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The TCP/IP Guide  9  TCP/IP Lower-Layer (Interface, Internet and Transport) Protocols (OSI Layers 2, 3 and 4)       9  TCP/IP Transport Layer Protocols            9  Transmission Control Protocol (TCP) and User Datagram Protocol (UDP)                 9  TCP/IP Transmission Control Protocol (TCP)                      9  TCP Message Formatting and Data Transfer

TCP Checksum Calculation and the TCP "Pseudo Header" 1 2 3 TCP Sliding Window Data Transfer and Acknowledgement Mechanics

Maximum Segment Size Selection

The selection of the MSS is based on the need to balance various competing performance and implementation issues in the transmission of data on TCP/IP networks. The main TCP standard, RFC 793, doesn't discuss MSS very much, which opened the potential for confusion on how the parameter should be used. RFC 879 was published a couple of years after the TCP standard to clarify this parameter and the issues surrounding it. Some issues with MSS are fairly mundane; for example, certain devices are limited in the amount of space they have for buffers to hold TCP segments, and therefore may wish to limit segment size to a relatively small value. In general, though, the MSS must be chosen by balancing two competing performance issues:

• Overhead Management: The TCP header takes up 20 bytes of data (or more if options are used); the IP header also uses 20 or more bytes. This means that between them a minimum of 40 bytes are needed for headers, all of which is non-data “overhead”. If we set the MSS too low, this results in very inefficient use of bandwidth. For example, suppose we set it to 40; if we did, a maximum of 50% of each segment could actually be data; the rest would just be headers. Many segment datagrams would be even worse in terms of efficiency.
  
  
• IP Fragmentation: TCP segments will be packaged into IP datagrams. As we saw in the section on IP, datagrams have their own size limit issues: the matter of the maximum transmission unit (MTU) of an underlying network. If a TCP segment is too large, it will lead to an IP datagram is too large to be sent without fragmentation. Fragmentation reduces efficiency and increases the chances of part of a TCP segment being lost, resulting in the entire segment needing to be retransmitted.

The solution to these two competing issues was to establish a default MSS for TCP that was as large as possible while avoiding fragmentation for most transmitted segments. This was computed by starting with the minimum MTU for IP networks of 576. All networks are required to be able to handle an IP datagram of this size without fragmenting. From this number, we subtract 20 bytes for the TCP header and 20 for the IP header, leaving 536 bytes. This is the standard MSS for TCP.

The selection of this value was a compromise of sorts. When this number is used, it means that most TCP segments will be sent unfragmented across an IP internetwork. However, if any TCP or IP options are used, this will cause the minimum MTU of 576 to be exceeded and fragmentation to happen. Still, it makes more sense to allow some segments to be fragmented rather than use a much smaller MSS to ensure that none are ever fragmented. If we chose, say, an MSS of 400, we would probably never have fragmentation, but we'd lower the data/header ratio from 536:40 (93% data) to 400:40 (91% data) for all segments.

TCP Checksum Calculation and the TCP "Pseudo Header" 1 2 3 TCP Sliding Window Data Transfer and Acknowledgement Mechanics