CS 557 - Lecture 1 Review of Basic Protocols IP - RFC 791, 1981 TCP - RFC 793, 1981 Spring 2013
These slides are a combination of two great sources: Kurose and Ross Textbook slides Steve Deering IETF Plenary Talk
IP Datagram Format IP protocol version number header length (bytes) type of data max number remaining hops (decremented at each router) upper layer protocol to deliver payload to how much overhead with TCP? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer overhead ver head. len 16-bit identifier time to live type of service upper layer 32 bits flgs length data (variable length, typically a TCP or UDP segment) Internet checksum 32 bit source IP address fragment offset 32 bit destination IP address Options (if any) total datagram length (bytes) for fragmentation/ reassembly E.g. timestamp, record route taken, specify list of routers to visit.
IP Fragmentation network links have MTU (max.transfer size) - largest possible link-level frame. different link types, different MTUs large IP datagram divided ( fragmented ) within net one datagram becomes several datagrams reassembled only at final destination IP header bits used to identify, order related fragments reassembly fragmentation: in: one large datagram out: 3 smaller datagrams
TCP RFCs: 793, 1122, 1323, 2018, 2581 socket door point-to-point: one sender, one receiver reliable, in-order byte steam: no message boundaries pipelined: TCP congestion and flow control set window size send & receive buffers application writes data TCP send buffer segment application reads data TCP receive buffer socket door full duplex data: bi-directional data flow in same connection MSS: maximum segment size connection-oriented: handshaking (exchange of control msgs) init s sender, receiver state before data exchange flow controlled: sender will not overwhelm receiver
TCP segment structure URG: urgent data (generally not used) ACK: ACK # valid PSH: push data now (generally not used) RST, SYN, FIN: connection estab (setup, teardown commands) Internet checksum (as in UDP) 32 bits source port # dest port # head len sequence number acknowledgement number not used U A P R S F checksum Receive window Urg data pnter Options (variable length) application data (variable length) counting by bytes of data (not segments!) # bytes rcvr willing to accept
TCP Connection Management Three-Way Handshake TCP server lifecycle TCP client lifecycle
TCP Flow control (Suppose TCP receiver discards out-of-order segments) spare room in buffer = RcvWindow = RcvBuffer-[LastByteRcvd - LastByteRead] Rcvr advertises spare room by including value of RcvWindow in segments Sender limits unacked data to RcvWindow guarantees receive buffer doesn t overflow
TCP Congestion Control Review When CongWin is below Threshold, sender in slowstart phase, window grows exponentially. When CongWin is above Threshold, sender is in congestion-avoidance phase, window grows linearly. When a triple duplicate ACK occurs, Threshold set to CongWin/2 and CongWin set to Threshold. When timeout occurs, Threshold set to CongWin/2 and CongWin is set to 1 MSS.
TCP: retransmission scenarios Host A Host B Host A Host B Seq=92, 8 bytes data timeout SendBase = 100 time Seq=92, 8 bytes data X loss ACK=100 Seq=92, 8 bytes data ACK=100 lost ACK scenario Sendbase = 100 SendBase = 120 SendBase = 120 Seq=92 timeout Seq=92 timeout time Seq=100, 20 bytes data Seq=92, 8 bytes data premature timeout
Setting the timeout TCP Timeouts EstimtedRTT plus safety margin large variation in EstimatedRTT -> larger safety margin first estimate of how much SampleRTT deviates from EstimatedRTT: DevRTT = (1-β)*DevRTT + β* SampleRTT-EstimatedRTT (typically, β = 0.25) Then set timeout interval: TimeoutInterval = EstimatedRTT + 4*DevRTT
Example RTT estimation: RTT: gaia.cs.umass.edu to fantasia.eurecom.fr 350 300 RTT (milliseconds) 250 200 150 100 1 8 15 22 29 36 43 50 57 64 71 78 85 92 99 106 time (seconnds) SampleRTT Estimated RTT
TCP Window Size Over Time 24 Kbytes congestion window 16 Kbytes 8 Kbytes time Long-lived TCP connection
Event State TCP Sender Action Commentary ACK receipt for previously unacked data ACK receipt for previously unacked data Loss event detected by triple duplicate ACK Slow Start (SS) Congestion Avoidance (CA) SS or CA CongWin = CongWin + MSS, If (CongWin > Threshold) set state to Congestion Avoidance CongWin = CongWin+MSS * (MSS/CongWin) Threshold = CongWin/2, CongWin = Threshold, Set state to Congestion Avoidance Timeout SS or CA Threshold = CongWin/2, CongWin = 1 MSS, Set state to Slow Start Duplicate ACK SS or CA Increment duplicate ACK count for segment being acked Resulting in a doubling of CongWin every RTT Additive increase, resulting in increase of CongWin by 1 MSS every RTT Fast recovery, implementing multiplicative decrease. CongWin will not drop below 1 MSS. Enter slow start CongWin and Threshold not changed
Summary
IPv6 Initial motivation: 32-bit address space soon to be completely allocated. Additional motivation: header format helps speed processing/ forwarding header changes to facilitate QoS IPv6 datagram format: fixed-length 40 byte header no fragmentation allowed
IPv6 Header (Cont) Priority: identify priority among datagrams in flow Flow Label: identify datagrams in same flow. (concept of flow not well defined). Next header: identify upper layer protocol for data
Other Changes from IPv4 Checksum: removed entirely to reduce processing time at each hop Options: allowed, but outside of header, indicated by Next Header field ICMPv6: new version of ICMP additional message types, e.g. Packet Too Big multicast group management functions