Configuring The Tcp Mss Value Of Tcp Packets - Cisco ASR 9000 Series Configuration Manual

Configuring Subscriber Features
reading the MSS option, and adjusting the value so that the server does not send packets larger than the
configured size (plus headers).
Note that the TCP MSS value is only adjusted downward. If the clients request an MSS value lower than the
configured value, then no action is taken.
In the case of PPPoE, an extra 8 bytes and in the case of L2TP, an extra 40 bytes is added to the packet. The
recommended MSS adjust values are 1452 for PPPoE, and 1420 for L2TP scenarios, assuming a minimum
MTU of 1500 end-to-end.
Separate unique global values for PTA and L2TP are supported, which once configured allows all future
sessions to be TCP MSS adjustment; however, the sessions already established will not be TCP adjusted. If
the global value is changed, then all new TCP subscriber sessions, will get the new global value.
For more information about configuring the TCP MSS value of packets, see
of TCP Packets, on page
To disable this on a session, you must first disable the global configuration, then delete the session and
Note
recreate it.
TCP encapsulated in both IPv4 and IPv6 are supported.
Restrictions
These restrictions are applicable for TCP MSS Adjustment:
• Because the MSS is TCP-specific, the TCP MSS Adjustment feature is applicable only to (transit) TCP
• TCP MSS Adjustment configuration affects only the PPPoE PTA and LAC sessions types. It does not
• The MSS option must be the first option in the TCP header.
• The router uses the MSS value that the user configures for checking TCP/IPV4 packets. When checking

Configuring the TCP MSS Value of TCP Packets

Perform this task to configure the TCP MSS value of TCP packets in order to prevent TCP sessions from
being dropped.
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packets and the UDP packets are unaffected.
affect IP sessions or any non-BNG interfaces.
TCP/IPv6 packets, the router automatically adjusts the configured MSS value down by 20 bytes to
account for the larger IPv6 header. For example, if the TCP MSS value is configured to 1450, then the
router adjusts the TCP MSS in an IPV4 packet down to 1450 and down to 1430 for an IPv6 packet.
Cisco ASR 9000 Series Aggregation Services Router Broadband Network Gateway Configuration Guide, Release
Configuring the TCP MSS Value of TCP Packets
Configuring the TCP MSS Value
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