prentice hallhigh performance tcp/ip networking, hassan-jain chapter 13 tcp implementation

Prentice HallHigh Performance TCP/IP Networking, Hassan-Jain

Chapter 13

TCP Implementation


Objectives

Understand the structure of typical TCP implementation Outline the implementation of extended standards for TCP

over high-performance networks Understand the sources of end-system overhead in typical

TCP implementations, and techniques to minimize them Quantify the effect of end-system overhead and buffering

on TCP performance Understand the role of Remote Direct Memory Access

(RDMA) extensions for high-performance IP networking


Contents

Overview of TCP implementation High-performance TCP End-system overhead Copy avoidance TCP offload


Implementation

Overview


Overall Structure (RFC 793)

Internal structure specified in RFC 793Fig. 13.1


Data Structure of TCP Endpoint Data structure of TCP endpoint

Transmission control block: Stores the connection state and related variables

Transmit queue: Buffers containing outstanding data Receiver queue: Buffers for received data (but not yet forwarded

to higher layer)


Buffering and Data Movement

Buffer queues reside in the protocol-independent socket layer within the operating system kernel TCP sender upcalls to the transmit queue to obtain data TCP receiver notifies the receive queue of correct arrival of

incoming data BSD-derived kernels implement buffers in mbufs

Moves data by reference Reduces the need to copy

Most implementations commit buffer space to the queue lazily Queues consume memory only when the bandwidth of the network

does not match the rate at which TCP user produces/consumes data


User Memory Access

Provides for movement of data to and from the memory of the TCP user

Copy semanticsSEND and RECEIVE are defined with copy semanticsThe user can modify a send buffer at the time the

SEND is issued

Direct accessAllows TCP to access the user buffers directlyBypasses copying of data


TCP Data Exchange TCP endpoints cooperate by exchanging segments Each segment contains:

Sequence number seg.seq, segment data length seg.len, status bits, ack seq number seg.ack, advertised receive window size seg.wnd

Fig. 13.3


Data Retransmissions

TCP sender uses retransmission timer to derive retransmission of unacknowledged dataRetransmits a segment if the timer fires

Retransmission timeout (RTO)RTO<RTT: Aggressive; too many retransmissionsRTO>RTT: Conservative; low utilisation due to

connection idle

In practice, adaptive retransmission timer with back-off is used (Specified in RFC 2988)


Congestion Control

A retransmission event indicates (to TCP sender) that the network is congested

Congestion management is a function of the end-systems RFC 2581 requires TCP end-systems respond to

congestion by reducing sending rate AIMD: Additive Increase Multiplicative Decrease

TCP sender probes for available bandwidth on the network path Upon detection of congestion, TCP sender multiplicatively

reduces cwnd Achieves fairness among TCP connections


High Performance

TCP


TCP Implementation with High Bandwidth-Delay Product

High bandwidth-delay product:High speed networks (e.g. optical networks)High-latency networks (e.g. satellite network)Collectively called Long Fat Networks (LFNs)

LFNs require large window size (more than 16 bits as originally defined for TCP)

Window scale option allows TCP sender to advertise large window size (e.g. 1 Gbyte)Specified at connection setupLimits window sizes in units of up to 16K


Round Trip Time Estimation

Accuracy of RTT estimation depends on frequent sample measurements of RTTPercentage of segments sampled decreases with larger

windowsMay be insufficient for LFNs

Timestamp optionEnables the sender to compute RTT samplesProvides safeguard against accepting out-of-sequence

numbers


Path MTU Discovery

Most efficient by using the largest MSS without segmentation

Enables TCP sender to automatically discover the largest acceptable MSS

TCP implementation must correctly handle dynamic changes to MSSNever leaves more than 2*MSS bytes of data

unacknowledgedTCP sender may need to segment data for

retransmission


End-System

Overhead


Reduce End-System Overhead

TCP imposes processing overhead in operating systemAdds directly to latencyConsumes a significant share of CPU cycles

and memory

Reducing overhead can improve application throughput


Relationship Between Bandwidth and CPU Utilization


Achievable Throuput for Host-Limited Systems


Sources of Overhead for TCP/IP Per-transfer overhead Per-packet overhead Per-byte overhead Fig. 13.5


Per-Packet Overhead

Increasing packet size can mitigate the impact of per-packet and per-segment overhead

Fig. 13.6 Increasing segment size S increases achievable

bandwidthAs packet size grows, the effect of per-packet overhead

becomes less significant

InterruptsA significant source of per-packet overhead


Relationship between Packet Size and Achievable Bandwidth


Relationship between Packet Overhead and Bandwidth


Checksum Overhead

A source of per-byte overhead Ways for reducing checksum overhead:

Complete multiple steps in a single traversal to reduce per-byte overhead

Integrate chechsumming with the data copyCompute the checksum in hardware


Copy Avoidance


Copy Avoidance for High-Performance TCP

Page remapping Uses virtual memory to reduce copying across the TCP/user

interface Typically resides at the socket layer in the OS kernel

Scatter/gather I/O Does not require copy semantics Entails a comprehensive restructuring of OS and I/O interfaces

Remote Direct Memory Access (RDMA) Steers incoming data directly into user-specified buffers IETF standards under way


TCP Offload


TCP Offload

Supports TCP/IP protocol functions directly on the network adapter (NIC)ProcessingTCP checksum offloading

Significantly reduces per-packet overheads for TCP/IP protocol processing

Helps to avoid expensive copy operations

prentice hallhigh performance tcp/ip networking, hassan-jain chapter 13 tcp implementation

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