Lesson plan
Maximum segment size
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). 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. Though there is no minimum required MSS defined in IETF RFCs, there is a minimum MTU, and so a default MSS is calculated by subtracting the minimum IP and TCP header sizes from the MTU. For example, IPv4 hosts typically default to an MSS of 536 octets (= 576 − 20 − 20) and IPv6 hosts generally are able to handle an MSS of 1220 octets (= 1280 − 40 − 20). Small MSS values will reduce or eliminate IP fragmentation but will result in higher overhead. Each direction of data flow can use a different MSS. For most computer users, the MSS option is established by the operating system.…
Article summary
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). 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. Though there is no minimum required MSS defined in IETF RFCs, there is a minimum MTU, and so a default MSS is calculated by subtracting the minimum IP and TCP header sizes from the MTU. For example, IPv4 hosts typically default to an MSS of 536 octets (= 576 − 20 − 20) and IPv6 hosts generally are able to handle an MSS of 1220 octets (= 1280 − 40 − 20). Small MSS values will reduce or eliminate IP fragmentation but will result in higher overhead. Each direction of data flow can use a different MSS. For most computer users, the MSS option is established by the operating system.…
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Discussion questions
- Summarise the article in your own words. Which single claim would you verify first, and why?
- Which claim in this article is most likely to be revised as the sources update? How would you check its current status?
- If two sources disagreed about one of these facts, what would you need in order to trust one over the other?
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