Subject: Questions on Configured Tunnel MTU and TOS Byte Settings

From: Roy Brabson <[email-masked]>
Date: Mon, 11 Nov 2002 09:44:27 -0500

I've got a few of questions on configured tunnels, as described in 
draft-ietf-ngtrans-mech-v2-01.txt. 

- Section 3.2 discusses how to set the tunnel MTU.  It covers the case 
  where the tunnel MTU size is manually configured, with a default of 
  1280.  While it discusses capping the tunnel MTU at 4400 when IPv4 
  path MTU discovery is used, it doesn't discuss a maximum configured
  value for the tunnel.  Does this mean there is no cap when manually 
  configuring the tunnel MTU (i.e., the configured tunnel MTU may be as 
  large as 65515, or even larger if jumbograms are used?)

- The same section does not discuss what a host should do if it receives a 

  Router Advertisement with an MTU option.  Should the MTU value received 
  be used?  If so, is there a cap associated with this MTU value?  In 
  other words, if it exceeds 4400 bytes should the value be used?

- In section 3.5, the TOS byte is defined as being set to 0 unless 
  otherwise specified.  What exactly does this mean?  That, if RFC 2893 is 

  followed the DSCP in the TOS byte may be set to a non-zero value?  Or 
  that RFC 2893 and RFC 3168 should explicitly NOT be implemented for 
  configured tunnels?  If the latter, I think some discussion on exactly
  WHY these two RFCs are not to be implemented would be helpful.

- In section 3.6, the TOS byte of the inner packet is left unmodified at 
  the tunnel egress.  This seems to contradict some of the referenced. 
  For instance, RFC 3168 defines both limited-functionality and 
  full-functionality support for ECN support over tunnels.  For 
  limited-functionality, which seems to most closely match what is 
  described in this draft, it discusses what to do at the tunnel egress if 

  the CE option is set in the outer packet header but not the inner packet 

  header.  This processing does not seem to match what is described in 
  this draft.  Is this intentional?

Roy
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