This document provides a sample configuration for adjusting the TCP Maximim Segmet Size (MSS) on the ASR9000 routers.
TCP MSS adjustment is required when the network operators want to ensure that the TCP packets traversing the network will not exceed a predetermined size, thus preventing any unnecessary fragmentation of TCP packets on links with small MTU. Typically this may happen on links with the MTU of 1500 bytes, when the original IP/TCP packet is encapsulated into PPPoE, GRE or any other encapsulation.
Such modification of TCP MSS size doesn't cause any observable impact on TCP session throughput because the throughput of a TCP session is predominantly dictated by the TCP window size.
When TCP sessions establish, they signal 2 key parameters that dictate the transfer speeds of the session. That is the MSS, the maximum size of a TCP packet and the Window size. In TCP transmission, in its basic operation is to be acknowledged. So packet transferred, receive ACK and send the next packet. This is very slow especially on long end to end delays. Windowing allows for multiple packets to be sent and using a single ACK on that window. If one packet is lost of that window, the whole window is to be retransmitted. The larger the window size is, the better for thruput, but on unreliable links, this can have an adverse affect if large windows have to be retransmitted. Also large windows require larger receive buffers on the receiving side.
So the size of the packet is dictated by the MSS. The number of packets at that size to be sent without direct ACK on each packet is the window size.
What does MSS adjust do?
MSS is the value that a TCP client signals to a server indicating what the maximum size of a packet is that it can receive. Generally this value is derived from the client's local MTU size minus 40 bytes. These 40 are composed of 20 bytes for ip header and 20 bytes for the tcp header.
The segment size refers to the actual TCP payload carried.
If there are mtu changes in the network, which can easily be seen for instance with PPPoE, which adds 8 bytes of additional header, it could lead to a client with a local MTU of 1500, signaling a TCP mss of 1460, but actually requiring 1508 bytes on a transmission line (1460 + 20 ip + 20 tcp + 8 pppoe). This will cause some hops to do fragmentation.
The TCP MSS adjust feature intercepts a TCP syn packet and scans for the first option after the tcp header. If that option is the mss size, it can re-adjust the value to the desired value configured and update the checksum along with it.
Basic understanding of the ASR9000 architecture and IOS-XR operating system.
Feature is supported on Typhoon and Tomahawk line card generations.
Feature is supported on BNG subscriber interfaces, on L3 dot1q sub-interfaces and on GRE tunnels. (You can't configure the ipv4 mss enable on main interfaces).
Range of supported sizes for TCP MSS adjustment is from 1280 to 1535 bytes.
UDP packets are obviously not subject to MSS rewrites, only the TCP SYN packet is intercepted and rewritten.
The implementation of TCP MSS adjust on ASR9000 was dictated by two factors:
minimal performance impact when applied to an interface
performance doesn't change with deployment scale (i.e. number of interfaces where the feature is applied)
To meet these requirements, the chosen implementation requires two configuration steps:
specify the TCP MSS size at the network processor (NP) level.
specify the interfaces on which the configured TCP MSS size should be applied
One consequence of this approach is that all interfaces on a given NP must use the identical value for TCP MSS adjustments.
When enabled on an interface, TCP MSS feature is applies in both directions. i.e. adjustment is applied on ingress and egress packets. Feature is applied to both IPv4 and IPv6 even if only one of them is configured.
To determine which NP controls the interface in question, use the show controllers np ports exec command.
For BNG which first supported the mss-adjust capability an different configuration can be used:
subscriber pta tcp mss-adjust 1410 !
The value range for the BNG command is 1280-1536. The reason for that is that as you can see we only adjust for the higher order byte to save some space in the ucode variable.
Fragmentation can be identified by looking at the NP counters.
Note that for BNG and non BNG the commands and counters are different.
The reason for that is that Fragmentation is handled by NETIO for regular L3 interfaces. However BNG subscriber interfaces don't register with NETIO. This was done for scalability reasons since BNG subs don't require all the services that NETIO provides. For BNG subs, a "lite-weight" NETIO was designed called SINT (subscriber interface) that provides some basic L3 services such as access to the TCP stack, some basic ICMP support etc.
Verifying BNG subscriber fragmentation:
Looking at fragmentation at the SPP (the interupt switching path) level.
RP/0/RSP0/CPU0:A9K-BNG#show spp node-counters | i frag Sun Mar 8 11:08:17.501 EST ipv4_frag Drop: Dont-fragment set: 3125 <<<< packets that have DF bit set ipv4-frag: 3854 <<<<< packets fragmented
Verifying the NP counters with show controller np counters NP<x> location 0/<y>/CPU0 16 MDF_TX_LC_CPU 11037 107 <<<<<<<<<< 100pps to the LC CPU 17 MDF_TX_WIRE 17423 201 <<<<< 200 packets to the wire (2 frags) 21 MDF_TX_FABRIC 24798 299 <<<<<<<<<<<200 pps injected from the fab 41 PARSE_INJ_RECEIVE_CNT 17079 201 <<<<<< 200pps injected 45 PARSE_ENET_RECEIVE_CNT 8969 101 <<<<received on wire from the tester 416 RSV_PUNT_IP_MTU_EXCEEDED 9615 99 <<<<<100pps requiring frag
with the same command show controller np counters NP<x> loc 0/<y>/cpu0 the following can be observed for fragmentation:
16 MDF_TX_LC_CPU 718 106 << sent to the LC CPU for frag 33 PARSE_FAB_RECEIVE_CNT 681 100 << packets received from the fabric 416 RSV_PUNT_IP_MTU_EXCEEDED 677 100 << packets needing punt because of MTU 842 IPV4_FRAG_NEEDED_PUNT 677 100 << packets punted for FRAG reasons
Note that in both cases the DF bit is NOT assessed in the hardware, this is handled by the controlling agent, whether it be SPP for BNG subs or NETIO for regular L3 interfaces.
In this example the TCP MSS adjust is applied to interface Bundle-Ether48.10.
Step 1: Determine the NP on which the TCP MSS adjust is to be configured:
RP/0/RSP0/CPU0:av-asr9001#sh bundle bundle-ether 48
Port Device State Port ID B/W, kbps
-------------------- --------------- ----------- -------------- ----------
Gi0/0/1/6 Local Active 0x8000, 0x0003 1000000
Link is Active
Gi0/0/1/7 Local Active 0x8000, 0x0004 1000000
Link is Active
RP/0/RSP0/CPU0:av-asr9001#show controllers np ports all location 0/0/CPU0
NP Bridge Fia Ports
-- ------ --- ---------------------------------------------------
0 0 0
0 0 0 TenGigE0/0/2/0, TenGigE0/0/2/1
1 1 1 GigabitEthernet0/0/1/0 - GigabitEthernet0/0/1/19
1 1 1 TenGigE0/0/2/2, TenGigE0/0/2/3
Step 2: Configure the desired TCP MSS value and activate the feature on the interface Bundle-Ether48.10.
To verify that the feature is enabled on a selected interface, check the microcode interface descriptor block (uidb) setting:
RP/0/RSP0/CPU0:asr9001#sh uidb data location 0/0/CPU0 Bundle-Ether 48.10 ing-extension | i TCP
TCP MSS ADJ Enable 0x1
RP/0/RSP0/CPU0:asr9001#sh uidb data location 0/0/CPU0 Bundle-Ether 48.10 extension | i TCP
TCP MSS ADJ Enable 0x1
You can verify the rewrite with a simple TCP session on an IOS router connected to the device that is doing the MSS rewrite.
Enable debug tcp transactions and packet.
Feb 6 14:17:26.112: tcp0: I LISTEN 184.108.40.206:1234 220.127.116.11:23 seq 0 OPTS 4 SYN WIN 0 Feb 6 14:17:26.112: TCP0: state was LISTEN -> SYNRCVD [23 -> 18.104.22.168(1234)] Feb 6 14:17:26.112: TCP0: tcb D333A2EC connection to 22.214.171.124:1234, received MSS 1300, MSS is 516
126.96.36.199 is the source and 188.8.131.52 is the destination. The debug is taken from 184.108.40.206
You can see that the decoded MSS value received is 1300 as per configuration on the asr9000 which sits in between client and destination. Our local MSS value is 516 which we advertise. This is merely to illustrate a possible verification of the MSS rewrite rather then focusing on the actual values.
It works similar in case of GRE Tunnel:
enable under the tunnel-ip the command 'ipv4 tcp-mss-adjust enable'
locate the exit interface the tunnel uses ((whether it be before -> traffic going into the tunnel, or after decap -> coming out of the tunnel) and then apply the corresponding 'hw-module location <> tcp-mss-adjust np X value <number>'
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