TCP payload size: 1358 bytes. ----- [bash]> iperf3 -c bouygues.iperf.fr --set-mss 900 Traffic observed in wireshark: MSS option in SYN packet: 900(As expected) Total frame size: 974 bytes TCP payload size: 888 bytes --set-mss flag seems clear effect on the generated traffics as it reduces the TCP payload within 900bytes.
MSS is Maximum TCP segment size. MTU is used for fragmentation i.e packet larger than MTU is fragmented.But in case of MSS, packet larger than MSS is discarded. MSS is specified during TCP handshake basically in SYN and its value can't be changed after the connection is established. MSS=MTU-40 (IP header (20 bytes) + TCP header (20 bytes)) In my last article, I explained how TCP SYN analysis reveals accurate round-trip times. In this article, I will discuss another TCP analysis option for network analysis and troubleshooting: maximum segment size (MSS), which is advertised as part of the SYN process. Jan 17, 2018 · Using Wireshark, we look at a TCP handshake and explain the MSS option. If you like these videos, please subscribe and like!----- WIRESHARK TRAINING ----- Check out my hands-on courses on A "gocha", is the standard L2 and L3 headers are 20 bytes each, which means one or both, due to additional L2 or L3 option fields, might be bigger. The sending source will "know" this, but a transit tcp adjust-mss doesn't. This means it's possible TCP packets might be too large when tcp adjust-mss is only configured to MTU less 40.
MSS option is used to define the maximum segment size that will be used during a connection between two hosts. It should be seen only 3-way handshake in SYN and SYN/ACK packets. MSS option fields is 4 bytes long. MSS is also helps to define MTU values.
The Short Statement When calculating the value to put in the TCP MSS option, the MTU value SHOULD be decreased by only the size of the fixed IP and TCP headers and SHOULD NOT be decreased to account for any possible IP or TCP options; conversely, the sender MUST reduce the TCP data length to account for any IP or TCP options that it is including in the packets that it sends. The rest of this document just expounds on that statement, and the goal is to avoid IP-level fragmentation of TCP packets. Borman Informational [Page 1] RFC 6691 TCP Options and MSS July 2012 1. Introduction There has been some confusion as to what value to use with the TCP MSS option when using IP and TCP options. RFC 879 [RFC879] states: The MSS counts only data octets in the segment, it does not count the TCP header or the IP header. A device can inform the other of the MSS it wants to use during the connection establishment process. A device that chooses to do so includes in its SYN message the TCP option called, appropriately, Maximum Segment Size. The other device receives this option and records the MSS for the connection. The TCP maximum segment size (MSS) specifies the maximum number of bytes in the TCP segment’s Data field, regardless of any other factors that influence segment size. The default MSS for TCP is 536, which results from taking the minimum IP MTU of 576 and subtracting 20 bytes each for the IP and TCP headers.
If someone thinks of the sending of TCP MSS options as a negotiation, he will think that when A announced a MSS to B of 1460 (as seen at host B) and B announced a MSS of 1380 back to B, it is faulty for B to send a packet with 1460 bytes of TCP payload to A, while in fact it is perfectly OK.
new TCP option, "Window Scale", to allow windows larger than 2**16. This option defines an implicit scale factor, which is used to multiply the window size value found in a TCP header to obtain the true window size. (2) Recovery from Losses This is a field in the TCP Options part of a SYN packet. Recent Linux kernels, and a few PPPoE drivers (notably, the excellent Roaring Penguin one), feature the possibility to 'clamp the MSS'. The good thing about this is that by setting the MSS value, you are telling the remote side unequivocally 'do not ever try to send me packets bigger than MSS is Maximum TCP segment size. MTU is used for fragmentation i.e packet larger than MTU is fragmented.But in case of MSS, packet larger than MSS is discarded. MSS is specified during TCP handshake basically in SYN and its value can't be changed after the connection is established. MSS=MTU-40 (IP header (20 bytes) + TCP header (20 bytes))