Amazon EC2 m3 instances MTU problems

If your transfers from outside the EC2 cloud to the new m3 instances fail, then it might be due to jumbo frames configured by default on this new type of instance.

I have an external machine which also uses jumbo frames and during the tcp handshake the machines advertise maximum segment sizes (MSS) of 8960 and 8961bytes respectively.

I’m not really sure if this is Amazon’s fault or a misconfiguration on my side but the fix is to lower the MTU back to 1500 on the instance. Sure, intra-area transfers might be slower but at least I can copy stuff from outside.

ip link set dev eth0 mtu 1500

2 Comments

  1. I found this today, after trying to work out why apparently modern windows boxes were advertising a maximum send window size of 64k when UPLOADING data to AWS.
    Needless to say, 64k limits the upload speed unless there is very low latency.

    The tcptrace shows 10% “rexmit data pkts” and a requested MSS of 8961.
    The client advertises a *maximum* win advertised of 65280 bytes, and a mss requested of 1460.

    I’m thinking that perhaps the MSS requested by the AWS server is the cause of the 65k maximum send window. Windows 8.1+ is capable of adapting and scaling window sizes when receiving data it makes no sense to me that it would cap it for sending data.

  2. I don’t have the setup up and running anymore but at some point I thought of some firewall somewhere filtering out ICMP path mtu discovery.

Leave a Reply

Your email address will not be published. Required fields are marked *