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

User · Answer

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

User · Answer

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

User · Answer

According to http   en wikipedia org wiki Maximum segment size  the default largest size for a IPV4 packet on a network is 536 octets  bytes of size 8 bits   See RFC 879

User · Answer

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

User · Answer

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

User · Answer

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

User · Answer

The absolute limitation on TCP packet size is 64K  65535 bytes   but in practicality this is far larger than the size of any packet you will see  because the lower layers  e g  ethernet  have lower packet sizes   The MTU  Maximum Transmission Unit  for Ethernet  for instance  is 1500 bytes  Some types of networks  like Token Ring  have larger MTUs  and some types have smaller MTUs  but the values are fixed for each physical technology

User · Answer

It seems most web sites out on the internet use 1460 bytes for the value of MTU  Sometimes it s 1452 and if you are on a VPN it will drop even more for the IPSec headers   The default window size varies quite a bit up to a max of 65535 bytes  I use http   tcpcheck com to look at my own source IP values and to check what other Internet vendors are using

User · Answer

This is an excellent question and I run in to this a lot at work actually   There are a lot of  technically correct  answers such as 65k and 1500   I ve done a lot of work writing network interfaces and using 65k is silly  and 1500 can also get you in to big trouble   My work goes on a lot of different hardware   platforms   routers  and to be honest the place I start is 1400 bytes   If you NEED more than 1400 you can start to inch your way up  you can probably go to 1450 and sometimes to 1480 ish   If you need more than that then of course you need to split in to 2 packets  of which there are several obvious ways of doing    The problem is that you re talking about creating a data packet and writing it out via TCP  but of course there s header data tacked on and so forth  so you have  baggage  that puts you to 1500 or beyond   and also a lot of hardware has lower limits   If you  push it  you can get some really weird things going on   Truncated data  obviously  or dropped data I ve seen rarely   Corrupted data also rarely but certainly does happen

User · Answer

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

[networking] Maximum packet size for a TCP connection

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