Subject: [Softwires] TCP MSS clamping to try to deal with MTU issues in Dual-Stack Lite

From: Magnus Westerlund <[email-masked]>
Date: Fri, 27 Mar 2009 10:15:46 -0700

Hi,

There is a proposal to use TCP MSS clamping to deal with MTU issues that
comes from Dual-stack lite's tunnel encapsulation.

I think it would be good if TCPM could provide some feedback on this
proposal.

The relevant document and section:
http://tools.ietf.org/html/draft-ietf-softwire-dual-stack-lite-00

7.4. MTU

   Using an encapsulation (IP in IP or L2TP) to carry IPv4 traffic over
   IPv6 will reduce the effective MTU of the datagrams.  Unfortunately,
   path MTU discovery is not a reliable method to deal with this.  As
   such a combination of solutions is suggested:

   o  For TCP traffic, let the carrier-grade NAT rewrite the MSS in the
      first SYN packet to a lower value.

   o  For non-TCP traffic, perform fragmentation and reassembly over the
      tunnel between the home gateway and the carrier grade NAT.  In
      practice, this means put the IPv4 packet into a large IPv6 packet
      and fragment/reassemble the IPv6 packet at each endpoint of the
      tunnel.  There is a performance price to pay for this.
      Fragmentation is not very expensive, but reassembly can be,
      especially on the carrier-grade NAT that would have to keep track
      of a lot of flows.  However, such a carrier-grade NAT would only
      have to perform reassembly for large UDP packets sourced by
      customers, not for large UDP packets received by customers.  In
      other words, streaming video to a customer would not have a
      significant impact on the performance of the carrier-grade NAT,
      but will require more work on the home gateway side.

Cheers

Magnus Westerlund

IETF Transport Area Director & TSVWG Chair
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