TCP Services

TCP picks up where IP leaves off
Message fragmentation and reassembly
Packet sequence numbers
Guarantees delivery
Error detection
ACKs and retransmits
Maintains conversational context
Sessions
Specifies process at destination
Port numbers in TCP packet header

Layered Headers

Each layer adds its own header to the message that it receives from the layer above
As far as the lower layer is concerned, the higher layers’ headers are just a part of the message to be delivered
The higher layers never see the lower layer headers because the lower layers remove them before passing the message up


Layered Headers In TCP/IP

The TCP layer fragments a message into packets, puts headers on the packets, and passes them to the IP layer
The IP layer adds its own header to each packet that it receives from TCP
The IP layer removes its header before passing the delivered packet back to the TCP layer
The TCP layer removes its headers and reassembles the message before passing the result back to the layer above


TCP Protocol: Message Fragmentation And Reassembly

TCP client at source divides message into packets
Each packet gets a sequence number
Stored in the header
TCP packet becomes payload of IP packet
TCP software at destination reassembles
If arrive out of order, use sequence number


TCP Fragmentation And Sequencing Demo

Same routing tables as on Thursday
Don’t forget to decrement HopLimit
New message takes several index cards
Each index card is a TCP fragment
The card is placed in a small envelope that has a TCP header
The small envelope is placed in a big envelope with an IP header