?Fast Retransmit: same as aboveRetransmission Timeout (RTO)

Round-Trip Time (RTT) MeasurementTradeoff: RTT vs. RTO

Karn’s Algorithm

Towards Better RTO: TCP Timestamp Option


Retransmit Which Packet?

Fast RetransmitDuplicate ACKs

Packet ReorderingPacket LossInternet Route Change

TCP Receiver ACK the First “Hole”

Triple Duplicate ACKs (TDA)4 Same ACKs (ACK field=X)TCP Sender Infer TDA as CongestionRetransmit the Packet with SeqNum=XHalve Its Sending Rate

2 3 6 7

3 4 4 4

8

4

2 3 6 7 8

Time at Receiver

ACK

DATA


When to Retransmit?

Retransmission TimeOut (RTO)Round-Trip Time (RTT) Measurement vs. RTO

RTT: Varying DramaticallySmoothed RTT (SRTT) : Exponential Weighted Moving AverageMdev: Mean Deviation of RTT

RTO=SRTT+4*Mdev

Karn’s AlgorithmDon’t Update RTO When Retransmission is Also Lost


Per-Flow Flow/Congestion Control

How Fast to Send?Fast Sender vs. Slow Receiver

How to Know?Feedback RWND (Receiver Advertised Window) in ACK by Receiver

Fast Sender vs. Congested NetworkHow to Know?

Feedback Loss Events by NetworkRe-adjust (Congestion Window) CWND

How Fast? Satisfy Both: min (RWND, CWND)



Sliding Window

3 9 10

TCP Window Size( = min(RWND, CWND) )

DATA 8

DATA 7

DATA 6

Next=6ACK

Next=5ACK

Receiver

2Sending Stream

Sent & ACKed To be sentWhen window moves

Network Pipe

sliding



Opening & Shrinking of Window Size

3 9 10

TCP Window Size( = min(RWND, CWND) )

2

Open Shrink Close


TCP Congestion Control (Linux 2.2.17)

05

10152025303540

0 0.5 1 1.5 2 2.5 3

Cw

nd (

pack

et)

Time (sec)

(a) Window Variation

cwndrwndssth

0

50

100

150

200

250

0 0.5 1 1.5 2 2.5 3

seq_

num

(K

B)

Time (sec)

(b) Sending bytes

sequence numberacknowledgement

slow-start

congestion avoidance

triple-duplicate ACKs

fast retransmit

pipe limitssth reset

fast recovery


Summary: Properties of TCP

Per-Flow Reliability Through ACKsWindow-based Flow ControlSelf-clocking using ACKs


TCP Performance & Enhancement

Interactive ConnectionsSide Effect: Silly Window Syndrome (SWS)

Problem: transactions using small packetsSolution: Clark & Nagle

Bulk Data TransfersBandwidth Delay Product (BDP)Modeling TCP Throughput


Performance of Interactive Connections

Problem: Silly Window Syndrome (SWS)Sender transmits small packetsReceiver advertises small window

SolutionSend either

Data Accumulated to Full-sized SegmentData Accumulated to ½ RWNDNagle’s Algorithm Disabled/Not Applied

(Nagle:) Don’t send small packets until no un-ACKed packetsSmall RTT: Nagle is rarely used (many small packets)Large RTT: Nagle is oftern used (few large packets)

Receiver eitherBuffer available to Full-sized SegmentBuffer available to ½ of receiver’s buffer space


Performance of Bulk Data Transfers

Bandwidth Delay Product (BDP)

Horizontal Dimension: TimeVertical Dimension: Bandwidth Shaded Area: Packet SizeBDP=pipe size=Bandwidth x Delay

Problem: ACK-Compression when 2-Way Traffic => Bursty TrafficAsymmetric Paths



Animation: Fill the Pipe Using TCP Congestion AvoidanceExample: Pipe size=6, send from left to right across the pipe

Upper pipe: data

Lower pipe: ACK



What have you observed?

When RTTs are heterogeneous……



Modeling TCP ThroughputGiven RTT, segment size s, loss rate p:

where c is a constant valueGiven additional information: Max Window Size Wm, # delayed ACK b, RTO

pt

scpstT

RTT

RTT

⋅⋅=),,(

+

+⋅

⋅=)321(

83

3,1min3

2,min),,,(

2ppbp

tbp

t

s

t

sWpsttT

RTORTTRTT

mRTORTT


UDP Header Format


TCP Header Format


TCP Options


4.4 Socket Programming Interface


4.4 Socket Programming Interface

Issue: Programming Interface to End-to-End ProtocolsProtocol Stack vs. Interfaces

BSD SocketINET Socket

TCP/UDPIP

NIC DriverEthernet

ARPICMP …

Socket Library


Bridging Applications & End-to-End Protocols

socket(domain, type, protocol)INET domain: PF_INETtype

UDP: SOCK_DGRAMTCP: SOCK_STREAM

Protocol: NULL

Typical Applications:telnetftpHTTP


Elementary Socket: TCP Client/Server


Elementary Socket: UDP Client/Server


Socket R/W in Linux: Kernel vs. User Space


Bridging Applications to Internetworking Protocols

socket(domain, type, protocol)PACKET domain: PF_PACKETtype: SOCK_DGRAMProtocol: NULL

Typical Applications:pingtraceroute


Bridging Applications to Node-to-Node Protocols

socket(domain, type, protocol)PACKET domain: PF_PACKETtype: SOCK_RAWEthernet Encapsulated IP packet: ETH_P_IP

Or others in “/usr/include/linux/if_ether.h”

Typical Applications:Packet sniffersHacking tools


Packet Capturing & Filtering

Capture until what header?

Towards Efficient Packet Filtering: Layered ModelUser-Space Tool: tcpdumpUser-Space Packet Filter: libpcap (portable)Kernel-Space Packet Filter: Linux Socket Filter


Linux Socket Filter (since post-2.0 releases)

Similar to BPF (Berkley Packet FIilter)


4.5 Real-time Transport Protocols


4.5 Real-time Transport Protocols

Issues: Codec Encapsulation & Path Quality ReportData-Plane: Video/Voice Codecs

Video: H.263…Voice: G.729…

Control-Plane: Delay/Jitter/Loss Report

RFC Standards: RTP & RTCPRTP: Data-Plane, Encapsulating the Chosen CodecRTCP: Control-Plane, Reporting Delay/Jitter/Loss to Senders


RTP (Real-Time Protocol)

ObjectivesEliminating Packet Reorder & Loss Detection: Sequence #TimestampSynchronization Source IdentifierContributing Source Identifier

Header Format


RTCP (Real-Time Transport Protocol)

ObjectivesReporting End-to-End DelayReporting Delay JitterReporting Loss Rate


Pitfalls & Fallacies

Throughput vs. GoodputGoodput is effective throughput

Window Size: Packet Count vs. Byte ModeRSVP vs. RTP vs. RTCP vs. RTSP

RSVP: signaling protocolRTP: transport protocolRTCP: control of RTPRTSP: initiates and directs the media delivery from media servers


Further Readings

TCP BasicsRFC 793R. Stevens, TCP/IP Illustrated Volume 1, Addison Wesley, 1994.

TCP VersionsK. Fall, and S. Floyd, Simulation-based Comparisons of Tahoe, Reno, and SACK TCP, ACM Computer

Communication Review, Vol. 26 No. 3, pp.5-21, Jul. 1996.J. Padhye, and S. Floyd, On Inferring TCP Behavior, ACM SIGCOMM'2001, San Diego, USA, Aug. 2001.

Modeling TCP ThroughputJ. Padhye, V. Firoiu, D. Towsley, and J. Kurose, Modeling TCP Throughput: A Simple Model

and its Empirical Validation, ACM SIGCOMM'98, Vancouver, British Columbia, Sep. 1998.


Exercise

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