Editing Maximum segment size (section)

==Inter-Layer Communication==

<!-- This is the description taken from the obsolete RFC879, which describes only IPv4. This should be rewritten from RFC 9293 sec. 3.7.1. -->
In order to notify MSS to the other end, an inter-layer communication is done as follows:{{Ref RFC|879|rsection=11}}
* The Network Driver (ND) or interface should know the [[Maximum transmission unit|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:{{Ref RFC|879|rsection=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.