TCP - Header Format

• Acknowledgement, SequenceNum, and AdvertisedWindow fields are all involved in TCP’s sliding window algorithm
• flags - 6-bits - SYN FIN RESET PUSH URG ACK
  • SYN and FIN are used for establishing and terminating a TCP connection
  • ACK is set any time Acknowledgement field is valid
  • URG signifies that this segment contains urgent data. when set UrgPtr field indicates where the non-urgent data contained in this segment begins
  • PUSH signifies that sender invoked the push operation
  • RESET signifies that receiver has become confused
• HdrLen
  • gives length of header in 32-bit words
  • also known as offset, measuring the offset from start of packet to the start of data

TCP Extensions (Header Options)

1. timestamp option (32-bit)
   • helps to improve TCP’s timeout mechanism
     • sender reads system clock and puts value in 32-bit timestamp option. the receiver then echos back this timestamp in its acknowledgements, the sender then subtracts this timestamp from the current timestamp to measure RTT
   • addresses the problem of TCP’s 32-bit SequenceNum field from wrapping around too soon on high-speed network
     • TCP decides whether to accept or reject a segment based on a 64-bit identifier that has the SequenceNum field in the low-order 32 bits and the timestamp in the high-order 32 bits
     • since timestamp is always increasing it serves to distinguish between 2 different incarnations of the same number
2. scaling factor option
   • allows TCP to advertise a larger window
   • with this option, rather than interpreting the AdvertisedWindow field as the number of individual bytes, both sides can agree that the AdvertisedWindow counts larger chunks
3. selective acknowledgement option (SACK)