8. TCP MSS Adjustment

8.1. In This Section

This section describes how to configure TCP MSS adjustments.

Topics include:

  1. Overview
  2. TCP MSS Adjustment for ESM Hosts
  3. TCP MSS Adjustment for NAT Services

8.2. 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 are supported:

  1. TCP MSS Adjustment for ESM Hosts
  2. TCP MSS Adjustment for NAT Services

8.3. 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.

8.4. 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.
    configure>service>nat
       nat-policy "policy-for-mss-adjust" crate
          tcp-mss-adjust 1452

8.5. TCC MSS Adjustment Commands

8.5.1. Command Hierarchy

config
— filter
— ip-filter
— entry
— action
— [no] tcp-mss-adjust
— filter
— ipv6-filter
— entry
— action
— [no] tcp-mss-adjust
config
— router
— mss-adjust-group nat-group-id segment-size segment-size
config
— service
— vprn
— mss-adjust-group nat-group-id segment-size segment-size
config
— service
— nat
— policy
— tcp-mss-adjust segment-size
— no tcp-mss-adjust

8.5.1.1. TCC MSS Adjustment Command Descriptions

mss-adjust-group

Syntax 
mss-adjust-group nat-group-id segment-size segment-size
no mss-adjust-group
Context 
config>router
config>service>vprn
Description 

This command associate the MSS adjust group consisting of multiple ISAs with the routing context in which the application requiring TCP MSS adjust resides.

Default 

None

Parameters 
nat-group-id—
specifies the NAT group used for a TCP MSS adjustment
segment-size—
specifies the value to put into the TCP Maximum Segment Size (MSS) option if not already present, or if the present value is higher
Values—
160 to 10240

tcp-mss-adjust

Syntax 
tcp-mss-adjust
tcp-mss-adjust segment-size
no tcp-mss-adjust
Context 
config>filter>ip-filter>entry
config>filter>ipv6-filter>entry
config>service>nat>nat-policy
Description 

This command configures the Maximum Segment Size (MSS) adjustment for TCP packets.

The no form of the command disables adjusting tcp-mss values.

Default 

None

Parameters 
segment-size—
Specifies the segment size. Applicable only in the config>service>nat>nat-policy context.
Values—
160 to 10240