Transcript lecture8
EE 122: Lecture 8/9 Ion Stoica September 20/25, 2001 Overview TCP/IP architecture TCP • • • • IP and TCP headers Connection establishment Flow control RTT estimation [email protected] 2 Before Internet… Different packet-switching networks (e.g., ARPANET, ARPA packet radio) - only nodes on the same network could communicate [email protected] 3 Goal and Challenges Share resources of different packet switching networks interconnect existing networks … but, packet switching networks differ widely - Different services • E.g., degree of reliability - Different interfaces • E.g., length of the packet that can be transmitted, address format - Different protocols • E.g., routing protocols [email protected] 4 Possible solutions Reengineer and develop one global packet switching network standard - Not economically feasible Have every host implement the protocols of any network it wants to communicate with - Too complex, very high engineering cost [email protected] 5 Solution Add an extra layer: internetworking layer, also called Internet Protocol (IP) - Hosts implement one higher-level protocol - Networks interconnected by nodes that run the same protocol - Provide the interface between the new protocol and every network [email protected] 6 Solution Gateways (routers) [email protected] 7 Challenge 1: Different Address Formats Provide one common format map lower level addresses to a common format IPv4 address format: 32 bit length, hierarchical organization Class A Class B Class C Class D Class E 0 24 7 Network 1 0 Host 14 Network 1 1 0 1 1 1 0 1 1 1 1 0 16 Host 21 Network 8 Host 28 Multicast address 27 Unused [email protected] 8 Challenge 2: Different Packet Sizes Define a maximum packet size over all networks. Why not? Implement fragmentation/re-assembly - who is doing fragmentation? - who is doing re-assembly? [email protected] 9 Other Challenges Errors require end-to-end reliability - Add a transport layer on top of IP Different (routing) protocols coordinate these protocols [email protected] 10 Transport Layer: TCP and UDP TCP (Transmission Control Protocol) TCP/UDP - Unbounded but finite length messages • Byte streaming - Reliable and in-sequence delivery - Full duplex - Application example: file transfer, telnet IP Transport Layer Networking Layer Link Layer Physical Layer UDP (User Datagram Protocol) - Unreliable datagram delivery - Adds multiplexing/demultiplexing to IP - Application example: video/audio streaming Note: The need for UDP was the main reason behind splitting TCP and IP (at the beginning there were only one TCP/IP layer) [email protected] 11 Overview TCP/IP architecture TCP • • • • IP and TCP headers Connection establishment Flow control RTT estimation [email protected] 12 Headers IP IP header used for IP routing, fragmentation, error detection… UDP header used for multiplexing/demultiplexing, data error correction TCP header used for multiplexing/demultiplexing, flow and congestion control Receiver Sender data Application Application TCP/UDP data TCP UDP TCP UDP TCP/UDP data IP IP [email protected] data IP TCP/UDP data TCP/UDP data 13 IP Header 0 4 Version HLen 8 16 TOS Identification TTL 19 31 Length Flags Fragment offset Protocol Header checksum Source address Destination address 20 bytes Options (variable) Comments - HLen – header length only in 32-bit words (5 <= HLen <= 15) - TOS (Type of Service): now split in • Differentiated Service Field (6 bits) • remaining two bits used by ECN (Early Congestion Notification) - Length – the length of the entire datagram/segment; header + data - Flags: Don’t Fragment (DF) and More Fragments (MF) - Fragment offset – all fragments excepting last one contain multiples of 8 bytes - Header checksum - uses 1’s complement [email protected] 14 TCP Header 0 4 10 Source port 16 31 Destination port Sequence number Acknowledgement HdrLen Flags Advertised window Checksum Urgent pointer Options (variable) Sequence number, acknowledgement, and advertised window – used by sliding-window based flow control Flags: - SYN, FIN – establishing/terminating a TCP connection ACK – set when Acknowledgement field is valid URG – urgent data; Urgent Pointer says where non-urgent data starts PUSH – don’t wait to fill segment RESET – abort connection [email protected] 15 TCP Header (Cont) Checksum – 1’s complement and is computed over - TCP header - TCP data - Pseudo-header (from IP header) • Note: breaks the layering! 0 Source address Destination address TCP Segment length Protocol (TCP) [email protected] 16 Overview TCP/IP architecture TCP • • • • IP and TCP headers Connection establishment Flow control RTT estimation [email protected] 17 TCP Connection Establishment Three-way handshake - Goal: agree on a set of parameters: the start sequence number for each side - Staring sequence numbers are random. Why? Server Client (initiator) [email protected] 18 Overview TCP/IP architecture TCP • • • • IP and TCP headers Connection establishment Flow control RTT estimation [email protected] 19 Basic Problem How much traffic do you send? Two components - Flow control • Make sure that the receiver can receive as fast as you send - Congestion control (see Lecture 10) • Make sure that the network delivers the packets to the receiver [email protected] 20 TCP Flow Control vs. Link Layer Flow Control Flow control runs end-to-end, instead of on the same link RTT can vary widely Receiver’s maximum window size can vary - Because resources allocated to a connection varies Sender’s maximum window size varies - Because it is used to throttle back the sender where there is congestion (see Lecture 10) Others: - Sequence numbers counts bytes not packets - Packets usually called segments [email protected] 21 Sliding-window based flow control: - Higher window higher throughput • Throughput = wnd/RTT - Need to worry about sequence number wrapping RTT (Round Trip Time) Flow control: Window Size and Throughput wnd = 3 Remember: window size control throughput 1/18/2000 22 TCP Flow Control Receiver window (MaxRcvBuf – maximum buffer size at receiver) AdvertisedWindow = MaxRcvBuffer – (LastByteRcvd – LastByteRead) Sender window (MaxSendBuf – maximum buffer size at sender) EffectiveWindow = AdvertisedWindow – (LastByteSent – LastByteAcked) MaxSendBuffer >= LastByteWritten - LastByteAcked Sending Application Receiving Application LastByteWritten LastByteAcked LastByteRead LastByteSent NextByteExpected sequence number increases LastByteRcvd sequence number increases [email protected] 23 Overview TCP/IP architecture TCP • • • • IP and TCP headers Connection establishment Flow control RTT estimation [email protected] 24 RTT Estimation Use exponential averaging: SampleRTT AckRcvdTime SendSegmentTime EstimatedRTT EstimatedRTT (1 ) SampleRTT TimeOut 2 EstimatedRTT 0 1 EstimatedRTT SampleRTT [email protected] Time 25 Problem How to differentiate between the real ACK, and ACK of the retransmitted packet SampleRTT Sender Receiver Sender Receiver SampleRTT [email protected] 26 Karn/Partridge Algorithm Measure SampleRTT only for original transmissions Exponential backoff for each retransmission, double EstimatedRTT [email protected] 27 Jacobson/Karels Algorithm Problem: take into account the variance Difference SampleRTT EstimatedRTT EstimatedRTT EstimatedRTT Difference Deviation Deviation (| Difference | Deviation) TimeOute EstimatedRTT Deviation 0 1 1 4 [email protected] 28 Summary TCP/IP architecture - Add a layer (i.e., the IP layer) to interconnect existing networks - Add a layer (i.e., transport layer) to provide multiplexing/demultiplexing, reliability, and in-order delivery TCP - Connection establishment three way handshake - Flow control based on sliding window protocol - Congestion control next lecture [email protected] 29