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

Tom Herbert <tom@herbertland.com> Mon, 11 March 2019 18:01 UTC

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From: Tom Herbert <tom@herbertland.com>
Date: Mon, 11 Mar 2019 11:01:27 -0700
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To: Gorry Fairhurst <gorry@erg.abdn.ac.uk>
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Subject: Re: [Int-area] Comments on draft-ietf-intarea-gue-07 (and fllow-up from last IETF meeting)
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Hi Gorry,

Thanks for the comments. Some replies inline.

On Mon, Mar 11, 2019 at 2:40 AM Gorry Fairhurst <gorry@erg.abdn.ac.uk> wrote:
>
>
> 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.
>
The [GUEEXTEN] reference is incorrect in the version 7. This should
refer to draft-ietf-intarea-gue-extensions-06 which is a WG item.

> 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.
>
As I said, reference is incorrect. It will be fixed in next version.

> The security issues relating to this document seem to be partly in
> another non-working-group document.
>
draft-ietf-intarea-gue-extensions-06 defines security options and
security payload transforms (i.e. DTLS).

> I would expect the fate of these other documents needs to be decided
> before the present document progresses.
>
Right, all normative references are RFCs or the WG item.

> (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.

Again see draft-ietf-intarea-gue-extensions-06. That defines eight
initial options. The protocol is extensible, however beyond the
initial set of options I would expect at most one new option to be
introduced every couple of years. Note this is markedly different from
a protocol like Geneve that has a 24 bit TLV space allowing thousands
of option (including proprietary options),and as AFAICT haven't
proposed a single one as standard.

GUE is implemented and in deployment. It has been in upstream Linux since 2014.

>
> --
>
> 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.
>
This is a very general comment. Can you give a specific pathological
case not properly handled by the protocol?

> 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).
>

Tunnels and encapsulation are separate concepts in the draft. Section
5.1 defines network tunnels. We can add definition in the terminology
section of 1.2.

Exactly where is the draft misquoting RFC1122?

Exactly how is this incompatible with RFC6935 (note David's point was
that the protocol needs to show how requirements of RFC6935 and
RFC6936 are met).

> 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.
>
Again, a very general comment for which it is hard to provide a
specific response. GUE has been discussed in depth in two working
groups and in being run in the wild for a while. For instance, if this
is just "another way of doing many things, many of which are already
specified in existing RFCs", then my question is what exactly are
those RFCs? Note that section 6 gives a comparison with similar
protocols and describes differentiating features.

> 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?

IMO, they're actually quite simple to allow variable length fields.
The example in the draft is:

 "Two flag bits are paired, a field can possibly be three different
lengths-- that is bit value of 00  indicates no field present; 01, 10,
and 11 indicate three possible lengths for the field."

This is used in the security extension field described in

The values in the SEC flags are:
      o 000b - No security field
      o 001b - 64 bit security field
      o 010b - 128 bit security field
      o 011b - 256 bit security field
      o 100b - 320 bit security field (HMAC)
      o 101b, 110b, 111b - Reserved values

> 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?

That was comment David also made. The point will be clarified.

>
> - 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.

In this case GUE is carrying IP protocol. Per RFC 8085 congestion
control is generally assumed when encapsulation carries an IP
protocol. Technically, GUE always carries an IP protocol (as opposed
to GREoUDP that carries an EtherType). The congestion considerations
in GUE are to cover cases where GUE would carry something like EtherIP
or GRE. Those can be  encapsulations of non-IP protocols hence should
have their own congestion considerations, however neither RFC2784 nor
RFC3378 mention congestion. I think it is valuable to have congestion
considerations in this doc to cover those cases.

>
> 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.
>
An example can be added should a simple TCP/GUE and the pseudo header
for TCP checksum calculation.

> * 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]."
>
This is referring to receiver processing. It is allowed by requirement
in RFC1122:

"An application MAY optionally be able to control whether UDP
datagrams without checksums should be discarded or passed to the
application".

> 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.
>
David made same comment, it will be addressed in next draft version.

> * 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 5.9 is mistitled, it should be " Congestion Considerations".
It is based on section 8 in RFC8086 (GRE-UDP). More text can be
imported from RFC8086 if it helps to clarify.

> * 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.
>
Which section are you referring to (there is no section 5.8.2).

> "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?
>
5.7 is about UDP checksum. NAT opertation with UDP checksums is
unaffected by use of GUE.

> Fragmentation
>
> - Elsewhere in the iRTF fragmentation has been described as fragile, why
> is it safe in this spec?
>
You interchanged the concepts of  "fragile" and "not safe"-- they are
not equivalent. draft-ietf-intarea-frag-fragiledescribes how IP
fragmentation is fragile in the Internet. This is because IP fragments
are often dropped by intermediate nodes. Fragmentation in an
encapsulation is hidden to intermediate nodes so for that reason it
may be more deployable (for instance, ECMP works better with GUE
fragmentation than IPv4 fragmentation). As for being safe, it is as
safe as any other fragmentation and DOS mitigations for fragmentation
learned over the years are applicable. The GUE fragment header also
has a larger ID field to avoid the problem that was seen in IPv4.

> * 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.
>
They are very different. UDP-options are being defined as part of the
transport layer to allow fragmentation of packets for a UDP
application. GUE does fragmentation in the encapsulation layer and
addressed the problems of tunnel fragmentation described in RFC4459.
UDP options have nothing to do with tunnels. I'd also point out that
UDP options are very preliminary, there is no deployment and it has
yet to be proved if they are even deployable.

> * 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?
>
They can be logged, will add text.

>
> 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."
>
Yes, only 0 to 127 are IANA assigned. Types 129 to 255 are site local.



> * 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
>
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