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Unformatted text preview: ytes for the
TCP header and 20 bytes for the IP header.
Some texts refer to this as a "negotiated" option. It is not negotiated in any way. When a
connection is established, each end has the option of announcing the MSS it expects to receive.
(An MSS option can only appear in a SYN segment.) If one end does not receive an MSS
option from the other end, a default of 536 bytes is assumed. (This default allows for a 20-byte
IP header and a 20-byte TCP header to fit into a 576-byte IP datagram.)
In general, the larger the MSS the better, until fragmentation occurs. (This may not always be
true. See Figures 24.3 and 24.4 for a counterexample.) A larger segment size allows more data
to be sent in each segment, amortizing the cost of the IP and TCP headers. When TCP sends a file:///D|/Documents%20and%20Settings/bigini/Docu...homenet2run/tcpip/tcp-ip-illustrated/tcp_conn.htm (8 of 37) [12/09/2001 14.47.16] Chapter 18. TCP Connection Establishment and Termination SYN segment, either because a local application wants to initiate a connection, or when a
connection request is received from another host, it can send an MSS value up to the outgoing
interface's MTU, minus the size of the fixed TCP and IP headers. For an Ethernet this implies
an MSS of up to 1460 bytes. Using IEEE 802.3 encapsulation (Section 2.2), the MSS could go
up to 1452 bytes.
The values of 1024 that we've seen in this chapter, for connections involving BSD/386 and
SVR4, are because many BSD implementations require the MSS to be a multiple of 512. Other
systems, such as SunOS 4.1.3, Solaris 2.2, and AIX 3.2.2, all announce an MSS of 1460 when
both ends are on a local Ethernet. Measurements in [Mogul 1993] show how an MSS of 1460
provides better performance on an Ethernet than an MSS of 1024.
If the destination IP address is "nonlocal," the MSS normally defaults to 536. While it's easy to
say that a destination whose IP address has the same network ID and the same subnet ID as
ours is local,...
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This test prep was uploaded on 04/04/2014 for the course ECE EL5373 taught by Professor Guoyang during the Spring '12 term at NYU Poly.
- Spring '12