Essentially IP is responsible for addressing the packet so that it conveys information
about the location of the destination and therefore can be routed.
TCP is responsible for establishing and maintaining reliable communications. This
means that if TCP A sends data to TCP B and this data is not acknowledged by TCP B,
TCP A will assume that it got lost and retransmit it. TCP is also connection oriented.
This means that a formal connection establishment process must be followed (this is
refered to as a 3-way handshake) before actual data can flow.
When you talk about virtual connection, don't confuse it with virtual circuit. A
virtual circuit is basically a path established through the network, through which all
traffic travels, although, it is not true to say that traffic cannot be re-routed if a
failure ocurs.
When TCP resides over IP, IP handles the routing of the datagrams. Packets are routed
on a packet by packet basis based on the destination IP address contained within the
packet. It is common for the path to a destination to differ from the path back. But,
it doesn't matter because TCP really doesn't care, although TCP and IP are related in
the following ways:
1) A TCP socket is used to identify the connection. A Socket is composed of the source
and destination IP addresses and Ports (80=HTTP, 25=SMTP, etc).
2) A TCP connection is associated with a routing table entry. Therefore, if the router
that you used to establish the connection goes away, you cannot defer to another
routing table entry and continue the same connection.
3) TCP will not send datagrams larger than the MTU (maximum transmission unit) of the
underlying media. IP can fragment datagrams larger than the MTU, but this is typically
not desireable, since IP is not as intelligent as TCP. TCP can also employ a
mechanism called Path MTU discovery whereby it will attempt to determine the smallest
MTU of a path and attempt to avoid creating segment larger than this.
Generally speaking, routers route packets via the layer 3 (IP) destination address
contained in the packet. The TCP connection is between the source and destination.
If you need a better explanation of the interraction of these 2 protocols, I would
suggest that you read TCP/Illustrated Volume 1 by Richard Stevens.
Hope that you found this helpful,
Tom Kager
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