[Discussion] MTU mismatch and impact of data-plane traffic

Mohamed Kamal mkamal at noor.net
Tue Oct 27 10:31:54 UTC 2015

Suppose you have the below network topology, where PE is connected to 
P1, P1 is connected to P2 and P2 is connected to GW, all through 1G links.


The numbers represent the MTU values configured in the following order; 
PE's egress interface to P1, P1 ingress interface, P1 egress interface, 
P2 ingress, P2 egress and eventually GW ingress.

Q1: What do you think would be the impact in terms of data-plane traffic 
(HTTP/s browsing, Video streaming etc), traversing this network, in the 
direction from the Internet and going to the PE router?

My answer is:

If there is a client running Win7 on a machine trying to access a web 
server out there, the TCP MSS would be adjusted to around 1260-1460 
bytes depending on the Operating System's MTU value. Hypothetically, the 
first packet from the web server destined to the client would be 
1460-bytes and will reach the ingress interface of the GW.

The GW would receive it in the input_buffer of the ingress interface, 
strip off the Ethernet header, and move it to the output buffer of the 
egress interface whose MTU is 1600. Since the largest MSS is 1460, and 
there is always a one-to-one mapping between segments received from the 
TCP module and the packets constructed in the IP module, I believe that 
the largest IP packet would be 1480. GW would cram the Ethernet frame 
with the 1480-bytes of IP payload data and send it to the P2, which 
would in the other end, pass it on its way.

Q2: However, what about larger MSS sizes? example; above 1500? and 
larges chunks of payload from a connectionless protocols that don't 
exchange MSS? UDP for example? or Google's QUIC (which is HTTP over UDP)?

Mohamed Kamal

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