The maximum segment size (MSS) is a parameter of the Options field of the TCP header that specifies the largest amount of data, specified in bytes, that a computer or communications device can receive in a single TCP segment. It does not count the TCP header or the IP header (unlike, for example, the MTU for IP datagrams).[1]: §3.7.1 The IP datagram containing a TCP segment may be self-contained within a single packet, or it may be reconstructed from several fragmented pieces; either way, the MSS limit applies to the total amount of data contained in the final, reconstructed TCP segment.
To avoid fragmentation in the IP layer, a host must specify the maximum segment size as equal to the largest IP datagram that the host can handle minus the IP and TCP header sizes. Therefore, IPv4 hosts are required to be able to handle an MSS of 536 octets (= 576 - 20 - 20) and IPv6 hosts are required to be able to handle an MSS of 1220 octets (= 1280 - 40 - 20).[1]: §3.7.1
Small MSS values will reduce or eliminate IP fragmentation but will result in higher overhead.[2]
Each direction of data flow can use a different MSS.
For most computer users, the MSS option is established by the operating system.
TCP options size (Variable 0–320 bits, in units of 32 bits) must be deducted from MSS size if TCP options are enabled. For example, TCP Time Stamps are enabled by default on Linux platforms.
The default TCP Maximum Segment Size is for IPv4 is 536. For IPv6 it is 1220.[1]: §3.7.1 Where a host wishes to set the maximum segment size to a value other than the default, the maximum segment size is specified as a TCP option, initially in the TCP SYN packet during the TCP handshake. The value cannot be changed after the connection is established.
Inter-Layer Communicationedit
In order to notify MSS to the other end, an inter-layer communication is done as follows:[3]: §11
The IP should ask the Network Driver for the Maximum Transmission Unit.
The TCP should ask the IP for the Maximum Datagram Data Size (MDDS). This is the MTU minus the IP header length (MDDS = MTU - IPHdrLen).
When opening a connection, TCP can send an MSS option with the value equal to: MDDS - TCPHdrLen. In other words, the MSS value to send is: MSS = MTU - TCPHdrLen - IPHdrLen
While sending TCP segments to the other end, an inter-layer communication is done as follows:[3]: §11
TCP should determine the Maximum Segment Data Size (MSDS) from either the default or the received value of the MSS option.
TCP should determine if source fragmentation is possible (by asking the IP) and desirable.
If so, TCP may hand to IP, segments (including the TCP header) up to MSDS + TCPHdrLen.
If not, TCP may hand to IP, segments (including the TCP header) up to the lesser of (MSDS + TCPHdrLen) and MDDS.
IP checks the length of data passed to it by TCP. If the length is less than or equal MDDS, IP attaches the IP header and hands it to the ND. Otherwise, the IP must do source fragmentation.
MSS and MTUedit
MSS is sometimes conflated with MTU/PMTU, which is a characteristic of the underlying link layer, while MSS applies specifically to TCP and hence the transport layer. The two are similar in that they limit the maximum size of the payload carried by their respective protocol data unit (frame for MTU, TCP segment for MSS), and related since MSS cannot exceed the MTU for its underlying link (taking into account the overhead of any headers added by the layers below TCP). However, the difference, in addition to applying to different layers, is that MSS can have a different value in either direction and also that frames exceeding the MTU may cause packets (which encapsulate segments) to be fragmented by the network layer, while segments exceeding the MSS are simply discarded.
Further readingedit
Comer, Douglas E. (2006). Internetworking with TCP/IP. Vol. 1 (5/E ed.). Upper Saddle River, NJ, USA: Prentice Hall.
Kozierok, Charles M. (2005-09-20). The TCP/IP Guide (3.0 ed.). Retrieved 2011-09-08.
^ abcW. Eddy, ed. (August 2022). Transmission Control Protocol (TCP). Internet Engineering Task Force. doi:10.17487/RFC9293. ISSN 2070-1721. STD 7. RFC 9293. Internet Standard 7. Obsoletes RFC 793, 879, 2873, 6093, 6429, 6528 and 6691. Updates RFC 1011, 1122 and 5961.
^The TCP/IP Guide, TCP Maximum Segment Size (MSS) and Relationship to IP Datagram Size
^ abJ. Postel (November 1983). The TCP Maximum Segment Size and Related Topics. Network Working Group. doi:10.17487/RFC0879. RFC 879. Obsolete. Obsoleted by RFC 7805 and 9293. Updated by RFC 6691.
January 01, 1970
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The maximum segment size MSS is a parameter of the Options field of the TCP header that specifies the largest amount of data specified in bytes that a computer or communications device can receive in a single TCP segment It does not count the TCP header or the IP header unlike for example the MTU for IP datagrams 1 3 7 1 The IP datagram containing a TCP segment may be self contained within a single packet or it may be reconstructed from several fragmented pieces either way the MSS limit applies to the total amount of data contained in the final reconstructed TCP segment To avoid fragmentation in the IP layer a host must specify the maximum segment size as equal to the largest IP datagram that the host can handle minus the IP and TCP header sizes Therefore IPv4 hosts are required to be able to handle an MSS of 536 octets 576 20 20 and IPv6 hosts are required to be able to handle an MSS of 1220 octets 1280 40 20 1 3 7 1 Small MSS values will reduce or eliminate IP fragmentation but will result in higher overhead 2 Each direction of data flow can use a different MSS For most computer users the MSS option is established by the operating system TCP options size Variable 0 320 bits in units of 32 bits must be deducted from MSS size if TCP options are enabled For example TCP Time Stamps are enabled by default on Linux platforms Contents 1 Default value 2 Inter Layer Communication 3 MSS and MTU 4 Further reading 5 See also 6 ReferencesDefault value editThe default TCP Maximum Segment Size is for IPv4 is 536 For IPv6 it is 1220 1 3 7 1 Where a host wishes to set the maximum segment size to a value other than the default the maximum segment size is specified as a TCP option initially in the TCP SYN packet during the TCP handshake The value cannot be changed after the connection is established Inter Layer Communication editIn order to notify MSS to the other end an inter layer communication is done as follows 3 11 The Network Driver ND or interface should know the Maximum Transmission Unit MTU of the directly attached network The IP should ask the Network Driver for the Maximum Transmission Unit The TCP should ask the IP for the Maximum Datagram Data Size MDDS This is the MTU minus the IP header length MDDS MTU IPHdrLen When opening a connection TCP can send an MSS option with the value equal to MDDS TCPHdrLen In other words the MSS value to send is MSS MTU TCPHdrLen IPHdrLen While sending TCP segments to the other end an inter layer communication is done as follows 3 11 TCP should determine the Maximum Segment Data Size MSDS from either the default or the received value of the MSS option TCP should determine if source fragmentation is possible by asking the IP and desirable If so TCP may hand to IP segments including the TCP header up to MSDS TCPHdrLen If not TCP may hand to IP segments including the TCP header up to the lesser of MSDS TCPHdrLen and MDDS IP checks the length of data passed to it by TCP If the length is less than or equal MDDS IP attaches the IP header and hands it to the ND Otherwise the IP must do source fragmentation MSS and MTU editMSS is sometimes conflated with MTU PMTU which is a characteristic of the underlying link layer while MSS applies specifically to TCP and hence the transport layer The two are similar in that they limit the maximum size of the payload carried by their respective protocol data unit frame for MTU TCP segment for MSS and related since MSS cannot exceed the MTU for its underlying link taking into account the overhead of any headers added by the layers below TCP However the difference in addition to applying to different layers is that MSS can have a different value in either direction and also that frames exceeding the MTU may cause packets which encapsulate segments to be fragmented by the network layer while segments exceeding the MSS are simply discarded Further reading editComer Douglas E 2006 Internetworking with TCP IP Vol 1 5 E ed Upper Saddle River NJ USA Prentice Hall Kozierok Charles M 2005 09 20 The TCP IP Guide 3 0 ed Retrieved 2011 09 08 See also editPath MTU DiscoveryReferences edit a b c W Eddy ed August 2022 Transmission Control Protocol TCP Internet Engineering Task Force doi 10 17487 RFC9293 ISSN 2070 1721 STD 7 RFC 9293 Internet Standard 7 Obsoletes RFC 793 879 2873 6093 6429 6528 and 6691 Updates RFC 1011 1122 and 5961 The TCP IP Guide TCP Maximum Segment Size MSS and Relationship to IP Datagram Size a b J Postel November 1983 The TCP Maximum Segment Size and Related Topics Network Working Group doi 10 17487 RFC0879 RFC 879 Obsolete Obsoleted by RFC 7805 and 9293 Updated by RFC 6691 Retrieved from https en wikipedia org w index php title Maximum segment size amp oldid 1176064172, wikipedia, wiki, book, books, library,
