Lectures notes for 2010 - KTH

TCP header continued
Reliability is provided by the 32 bit sequence number which indicates the byte
offset in a stream of the first byte in this segment and a 32 bit acknowledgement
number which indicates the next byte which is expected.
• The initial sequence number (ISN) is a random 32 bit number.
• Note that the acknowledgement is piggybacked in each TCP segment
• TCP maintains a timer for each segment. If an acknowledgement is not received before the
timeout, then TCP retransmits the segment
• When TCP receives data it sends an acknowledgement back to sender
• TCP applies an end-to-end checksum on its header and data
• The checksum is mandatory - but otherwise similar to the UDP checksum
• TCP resequences data at the receiving side ⇒ all the bytes are
delivered in order to the receiving application
• TCP discards duplicate data at the receiving side
Urgent pointer - specifies that the stream data is offset and that the data field
begins with "urgent data" which is to bypass the normal stream - for example ^C
Maguire TCP header continued 5: 7 of 77
maguire@kth.se 2010.03.21 Internetworking/Internetteknik