1 / 28

Chapter 13

Chapter 13. TCP Implementation. Objectives. Understand the structure of typical TCP implementation Outline the implementation of extended standards for TCP over high-performance networks

adriel
Télécharger la présentation

Chapter 13

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Chapter 13 TCP Implementation

  2. 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

  3. Contents • Overview of TCP implementation • High-performance TCP • End-system overhead • Copy avoidance • TCP offload

  4. Implementation Overview

  5. Overall Structure (RFC 793) • Internal structure specified in RFC 793 • Fig. 13.1

  6. 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)

  7. 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

  8. User Memory Access • Provides for movement of data to and from the memory of the TCP user • Copy semantics • SEND and RECEIVE are defined with copy semantics • The user can modify a send buffer at the time the SEND is issued • Direct access • Allows TCP to access the user buffers directly • Bypasses copying of data

  9. 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

  10. Data Retransmissions • TCP sender uses retransmission timer to derive retransmission of unacknowledged data • Retransmits a segment if the timer fires • Retransmission timeout (RTO) • RTO<RTT: Aggressive; too many retransmissions • RTO>RTT: Conservative; low utilisation due to connection idle • In practice, adaptive retransmission timer with back-off is used (Specified in RFC 2988)

  11. 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

  12. High Performance TCP

  13. 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 setup • Limits window sizes in units of up to 16K

  14. Round Trip Time Estimation • Accuracy of RTT estimation depends on frequent sample measurements of RTT • Percentage of segments sampled decreases with larger windows • May be insufficient for LFNs • Timestamp option • Enables the sender to compute RTT samples • Provides safeguard against accepting out-of-sequence numbers

  15. 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 MSS • Never leaves more than 2*MSS bytes of data unacknowledged • TCP sender may need to segment data for retransmission

  16. End-System Overhead

  17. Reduce End-System Overhead • TCP imposes processing overhead in operating system • Adds directly to latency • Consumes a significant share of CPU cycles and memory • Reducing overhead can improve application throughput

  18. Relationship Between Bandwidth and CPU Utilization

  19. Achievable Throuput for Host-Limited Systems

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

  21. 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 bandwidth • As packet size grows, the effect of per-packet overhead becomes less significant • Interrupts • A significant source of per-packet overhead

  22. Relationship between Packet Size and Achievable Bandwidth

  23. Relationship between Packet Overhead and Bandwidth

  24. 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 copy • Compute the checksum in hardware

  25. Copy Avoidance

  26. 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

  27. TCP Offload

  28. TCP Offload • Supports TCP/IP protocol functions directly on the network adapter (NIC) • Processing • TCP checksum offloading • Significantly reduces per-packet overheads for TCP/IP protocol processing • Helps to avoid expensive copy operations

More Related