maximum packet size for a TCP connection

Question

What is the maximum packet size for a TCP connection or how can I get the maximum packet size?

Answer 1

At the application level, the application uses TCP as a stream oriented protocol. TCP in turn has segments and abstracts away the details of working with unreliable IP packets.

TCP deals with segments instead of packets. Each TCP segment has a sequence number which is contained inside a TCP header. The actual data sent in a TCP segment is variable.

There is a value for getsockopt that is supported on some OS that you can use called TCP_MAXSEG which retrieves the maximum TCP segment size (MSS). It is not supported on all OS though.

I'm not sure exactly what you're trying to do but if you want to reduce the buffer size that's used you could also look into: SO_SNDBUF and SO_RCVBUF.

Answer 2

If you are with Linux machines, "ifconfig eth0 mtu 9000 up" is the command to set the MTU for an interface. However, I have to say, big MTU has some downsides if the network transmission is not so stable, and it may use more kernel space memories.

Answer 3

There're no packets in TCP API.

There're packets in underlying protocols often, like when TCP is done over IP, which you have no interest in, because they have nothing to do with the user except for very delicate performance optimizations which you are probably not interested in (according to the question's formulation).

If you ask what is a maximum number of bytes you can send() in one API call, then this is implementation and settings dependent. You would usually call send() for chunks of up to several kilobytes, and be always ready for the system to refuse to accept it totally or partially, in which case you will have to manually manage splitting into smaller chunks to feed your data into the TCP send() API.

Answer 4

One solution can be to set socket option TCP_MAXSEG (http://linux.die.net/man/7/tcp) to a value that is "safe" with underlying network (e.g. set to 1400 to be safe on ethernet) and then use a large buffer in send system call. This way there can be less system calls which are expensive. Kernel will split the data to match MSS.

This way you can avoid truncated data and your application doesn't have to worry about small buffers.

Answer 5

The packet size for a TCP setting in IP protocol(Ip4). For this field(TL), 16 bits are allocated, accordingly the max size of packet is 65535 bytes: IP protocol details

Answer 6

Generally, this will be dependent on the interface the connection is using. You can probably use an ioctl() to get the MTU, and if it is ethernet, you can usually get the maximum packet size by subtracting the size of the hardware header from that, which is 14 for ethernet with no VLAN.

This is only the case if the MTU is at least that large across the network. TCP may use path MTU discovery to reduce your effective MTU.

The question is, why do you care?