TCP

• Network Address Translation
• Sequence and Acknowledgement Numbers
• Reliable Transmission
• Windows
• TCP header

Network Address Translation

• IP addresses are expensive (a symptom of scarcity)
• multiple computers could share a single address as long as they use different TCP/UDP ports, e.g. computer A uses local port 12345 to connect to the web server at port 80 of 1.2.3.4, and computer B uses port 12346 to connect to the same web server
• the router from the Internet to A and B must be smart enough to route packets based not only on IP address, but also on TCP or UDP port number
• how to coordinate multiple computers' use of source ports?
• in particular, both A and B might be running a web server on port 80
• answer: each computer actually has its own internal address, e.g. 192.168.1.2, and communication on the internal network uses these addresses
• the Network address translator rewrites IP and TCP/UDP headers for outgoing packets to have the external IP address as the source address, and a randomly selected port as the source port
• replies are translated in the opposite way to have the internal address as the IP destination, and the correct internal port as the destination port
• a table is maintained for each TCP connection and for recently used UDP sessions

Transmission Control Protocol

• connection oriented
• reliable
• full duplex
• streaming
• point-to-point
• communication over IP

• reliable startup
• reliable shutdown (all data received by each receiver)
• end-to-end (not on routers)

Retransmission

• each packet sent carries a sequence number
• sequence numbers vary independently in the two directions
• each packet received causes the receiver to send an acknowledgement (ack) packet
• the sender starts a retransmission timer whenever it sends a data packet (not an ack)
• if the packet or ack was lost, the timer expires and the sender retransmits the data packet
• the receiver uses the sequence number to detect duplicate, out of order, or old packets
• in TCP, (almost) every packet carries an ACK (piggyback acks)

Adaptive Retransmission

• TCP monitors the time from sending to receiving an ack
• the average of this time is a good estimate for the retransmission timer
• also add in a factor (e.g. 4) times the deviation: |actual time - average time|, so we won't retransmit unnecessarily
• if a packet was retransmitted, don't use that packet's "RTT":
  • if the packet or ack was lost, the RTT would be measured from the retransmit
  • otherwise the ack could simply be the delayed ack from the original packet
  • more complicated if we retransmitted the packet more than once
  • so only use RTTs from packets that were only sent once

Computation of Running Average

• use a factor alpha, e.g. alpha = 1/8 (it is nice if alpha is a power of two -- why?)
• if our estimate is E and we get a new sample s, compute new estimate E' = alpha s + (1 - alpha) E
• example for round-trip time, E'_RTT = 1/8 RTT + 7/8 E_RTT
• example for deviation, E'_dev = 1/8 |RTT - E_RTT| + 7/8 E_dev,
• TCP sets its retransmission timer to E_RTT + 4 E_dev
• if E_RTT = 8 ms, E_dev = 4 ms, the retransmission time is set to 8ms+16ms = 24ms
• if with this example our latest RTT is 16 ms, the new values E'_RTT and E'_dev are: (in-class exercise)

TCP windows

• TCP sequence (and ack) numbers and window sizes are all in bytes, allowing us to segment differently during retransmission, if so desired
• ack number n means we have received all bytes up to and including sequence number n-1
• each ack also carries a window w: if a packet has ack n and window w, the sender is allowed to send up to byte number n+w before stopping
• the sequence number carried in a packet is the sequence number of its first byte
• example: packet with sequence number 100 and 22 bytes numbered 100..121
• the acknowledgement sent (ack number 121) carries a window of 500 bytes
• the sender may send up to sequence number 621, inclusive

TCP acks

• in case of bidirectional data flow, we want to delay acks so they can be piggybacked: carried for free
• in case of unidirectional data flow, we want to send acks as soon as possible, so the window on the sender accurately reflects the state of the receiver
• TCP usually doesn't know whether the data flow will be uni- or bi-directional, so TCP delays all acks by a small amount or until a second packet must be acked
• if additional buffer space becomes available (the application has read some of the data), an ack might be sent just to update the window
• an ack by itself is unreliable and might get lost, so a sender with more data to send must probe the window by sending 1-byte packets until it gets a response

TCP header

    0                   1                   2                   3   
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Source Port          |       Destination Port        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Sequence Number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Acknowledgment Number                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  Data |           |U|A|P|R|S|F|                               |
   | Offset| Reserved  |R|C|S|S|Y|I|            Window             |
   |       |           |G|K|H|T|N|N|                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Checksum            |         Urgent Pointer        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Options                    |    Padding    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             data                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
 
                            TCP Header Format

TCP Header fields

• Source and Destination port: demultiplexing
• Sequence and acknowledgement: reliable delivery
• Data Offset: header size, options
• Window: flow control
• Checksum: correctness
• Urgent Pointer: "special place" in the data stream