Digital Equipment Digital NetRider, Owner'S Manual

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Configuring and Managing SLIP Ports 15-7
Managing the Maximum Transmission Unit
Managing the Maximum Transmission Unit
Introduction
The maximum transmission unit (MTU) value specifies the maximum size of the
datagram that a given access server port accepts. The range is 64 to 1500 bytes. The
default is 1500 bytes.
Changing the MTU
You can change the MTU value using the SET/DEFINE/CHAGE PORT n MTU
command. If you use the SET or CHANGE command, the new value does not affect
an existing SLIP connection.
MTU Change Example:
The following example makes the access server compatible with attached hosts that
have fixed MTU values other than 1006 bytes:
Local> DEFINE PORT 4 SLIP MTU 500
Relationship of the TCP Maximum Segment Size and the MTU
An attached host announces the TCP maximum segment size (MSS) at connection
time. The MSS specifies the largest fragment of a datagram that the attached host is
willing to receive. The MSS is normally based on the MTU of the network connection
as recommended by RFC 879.
The MSS should always be smaller than the MTU value. The TCP and IP headers
account for the difference between the two values.
Fragmentation
When the TCP MSS announced by the SLIP host exceeds the MTU on the SLIP line,
IP fragmentation occurs. Therefore, an MSS value that greatly exceeds the MTU
causes excessive fragmentation and downgrades performance.
Setting the MTU adjusts the point at which IP fragmentation occurs when sending
datagrams. Setting the MTU also adjusts the maximum receive unit (MRU) packet
size. Since the access server considers packets larger than the MTU as framing errors,
it discards these packets.