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TDC561 Network Programming

This article discusses the performance aspects of end-to-end (transport) protocols, including common services, demultiplexing, and the issues related to TCP sliding window and flow control.

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TDC561 Network Programming

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  1. TDC561 Network Programming Week 10: Performance Aspects of End-to-End (Transport) Protocols and API Programming Camelia Zlatea, PhD Email: czlatea@cs.depaul.edu

  2. End-to-End (Transport) Protocols • Underlying best-effort network • drops messages • re-orders messages • delivers duplicate copies of a given message • limits messages to some finite size • delivers messages after an arbitrarily long delay • Common end-to-end services • guarantee message delivery • deliver messages in the same order they are sent • deliver at most one copy of each message • support arbitrarily large messages • support synchronization • allow the receiver to apply flow control to the sender • support multiple application processes on each host

  3. Simple Demultiplexor (UDP) 0 16 31 SrcPort DstPort Checksum Length Data • Unreliable and unordered datagram service • Adds multiplexing • No flow control • Endpoints identified by ports • servers have well-known ports • see /etc/services on Unix • Optional checksum • pseudo header + udp header + data • Header format

  4. Simple Demultiplexor (UDP) Application Application Application process process process Ports Queues Packets demultiplexed UDP Packets arrive

  5. Reliable Byte-Stream (TCP) • Connection-oriented • Byte-stream • sending process writes some number of bytes • TCP breaks into segments and sends via IP • receiving process reads some number of bytes • Full duplex • Flow control: keep sender from overrunning receiver • Congestion control: keep sender from overrunning network

  6. Application process Application process W rite Read bytes bytes … … TCP TCP Send buffer Receive buffer … Segment Segment Segment T ransmit segments Reliable Byte-Stream (TCP)

  7. End-to-End Issues Based on sliding window protocol used at data link level, but the situation is very different. • Potentially connects many different hosts • need explicit connection establishment and termination • Potentially different RTT (Round Trip Time) • need adaptive timeout mechanism • Potentially long delay in network • need to be prepared for arrival of very old packets • Potentially different capacity at destination • need to accommodate different amounts of buffering • Potentially different network capacity • need to be prepared for network congestion

  8. Segment Format Data (SequenceNum) Receiver Sender Acknowledgment + AdvertisedWindow • Each connection identified with 4-tuple: • <SrcPort, SrcIPAddr, DstPort, DstIPAddr> • Sliding window + flow control • Acknowledgment, SequenceNum, AdvertisedWindow • Flags: SYN, FIN, RESET, PUSH, URG, ACK • Checksum: pseudo header + tcp header + data

  9. TCP Sliding Window Revisited Sending application Receiving application TCP TCP LastByteWritten LastByteRead LastByteAcked LastByteSent NextByteExpected LastByteRcvd (a) (b) • Relationship between TCP send buffer (a) and receive buffer (b) • Each byte has a sequence number • ACKs are cumulative

  10. TCP Sliding Window Revisited • Sending side • LastByteAcked <= LastByteSent • LastByteSent <= LastByteWritten • bytes between LastByteAcked and LastByteWritten must be buffered • Receiving side • LastByteRead < NextByteExpected • NextByteExpected <= LastByteRcvd + 1 • bytes between NextByteRead and LastByteRcvd must be buffered

  11. Flow Control • Sender buffer size: MaxSendBuffer • Receive buffer size: MaxRcvBuffer • Receiving side • LastByteRcvd - NextByteRead Š MaxRcvBuffer • AdvertisedWindow = MaxRcvBuffer - (LastByteRcvd - NextByteRead) • Sending side • NextByteExpected Š LastByteRcvd + 1 • LastByteSent - LastByteAcked Š AdvertisedWindow • EffectiveWindow = AdvertisedWindow - (LastByteSent - LastByteAcked) • LastByteWritten - LastByteAcked Š MaxSendBuffer • block sender if (LastByteWritten - LastByteAcked) + y > MaxSendBuffer • Always send ACK in response to an arriving data segment • Persist when AdvertisedWindow=0

  12. Delay/Latency Bandwidth Network as a Pipe

  13. Keeping the Pipe Full • TCP Correctness and Performance Aspect • Size of the SequenceNum and AdvertizedWindow affects the correctness and performance of TCP • Wrap Around: 32-bit SequenceNum • Bandwidth & Time Until Wrap Around Bandwidth T1 (1.5Mbps) Ethernet (10Mbps) T3 (45Mbps) FDDI (100Mbps) STS-3 (155Mbps) STS-12 (622Mbps) STS-24 (1.2Gbps) Time Until Wrap Around 6.4 hours 57 minutes 13 minutes 6 minutes 4 minutes 55 seconds 28 seconds

  14. Delay x Bandwidth Product 18KB 122KB 549KB 1.2MB 1.8MB 7.4MB 14.8MB Bandwidth T1 (1.5Mbps) Ethernet (10Mbps) T3 (45Mbps) FDDI (100Mbps) STS-3 (155Mbps) STS-12 (622Mbps) STS-24 (1.2Gbps) Keeping the Pipe Full • TCP Correctness and Performance Aspect • Bytes in Transit: 16-bit AdvertisedWindow ( up to 64KBytes) • AdvertisedWindow large enough to allow sender to keep the pipe full (Delay x Bandwidth) Product • Bandwidth and (Delay x Bandwidth) Product dictates how big AdvertisedWindow needs to be • Required window size for 100ms RTT • TCP protocol extensions

  15. Application API Transport protocol Application Programming Interface • Separate the implementation of protocols from the interface they export. • Important at the transport layer since this defines the point where application programs typically access the network. • This interface is often called the application programming interface, or API. Notes • The API is usually defined by the OS. • Example API: sockets • Defined by BSD Unix, but ported to other systems

  16. Socket Operations • Creating a socket • int socket(int domain, int type, int protocol) • domain=AF_INET, PF_UNIX • type=SOCK_STREAM, SOCK_DGRAM • Passive open on server int bind(int socket, struct sockaddr *address, int addr_len) int listen(int socket, int backlog) int accept(int socket, struct sockaddr *address, int *addr_len) • Active open on client int connect(int socket, struct sockaddr *address, int addr_len) • Sending and receiving messages int write(int socket, char *message, int msg_len, int flags) int read(int socket, char *buffer, int buf_len, int flags)

  17. Performance

  18. Performance Overview • Bandwidth • a measure of of the width of a frequency band • voice grade telephone line supports a frequency band ranging from 300MHz to 3300MHz ; it is said to have”a bandwidth” of 3000MHZ; when given in Hz, it probably refers to the range of signals that can be accommodated. • Bandwidth of a communication link = number of bits per second that can be transmitted on that link. • ETH bandwidth is 10Mbps • available bandwidth • Measured bandwidth = number of bits per second that can be actually transmitted on that link, in practice • Throughput = Measured Performance of a system • A pair of nodes connected by a link with a bandwidth of 10Mbs might achieve a throughput of 2Mbps; an application on one host could sedn data to the other host at 2Mbps. • Bandwidth requirements for an application • Number of bits per second that it needs to transmit over the network to perform acceptably

  19. 10,000 5000 2000 1000 500 1-MB object, 1.5-Mbps link 200 1-MB object, 10-Mbps link Perceived latency (ms) 2-KB object, 1.5-Mbps link 100 2-KB object, 10-Mbps link 50 1-byte object, 1.5-Mbps link 1-byte object, 10-Mbps link 20 10 5 2 1 10 100 R TT (ms) Latency (Response Time, delay) vs. RTT Latency = Propagation + Transmit + Queue

  20. Performance Overview 1-Mbps crosscountry link Source Destination .1 Mb .1 Mb .1 Mb … .1 Mb (a) 84 pipes full of data = 8.4Mb file 1-Gbps crosscountry link Source Destination 8.4 Mb (b) 1/12 of one pipe full of data = 8.4Mb file Latency and Bandwidth

  21. Performance Overview Delay/Latency Bandwidth • 1Mbps and 1Gbps links have the same latency • limited by the speed of light • To transfer a 1MB file takes... • 100ms RTTs on a 1Mbps link • doesn't fill a 1Gbps link (12.5MB delay x bandwidth) • In other words: • 1MB file is to 1Gbps network what 1KB packet is to 1Mbps network

  22. Latency/Bandwidth Tradeoff • Throughput = TransferSize / TransferTime • if it takes 10ms to transfer 1MB, then the effective throughput is 1MB/10ms = 100MBps = 800Mbps • TransferTime = Latency + 1/Bandwidth x TransferSize • if network bandwidth is 1Gbps (it takes 1/1Gbps x 1MB = 0.8ms to transmit 1MB), an end-to-end transfer that requires 1 RTT of 100ms has a total transfer time of 100.8ms • effective throughput is 1MB/100.8ms = 79.4Mbps, not 1Gbps

  23. Round-Trip Latency (ms) App1 App2 TCP UDP IP ETH PHY Message size (bytes) 1 100 200 300 400 500 600 700 800 900 1000 UDP 297 413 572 732 898 1067 1226 1386 1551 1719 1878 TCP 365 519 691 853 1016 1185 1354 1514 1676 1845 2015 Per-Layer Latency (1 byte latency) • ETH + wire: 216ms • UDP/IP: 58ms

  24. Throughput (UDP/IP/ETH) 10 9.8 9.6 9.4 9.2 Throughput (Mbps) 9 8.8 8.6 8.4 8.2 8 2 4 8 16 32 1 Message size (KB) • Throughput improves as the message get larger, up to a limit when per-message overhead becomes insignificant = message overhead/number of bytes = ~16KB • Flattens at ~9.5Mbps < ETH 10Mbs

  25. Notes • transferring a large amount of data helps improve the effective throughput; in the limit, an infinitely large transfer size causes the effective throughput to approach the network bandwidth • having to endure more than one RTT will hurt the effective throughput for any transfer of finite size, and will be most noticeable for small transfers

  26. Implications • Congestion control • feedback based mechanisms require an RTT to adjust • can send 10MB in one 100ms RTT on a 1Gbps network • that 10MB might congest a router and lead to massive losses • can lose half a delay x bandwidth's of data during slow start • reservations work for continuous streams (e.g., video), but require an extra RTT for bulk transfers • Retransmissions • retransmitting a packet costs 1 RTT • dropping even one packet (cell) halves effective bandwidth • retransmission also implies buffering at the sender • possible solution: forward error correction (FEC) • Trading bandwidth for latency • each RTT is precious • willing to “waste” bandwidth to save latency • example: pre-fetching

  27. Host Memory Bottleneck • Issue • turning host-to-host bandwidth into application-to-application bandwidth • have to deliver data across I/O and memory buses into cache and registers

  28. Memory bandwidth • I/O bus must keep up with network speed (currently does for STS-12, assuming peak rate is achievable) • 114MBps (measured number) is only slightly faster than I/O bus; can't afford to go across memory bus twice • caches are of questionable value (rather small) • lots of reason to access buffers • user/kernel boundary • certain protocols (reassembly, check-summing) • network device and its driver • Same latency/bandwidth problems as high-speed networks

  29. Integrated Services • High-speed networks have enabled new applications • they also need “deliver on time” assurances from the network • Applications that are sensitive to the timeliness of data are called real-time applications • voice • video • industrial control • Timeliness guarantees must come from inside the network • end-hosts cannot correct late packets like they can correct for lost packets • Need more than best-effort • IETF is standardizing extensions to best-effort model

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