[Int-area] Comments on draft-ietf-intarea-gue-07 (and fllow-up from last IETF meeting)

[Gorry Fairhurst <[e-mail]>]

I've just looked at draft-ietf-intarea-gue-07again and have a few comments:


First, I spoke at the Mic in INTEAREA last IETF meeting, and I then had 
a few serious concerns about this draft:

(1) References. This ID is incomplete because it relies on what appears 
to be abandoned work for important details. It does not really talk 
about the security and deployment concerns, or the extensions that are 
required to implement this. Instead, it points to a set of expired 
drafts (at least one expired in 2015). I do not believe these dependent 
drafts are mature. As far as I know they have not been adopted by a 
working group and have no status - at the very least these need to be 
adopted, or appropriate parts incorporated. This is still the case.

I suggest it is bad practice to require mechanisms in a PS that are 
informative references, especially ones that are not adopted. e.g. 
draft-herbert-transportsand this needs working group review.

The security issues relating to this document seem to be partly in 
another non-working-group document.

I would expect the fate of these other documents needs to be decided 
before the present document progresses.

(2) I could not understand why this is a “PS”? I suggest that the 
proposal could be too flexible and extensible to be usefully 
standardised and demonstrated to interoperate. If this is a “PS”, I 
would like to be sure that all the functionality described is 
unambiguous, has been implemented somewhere and is really needed. There 
are many points of extensibility and likely many issues that will be 
raised as these extensions emerge. I do not grasp why we need to specify 
all these possibilities at this time, with no future use identified.

--

As David noted in his recent TSV ART Review, from a transport 
perspective, there are some more major issues. My thoughts on this 
should be probably read as additional comments following David's review:

Tunnels & Endpoints

There are concerns because of the wide applicability of various 
combinations of features. Overall, this flexibility makes it hard to 
analyse this extensible framework from a transport perspective. At some 
point I stopped trying to follow the various pathologies that can arise 
with different combinations of use (so I think there are more issues). I 
am therefore surprised that this is being proposed as a "PS", because it 
seems there are at least likely to be unknowns and very likely 
applicability concerns.

I think there are serious issues that emerge because the method lumps 
tunnels and endpoint encapsulation together. One example is that there 
is insufficient specification of how congestion collapse is to be 
avoided, when acting as an endpoint for a non-congestion controlled 
payload (section 5.9 does not address congestion control). Another is 
the mis-use of the zero checksum text in 5.7.1 (a misquotation of the 
spec. RFC1122 states in 4.1.3.4, and incompatible with RFC6935).

This is another way of doing many things, many of which are already 
specified in existing RFCs - albeit with some extra (and interesting) 
features and a lot more flexibility. The disadvantage is that by 
widening the applicability it is less clear on the implications of 
specific techniques and could itself be extended in arbitrary 
directions. I think the IETF should consider this when determining what 
to do with this document, and seek to understand why we need 
interoperability standards in this area.

Also:

* Section 3.3.1
I do not understand the operation of the paired flags. I suggest that 
some combinations can result in significant complexity - is this 
something that the WG has considered, and what do they think about this? 
e.g.:
" Flags can be paired together to allow different lengths for an
extension field. "

* I do not understand this statement:
" If a decapsulator receives a GUE packet with private data, it MUST
validate the private data appropriately. "
- How does it do that, or what does "appropriately" mean? What are the 
costs, and the issues if verification fails?

- How does the receiver know this is being done? (If we don't 
standardise it, then why would need to specify it in a RFC?)
"An implementation MAY place security data in GUE private data which if 
present MUST be verified for packet acceptance."

* Section 4:
" Variant 1 of GUE allows direct encapsulation of IPv4 and IPv6 in UDP."
- How is congestion control handled in this case, I expected text on the 
congestion safety of this approach for use in different scenarios, but 
found none.

Endpoint checksum v Tunnel Checksum

Section 4:
Variant 1: This variant appears to be a tunnel that places a packet 
directly in a UDP packet.

* Section 5.2 seems way under-specified with respect to the 
pseudo-header calculation. This could be contained in anothe ID, I did 
not check, because I suggest it needs to be in this particular document.

* Section 5.7.1:
"By default, a
decapsulator SHOULD accept UDP packets with a zero checksum. A node
MAY be configured to disallow zero checksums per [RFC1122]."

I read this is as a misquotation of the spec. RFC1122 states in 4.1.3.4 
that:

"An application MAY optionally be able to
control whether a UDP checksum will be generated, but it
MUST default to checksumming on."

- Instead, I read RFC 6935 for IPv6 explicitly stating:

"As an alternative, certain protocols that use UDP as a tunnel
encapsulation MAY enable zero-checksum mode for a specific port
(or set of ports) for sending and/or receiving. Any node
implementing zero-checksum mode MUST follow the node requirements
specified in Section 4 of "Applicability Statement for the use of
IPv6 UDP Datagrams with Zero Checksums" [RFC6936]."

- I do not yet understand how GUE can safely vary this. The text is 
insufficient.

* Section 5.9

This states

" In the case that the encapsulated traffic does not implement any or
sufficient control, or it is not known whether a transmitter will
consistently implement proper congestion control, then congestion
control at the encapsulation layer MUST be provided per [RFC5405].
Note that this case applies to a significant use case in network
virtualization in which guests run third party networking stacks
that cannot be implicitly trusted to implement conformant congestion
control."

It then states:
"Out of band mechanisms such as rate limiting, Managed Circuit
Breaker [RFC8084], or traffic isolation MAY be used to provide
rudimentary congestion control. "

This may be just lack of clarity in the text, but I think thisseems like 
a "magic trick" to escape doing congestion control.
- which may be heading in a good direction, but really does not address 
the issue. This is particularly worrying since 5.10 actually describes 
the use of the method for multicast, broadcast etc, but still has no 
explanation of how to provide prevent congestion collapse.

I think the following statement is currently flawed and needs more clarity:
"For finer-grained congestion control
that allows alternate congestion control algorithms, reaction time
within an RTT, and interaction with ECN, in-band mechanisms might be
warranted."
- I think this needs to be removed and replaced by something that is 
more specific. I'm concerned the interaction with ECN seems 
under-specified (or perhaps should be removed - or at least be replaced 
with a mechanism that has IETF consensus).

* Section 5.8.2
- I would like to see discussion on whether 5.8.2 is safe or unsafe. I 
do not know how the integrity will be managed.

"The GUE header checksum (in version 0x0) provides a UDP-lite
[RFC3828] type of checksum capability as an optional field of the
GUE header."

Endpoint - NAT

* Section 5.7
- This seems about NAT. Is this appropriate?

Fragmentation

- Elsewhere in the iRTF fragmentation has been described as fragile, why 
is it safe in this spec?

* GUE level fragmentation is mentioned, and interesting as a concept. 
However, there is so much discussion of IPv6 fragments that I think this 
needs detailed consideration by the WG. It also directly competes with 
the TSVWG work on UDP-Options, albeit the IETF can decide to do two more 
methods that both use UDP, but I'd hope that if it specified either, it 
would carefully consider the issues in accepting fragments at a receiver.

* Section 5.4:
"Note that set flags in a GUE
header that are unknown to a decapsulator MUST NOT be ignored. If a
GUE packet is received by a decapsulator with unknown flags, ..."

- Does that imply silently discarded, why not logged?


Other comments:

"If a received GUE packet in IPv6 contains a
protocol number that is an extension header (e.g. Destination
Options) then the extension header is processed after the GUE header
is processed as though the GUE header is an extension header."

* Section 3.2.2
I do not understand the intended IANA allocation method:
" Control messages will be defined in an IANA registry. Control message
types 1 through 127 may be defined in standards. "
- What is the difference between the two use below, and why are they
separately mentioned? Are these differentiated in the registry?>
" Types 128 through
255 are reserved to be user defined for experimentation or private
control messages."

* Section 3.4.
I don't understand the normative MUST, it could just be that this just a 
truism, that a receiver can not use data that it does not understand?

* Section 5.4:
"Such events MAY be logged subject to configuration and rate limiting of 
logging messages. "

- I don't understand the MAY here. I could see why "REQUIRED" or 
"RECOMMENDED" is stated for operational reasons.
- Why is rate limiting only permitted by a "MAY" - should that be 
required or recommended?

* This is another way of doing many things that are specified in 
existing RFCs - albeit with some extra features and a lot more flexibility.

Section 6.2

- This is an alternative proposal to using existing IETF specifications.
It states:
"A number of different encapsulation techniques have been proposed for
the encapsulation of one protocol over another." ...

- What follows is mainly a list of PS specifications from IETF, not 
proposals.
And states:
"Several proposals exist for encapsulating packets over UDP including
ESP over UDP [RFC3948], TCP directly over UDP [TCPUDP], VXLAN
[RFC7348], LISP [RFC6830] which encapsulates layer 3 packets,
MPLS/UDP [RFC7510], GENEVE [GENEVE], and GRE-in-UDP Encapsulation
[RFC8086]."
- Many of these are PS specifications from IETF, not proposals. If the 
WG thinks another spec is needed, this should not regard the existing PS 
as "proposals", but clearly differentiate the benefits of the new approach.

----------------

If this document is to be published, I would expect it needs significant 
changes and I would say this would certainly require a much more 
detailed transport review together with the other drafts that form a 
part of the spec.

Best wishes,

Gorry