Subject: Ambiguous Terminology (Re: Questions / comments regarding draft-voyer-6man-extension-header-insertion (was: Proposed revised Extension Header text for rfc2460bis))

From: Mark Smith <[email-masked]>
Date: Sat, 6 May 2017 10:31:18 +1000

Reading through various emails, I think there may be some confusion
about terminology in regards to "EH insertion".

This definitely needs to be covered in this draft. If there is
ambiguity, there is possibility for misunderstanding.

My definition of "EH insertion" is the following action, because
"insertion" is a variation of "in" or "into"

EH Insertion:

-  Pre-insertion, prefixed with SA to indicate originated by the
device in the SA field.

[SA. IPv6 Hdr][SA. IPv6 EHs][SA. IPv6 Payload]

-  Post-insertion, where the SR EH has been inserted into the existing
IPv6 packet

[SA. IPv6 Hdr][SRH EH][SA. IPv6 EHs][SA. IPv6 Payload]

The alternative is to add an EH via IPv6-in-IPv6 encapsulation, so
this could be given the term "EH addition" or "EH addition via
encapsulation" (the latter probably better because it is even more
explicit)

EH Addition via Encapsulation:

- Pre-addtion

[SA. IPv6 Hdr][SA. IPv6 EHs][SA. IPv6 Payload]

- Post-addtion,

[New IPv6 Hdr][SRH EH]([SA. IPv6 Hdr][SA. IPv6 EHs][SA. IPv6 Payload])

The main key difference between them is in "EH insertion", while the
inserted EH is present, the source address field in the [SA. IPv6 Hdr]
is wrong - the device recorded is not the source of all of the
information in the packet. This is what causes PMTUD, AH, ICMP etc. to
fail. This operation of EH insertion is what is not described by
RFC2460 and its ancestors and is what receiving IPv6 nodes are not
expecting. They expect that if a packet arrives, the SA field
identifies the device that originated the whole received packet.

"EH Addition via Encapsulation" preserves the semantics of the
original SA while the EH was added. The addition of the EH is entirely
transparent to the original IPv6 packet and its originating or
receiving hosts, as there is no modification of the original IPv6
packet. PMTUD, AH, ICMP won't fail in unexpected ways, because the
device that added the EH is identified in the new outer IPv6 packet
header's Source Address field.

The optional SRH Ingress Node TLV would capture the SRH header source
device information, however in addition to being optional, that source
device information is not recorded in a place where protocols and
methods such as PMTUD can use it.

The other point of note is that hosts "inserting EHs" when they
originate the IPv6 packet is not a case to support routers doing it
while the IPv6 packet is in-flight. The host "inserting" the EH is the
one that originates the IPv6 packet, and is the one that is identified
in the SA field. It is an operation described by RC2460 and its
ancestors, and if it triggers e.g., PMTUD, then the ICMP PTB will be
sent back to the correct packet source host.

Regards,
Mark.