TCP/IP protocol and fragmentation

Question

Using the TCP/IP protocol, given a connection between a client and a server, are the packets sent by the client to the server always received in the same order they were sent?

For example, if the client sends 3 packets of data, A, B and C, will the server always receive A first followed by B and C or is it possible for the server to receive C first, followed by A and B?

Answer 1

At IP level, packets may arrive in any order (if they arrive). At TCP level, the data stream is guaranteed to be ordered in the same manner on both ends.

That means yes, the server will always receive A then B then C . As long as you are using TCP.

Answer 2

TCP is a transmission protocol, and it transmits data by sending the data out in IP packets over the underlying IP network. TCP is responsible for ensuring the correct transmission of the data, which includes ordering the arriving packets, re-requesting missing ones and discarding duplicates.

TCP as such does not expose any notion of "packet" to the user; the fact that the data is chunked into IP packets is a detail of the "over IP" implementation. A different implementation, eg TCP-over-bicycle-courier, might employ an entirely different scheme.

Answer 3

When using TCP, data is received by the destination application in the same order as it is sent by the source application.

See the following for more details:

http://en.wikipedia.org/wiki/Transmission_Control_Protocol#Data_transfer

Answer 4

It cannot happen that you receive data in a different order on the application side over a TCP socket.

It may happen that packets are received in a different order by the networking layer of the OS, but TCP makes it a requirement that the upper levels get data in order. It is the OS' role to ask again for unreceived fragments etc and assemble these fragments. So, you need not worry.

UDP, on the other hand, offers no such guarantee.

Answer 5

The server (as the physical NIC of the machine) might receive them in any order. Your OS might receive them in any order again - that will mostly (but not allways) be the order of physical reception. Your client application is guaranteed to receive them in correct order, thats a property of TCP

Answer 6

In general, packets will be received in the same order they are transmitted. But the network may drop or reorder packets. For example, packets may take different routes and arrive out of order. Packets may be lost or even duplicated on the network. The TCP implementation is responsible for retransmitting packets that are lost, acknowledging packets that are received, ignoring duplicated packets, all with the objective of accurately reconstructing the transmitted byte stream at the receiver.

At the application level, you send a stream of bytes and receive a stream of bytes. TCP does whatever is needed to ensure the received stream of bytes is identical to the sent stream of bytes, regardless of what happens to the packets on the network.