I've reviewed this document as part of the transport area
directorate's ongoing effort to review key IETF documents. These
comments were written primarily for the transport area directors, but
are copied to the document's authors for their information and to
allow them to address any issues raised. The authors should consider
this review together with any other last-call comments they receive.
Please always CC tsv-dir(_at_)ietf(_dot_)org if you reply to or forward this
review.
In summary, this draft seems to be on the right track but has some
open issues, as discussed below.
The NAT keep-alive interval (Section 5.3 and 7) is set to 110 seconds
by default, unless configured by the home agent in a binding
acknowledgement. While the default interval seems a safe choice, it's
not clear why it was chosen, or when it's appropriate for a home
agent to signal an alternative interval. The draft would benefit from
an explanation of this choice, and further discussion of the issues
inherent in choosing an appropriate keep-alive interval (Section 3.5
of RFC 5405 considers this point, and might be appropriate to
reference).
The NAT detection mechanism specified in Section 5.2 relies on the
NAT rewriting the source address of the outer IPv4 header, but not
the IPv4 address carried in the binding update payload. I'm not
familiar with the way in which mobile IPv6 uses IPsec ESP, but if
it's used in a way that provides integrity protection but not
confidentiality, issues may arise with NAT devices that rewrite IPv4
addresses in the UDP payload, causing the integrity check to fail
(this is why STUN [RFC5389] provides an XOR-MAPPED-ADDRESS attribute
to obfuscate the IP address).
It's not clear that the use with ECN is fully specified. Section
5.1.1 notes that it's "RECOMMENDED that the ECN information is copied
between the inner and outer header as defined in [RFC3168]" - is the
intent that the full-functionality option defined in RFC 3168 is
recommended? If so, it would be useful to state that explicitly. It
also seems that UDP encapsulation will prevent the use of ECN over
the tunnel, since the UDP API doesn't allow access to the ECN bits,
and cause different ECN (and hence congestion) behaviour depending on
the encapsulation method used. This should be called out in the draft.
Processing of other IP header fields (e.g. the diffserv field and
TTL) when encapsulated in UDP tunnels also seems poorly defined. The
intent, presumably, is for the choice of tunnelling mechanism to be
hidden from higher-levels, but the exact en-/de-capsulation steps
needed to achieve this must be specified for the UDP encapsulation.
For example, when is the TTL of the encapsulated packet decremented?
Finally, the use of multiple UDP tunnelling schemes (vanilla or with
TLV-header) seems undesirable, and it would be useful to document why
this is necessary, for those not familiar with the working group
history.
Regards,
--
Colin Perkins
http://csperkins.org/
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