TCP MSS adjustment

Overview

This feature adds support for adjustment of MSS of TCP packets with SYN flag according to access/aggregation network 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 AQP for the AQP rules.

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 to prevent fragmentation of upstream and downstream TCP packets using ISA-BB diverted by IPv4/IPv6 filter actions.

The following ESM host types are supported:

IPv4 and IPv6 IPoE hosts
locally terminated PPPoE hosts (without L2TP LAC)
L2TP LNS hosts

The configuration steps are as follows:

Create a NAT group used for an MSS adjustment.

MD-CLI

[ex:/configure isa]
A:admin@node-2# info nat-group 1 {
        redundancy {
            active-mda-limit 1
        }
        mda 1/2 { }
        mda 1/2 { }
    }

classic CLI

A:node-2>config>isa# info
----------------------------------------------
        nat-group 1 create
            shutdown
            active-mda-limit 1
            mda 1/1
            mda 1/2
        exit
----------------------------------------------

Associate the NAT group with a routing instance or VPRN instance, and configure the MSS value.

Note: Unless there are dedicated ISAs or ESAs for MSS adjustment, an existing NAT group or WLAN-GW group can be referenced. If multiple NAT or WLAN-GW groups reference the same ISA or ESA, the NAT or WLAN-GW groups become inactive. MSS adjustment does not function correctly if it references an inactive NAT or WLAN-GW group.

MD-CLI

[ex:/configure router "Base"]
A:admin@node-2# info
    mss-adjust {
        nat-group 1
        segment-size 1452
    }

[ex:/configure service vprn "1"]
A:admin@node-2# info
    customer "1"
    mss-adjust {
        nat-group 1
        segment-size 1452
    }

classic CLI

A:node-2>config>router# info
----------------------------------------------
#--------------------------------------------------
echo "IP Configuration"
#--------------------------------------------------
...
        mss-adjust-group 1 segment-size 1452
----------------------------------------------

A:node-2>config>service>vprn$ info
----------------------------------------------
            no shutdown
            mss-adjust-group 1 segment-size 1452
----------------------------------------------

Create an IPv4 or IPv6 filter to perform an MSS adjust.

MD-CLI

[ex:/configure filter ip-filter "1"]
A:admin@node-2# info
    entry 1 {
        egress-pbr true
        match {
            protocol tcp
            tcp-flags {
                syn true
            }
        }
        action {
            tcp-mss-adjust
        }
    }

[ex:/configure filter ipv6-filter "1"]
A:admin@node-2# info
    entry 1 {
        match {
            next-header tcp
            tcp-flags {
                syn true
            }
        }
        action {
            tcp-mss-adjust
        }
    }

classic CLI

A:node-2>config>filter>ip-filter# info
----------------------------------------------
            entry 1 create
                match protocol tcp
                    tcp-syn true
                exit
                action
                    tcp-mss-adjust
                exit
                egress-pbr default-load-balancing
            exit
----------------------------------------------
A:node-2>config>filter>ipv6-filter# info
----------------------------------------------
            entry 1 create
                match next-header tcp
                    tcp-syn true
                exit
                action
                    tcp-mss-adjust
                exit
            exit
----------------------------------------------

Apply an IPv4/IPv6 filter to the SLA profile.

TCP MSS adjustment for NAT services

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

Create a NAT group used for NAT services with MSS adjustment.

MD-CLI

[ex:/configure isa]
A:admin@node-2# info nat-group 1 {
        redundancy {
            active-mda-limit 2
        }
        mda 1/2 { }
        mda 1/2 { }
    }

classic CLI

A:node-2>config>isa# info
----------------------------------------------
        nat-group 1 create
            shutdown
            active-mda-limit 1
            mda 1/1
            mda 1/2
        exit
----------------------------------------------

Create a NAT policy that also adjusts MSS.

MD-CLI

[ex:/configure service nat]
A:admin@node-2# info
...
    nat-policy "policy-for-mss-adjust" {
        tcp {
            mss-adjust 1452
        }
    }

classic CLI

A:node-2>config>service>nat# info
----------------------------------------------
            nat-policy "policy-for-mss-adjust" create
                tcp-mss-adjust 1452
            exit
----------------------------------------------