tcp - What is the difference between MSS and MTU

Urg indicates there is `urgent' data in the packet. Opts are TCP options (e.g., mss 1024). Len is the length of payload data. Iptype, Src, dst, and flags are always present. The other fields depend on the contents of the packet's TCP protocol header and are output only if appropriate. Here is the opening portion of an rlogin from host rtsg to tcp - How to calculate MSS - Stack Overflow Your correct in thinking the MSS shrinks when IP/TCP options are added. MSS = MTU - (20 (IP header) + len (IP Options)) - (20 (TCP Header) + len (TCP Options)) The other main reason it would be lowered is if the packet is being encapsulated in some way (IPsec/GTP) since that adds overhead to the packet. How MTU and MSS Affect You Network | Network Direction TCP MSS. Another option is to tune the MSS. As mentioned earlier, the MSS is like the MTU, but used with TCP at layer 4. Put simply, the MSS is the maximum size that the payload can be, after subtracting space for the IP, TCP, and other headers. So, if the MTU is 1500 bytes, and the IP and TCP headers are 20 bytes each, the MSS is 1460 bytes.

If the MSS value is less than the maximum, it may be due to a smaller IP MTU setting. For example, if the IP MTU is 200 bytes smaller to support VPN traffic, the TCP MSS will also be 200 bytes less. The diagram below illustrates the size dependency between the IPv4 MTU and the TCP MSS.

IPPROTO_IP socket options - Win32 apps | Microsoft Docs

Mar 25, 2020

MTU and MSS: Internet terms explained Jan 03, 2017 IP MTU, TCP MSS and TCP windows sizes defaults ETHERNET MTU > MPLS MTU > IP MTU > MSS Again, theoretically the MSS can be calculated with the following formula, defined in the RFC 879 MSS = MTU – IP Header (+ Options if present) – TCP Header (+ Options if present) IP Header = 20 / 60 bytes without / with Options