9. TCP MSS Adjustment

9.1. Overview

This feature adds support for adjustment of MSS of TCP packets with SYN flag according to access/aggregation network in order to prevent fragmentation of upstream and downstream TCP packets using ISA-BB.

There are two modes of adjustment operations supported: TCP MSS Adjustment for ESM Hosts, and TCP MSS Adjustment for NAT Services.

For TCP MSS adjust using ISA2-AA, see section Application QoS Policy (AQP) for the AQP rules.

9.2. TCP MSS Adjustment for ESM Hosts

This feature adds support for adjustment of the MSS size of TCP packets with SYN flag according to the access/aggregation network in order to prevent fragmentation of upstream and downstream TCP packets using ISA-BB diverted by IP/IPv6 filter actions.

The following ESM host types are supported:

  1. IPv4/IPv6 IPoE hosts
  2. Locally terminated PPPoE hosts (without L2TP LAC)
  3. L2TP LNS hosts

Configuration steps

  1. Create a NAT group used for an MSS adjustment.
    config>isa
    nat-group 1
    active-mda-limit 2
    mda 1/1
    mda 1/2
  2. Associate the NAT group with a routing instance and configure the MSS value.
    config>router
    config>service>vprn
    mss-adjust-group 1 segment-size 1452
  3. Create an IPv4/IPv6 filter to perform an MSS adjust.
    config>filter>ip-filter>entry
    egress-pbr default-load-balancing
    match tcp-syn
    action tcp-mss-adjust
    config>filter>ipv6-filter>entry
    match tcp-syn
    action tcp-mss-adjust
  4. Apply an IPv4/IPv6 filter to the SLA profile.

9.3. TCP MSS Adjustment for NAT Services

This feature provides MSS adjustment for TCP packets to be translated by NAT services.

Configuration steps

  1. Create a NAT-group used for NAT services with MSS adjustment.
    config>isa
       nat-group 1
          active-mda-limit 2
          mda 1/1
          mda 1/2
  2. Create a NAT-policy that also adjusts MSS.
    config>service>nat
       nat-policy "policy-for-mss-adjust" crate
          tcp-mss-adjust 1452