R2511-HP MSR Router Series Layer 3 - IP Services Configuration Guide(V5)

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• If you configure a TCP MSS on an interface, the size of each TCP segment received or sent on the
interface cannot exceed the MSS value.
• This configuration takes effect only for TCP connections that are established after the configuration
rather than the TCP connections that already exist.
• This configuration is effective only for IP packets. If MPLS is enabled on the interface, do not to
configure the TCP MSS on the interface.
To configure TCP MSS of the interface:
Step Command Remarks
1. Enter system view.
system-view N/A
2. Enter interface view.
interface interface-type
interface-number
N/A
3. Configure the TCP MSS of the
interface.
tcp mss value
Optional.
The TCP MSS is 1460 bytes by
default.
Configuring TCP path MTU discovery
CAUTION:
A
ll devices on the TCP path must be enabled to send ICMP error messa
g
es by usin
g
the ip unreachables
enable command.
TCP path MTU discovery (in RFC 1191) discovers the path MTU between the source and destination ends
of a TCP connection. It works as follows:
1. A TCP source device sends a packet with the Don't Fragment (DF) bit set.
2. A router that fails to forward the packet because it exceeds the MTU on the outgoing interface
discards the packet and returns an ICMP error message, which contains the MTU of the outgoing
interface.
3. Upon receiving the ICMP message, the TCP source device calculates the current path MTU of the
TCP connection.
4. The TCP source device sends subsequent TCP segments that each are smaller than the MSS (MSS
=path MTU–IP header length–TCP header length).
NOTE:
• If the TCP source device still receives ICMP error messa
g
es when the MSS is smaller than 32 bytes, the
TCP source device fragments packets.
• An ICMP error message received from a router that does not support RFC 1191 has the MTU of the
outgoing interface set to 0. Upon receiving the ICMP message, the TCP source device selects the path
MTU smaller than the current path MTU from the MTU table as described in RFC 1191 to calculate the
TCP MSS. The MTU table contains MTUs of 68, 296, 508, 1006, 1280, 1492, 2002, 4352, 8166, 17914,
32000, and 65535 bytes. Because the minimum TCP MSS specified by the system is 32 bytes, the actual
minimum MTU is 72 bytes.
After you enable TCP path MTU discovery, all new TCP connections detect the path MTU. The device uses
the path MTU to calculate the MSS to avoid IP fragmentation.