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Questions on Configured Tunnel MTU and TOS Byte Settings
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