Hi Stewart,
Thanks for the detailed review. I am responding after reading the email thread
that resulted, some issues were closed.
Several of the reviews have suggested significant changes to this document.
The working group was trying to make a few changes to bring it into alignment
with some changes to rfc2460bis (based on updating documents). It was not
attempting a major rewrite when advancing it to Internet Standard. The
alternative that the working group discussed was to file errata on RFC1981 and
leave it to a future document update.
I don’t think many of these changes can be done and still advance it to
Internet Standard. If we can’t advance the current document, then the w.g. may
want to just do the errata and withdraw the advancement request. Of course, if
people want to work on a revision of IPv6 Path MTU Discovery, it would be
welcomed, but it won’t be able to advanced to Internet Standard.
Comments below.
Thanks,
Bob
On Feb 9, 2017, at 7:19 AM, Stewart Bryant
<stewart(_at_)g3ysx(_dot_)org(_dot_)uk> wrote:
Reviewer: Stewart Bryant
Review result: Almost Ready
I am the assigned Gen-ART reviewer for this draft. The General Area
Review Team (Gen-ART) reviews all IETF documents being processed
by the IESG for the IETF Chair. Please treat these comments just
like any other last call comments.
For more information, please see the FAQ at
<https://trac.ietf.org/trac/gen/wiki/GenArtfaq>.
Document: draft-ietf-6man-rfc1981bis-04
Reviewer: Stewart Bryant
Review Date: 9/Feb/2017
IETF LC End Date: 1/Mar/2017
IESG Telechat date: unknown
Summary: This draft is on the right track but has open issues,
described in the review.
This review together with the lengthy discussion on the IETF list
suggest that this draft has a number of issues that need to be
addressed before publication.
I wonder if we would best serve both our future and our heritage
if we declared RFC1981 as historic, and either left the idea there,
or declared it as historic and wrote a new text from a clean start?
I think this was closed in the email thread.
Major issues:
Nits points out a number of faults with the document, but the only
one of substance is:
The text has a lot of RFC2119 language, but no RFC2119 declaration.
The document could use a thorough RFC2119 scrub
Based on in an earlier another review, our AD suggested an update to Shepard
Writeup instead of adding something to the document. It says:
This document is an update to RFC1981 that was published prior to
RFC2119 being published. Consequently while it does use "should/must"
style language in upper and lower case, the document does not cite the
RFC2119 definitions. Since the changes in this update were very
limited, the w.g. concluded to not change any of this language.
The document lists the three original authors one with an affiliation
change, but no email addresses. Has this been agreed with the
original
authors, and have arrangements been put in place for the RFC editor
to process auth48?
I contacted the original authors and they didn’t express any interest in being
involved in this update. They haven’t participated in the IETF for many years.
I can contact them during AUTH48, or the AD can override this requirement for
this document.
It is concerning that the draft does not talk in any detail about
how modern ECMP works, i.e. using the five tuple, and noting that
the PMTU may be different depending on the transport layer port
numbers.
I think this is out of scope.
Given that a very large fraction of packets will traverse an MPLS
network at some point, I am surprised that there is no text talking
about the importance of providing support for this feature in the
MPLS domain. RFC3988 talks to this point, but is only experimental.
I think this was closed in the email thread.
======
If flows [I-D.ietf-6man-rfc2460bis] are in use, an implementation
could use the flow id as the local representation of a path.
Packets
sent to a particular destination but belonging to different flows
may
use different paths, with the choice of path depending on the flow
id. This approach will result in the use of optimally sized
packets
on a per-flow basis, providing finer granularity than PMTU values
maintained on a per-destination basis.
SB> How widely is flow-id supported in networks? I thought that the
SB> current position was that it was unreliable as an ECMP indicator
SB> and thus routers tended to glean information from the packet
themselves.
I think this was closed in the email thread.
======
Note: if the original packet contained a Routing header, the
Routing header should be used to determine the location of the
destination address within the original packet. If Segments
Left
is equal to zero, the destination address is in the Destination
Address field in the IPv6 header. If Segments Left is greater
than zero, the destination address is the last address
(Address[n]) in the Routing header.
SB> So this has the effect that a traffic engineered packet and
SB> a non-traffic engineered packet will have the lower of the
SB> two PMTUs. This was all harmless when source routing was a
curiosity
SB> as far as mainstream networking was concerned, but may be
SB> more of a problem as a result of the SPRING work.
I think the onus is on SPRING to make sure it doesn’t break existing mechanisms.
=======
5.3. Purging stale PMTU information
Internetwork topology is dynamic; routes change over time. While
the
local representation of a path may remain constant, the actual
path(s) in use may change. Thus, PMTU information cached by a
node
can become stale.
If the stale PMTU value is too large, this will be discovered
almost
immediately once a large enough packet is sent on the path. No
such
mechanism exists for realizing that a stale PMTU value is too
small,
so an implementation should "age" cached values. When a PMTU
value
has not been decreased for a while (on the order of 10 minutes),
the
PMTU estimate should be set to the MTU of the first-hop link, and
the
packetization layers should be notified of the change. This will
cause the complete Path MTU Discovery process to take place again.
SB> Should that be an RFC2119 SHOULD?
SB> The impact of this advice is going to be a disruption to what
might
SB> be a critical service every 10 mins.
SB> Should there be some advice along the lines of noting the
SB> importance of service delivery as part of deciding whether to
SB> test for bigger PMTU vs improving efficiency?
=======
Minor issues:
IPv6 defines a standard mechanism for a node to discover the
PMTU of an arbitrary path.
SB> Do you mean "This document defines ....."? Otherwise this needs
SB> a reference.
Will fix.
=======
An extension to Path MTU Discovery defined in this document can be
found in [RFC4821]. It defines a method for Packetization Layer
Path
SB> Rather than have the reader figure out what "It" is, perhaps
SB> s/It/RFC4821/
The w.g. noodled on this text for a long time. I think the “it” is clear, but
will change to RFC4821.
=======
Upon receipt of such a
message, the source node reduces its assumed PMTU for the path
based
on the MTU of the constricting hop as reported in the Packet Too
Big
message.
SB> We should perhaps state up front that this procedure
SB> hunts for the worst case of the ECMP set associated with the
SB> ingress nodes PMTU classifier.
I think getting into ECMP is out of scope for moving this to IS.
=======
If a node receives a Packet Too Big message reporting a next-hop
MTU
that is less than the IPv6 minimum link MTU, it should discard it.
SB> Should that be an RFC2119 SHOULD?
See note above.
=======
5.2. Storing PMTU information
Ideally, a PMTU value should be associated with a specific path
traversed by packets exchanged between the source and destination
nodes. However, in most cases a node will not have enough
information to completely and accurately identify such a path.
Rather, a node must associate a PMTU value with some local
representation of a path. It is left to the implementation to
select
the local representation of a path.
SB> Is it worth noting the five tuple since that is how a lot of
SB> load balancers work?
I think getting into ECMP is out of scope for moving this to IS.
=======
The set of paths in use to a
particular destination is expected to be small, in many cases
consisting of a single path.
SB> I am not sure that remains true in modern networks.
Do we have any data on this? I wonder if there will be much difference in the
PMTU in this case.
=======
One approach to implementing PMTU aging is to associate a
timestamp
field with a PMTU value. This field is initialized to a
"reserved"
value, indicating that the PMTU is equal to the MTU of the first
hop
link. Whenever the PMTU is decreased in response to a Packet Too
Big
message, the timestamp is set to the current time.
Once a minute, a timer-driven procedure runs through all cached
PMTU
values, and for each PMTU whose timestamp is not "reserved" and is
older than the timeout interval:
- The PMTU estimate is set to the MTU of the first hop link.
- The timestamp is set to the "reserved" value.
- Packetization layers using this path are notified of the
increase.
SB> Such detailed implementation advice is uncommon in modern RFCs. It
has
SB> the disadvantage of de-facto standardizing something that should
be left to
SB> the innovation of the implementer.
Can you point to any harm this has caused? It is very widely implemented.
=======
5.4. TCP layer actions
SB> TCP implementations have moved on a lot since this section was
SB> written. Is this still current best practise?
=======
5.5. Issues for other transport protocols
Some transport protocols (such as ISO TP4 [ISOTP]) are not allowed
to
repacketize when doing a retransmission.
SB> How much TP4 is there going over IPv6? Doesn't this example
SB> show the IETF as not being in the modern age?
I have no information one way or another. I have heard there is some TP4 on
airplanes, may they will move it to IPv6 :-)
=======
Nits/editorial comments:
upper layer a protocol layer immediately above IPv6.
Examples are transport protocols such as TCP
and
UDP, control protocols such as ICMP, routing
protocols such as OSPF, and internet or lower-
layer protocols being "tunneled" over (i.e.,
encapsulated in) IPv6 such as IPX, AppleTalk,
or
IPv6 itself.
SB> Everything in the list above is in the well known list, except
SB> IPX, so technically it needs expansion. However it might be nice
SB> to use some modern example in common use.
I will add a reference to IPX. The best reference I have found to IPX is:
Novell, Inc., "Advanced NetWare V2.1 Internetwork Packet Exchange
Protocol (IPX) with Asynchronous Event Scheduler (AES)", October
1986.
Please suggest something better.
=======
link a communication facility or medium over which
nodes can communicate at the link layer, i.e.,
the layer immediately below IPv6. Examples
are
Ethernets (simple or bridged); PPP links;
X.25,
Frame Relay, or ATM networks; and internet (or
higher) layer "tunnels", such as tunnels over
IPv4 or IPv6 itself.
SB> Technically X.25 needs a reference, since it is not "well known”
Will add a reference, if you think it is necessary.
=======
path the set of links traversed by a packet between
a
source node and a destination node.
SB> Is it a set of links, or a set of links and nodes?
I think the current text is OK.
========
the value of MMS_S, the "maximum send transport-message size".
SB> The modern convention is full-name(abbreviation)
I don’t follow this.
========
The Sun Network File System (NFS) uses a Remote Procedure Call
(RPC)
protocol [RPC] that, when used over UDP, in many cases will
generate
payloads that must be fragmented even for the first-hop link.
This
might improve performance in certain cases, but it is known to
cause
reliability and performance problems, especially when the client
and
server are separated by routers.
SB> Perhaps this should point to RFC7530 (the current NFS Spec),
assuming
SB> the behaviour description is still correct.
Will do.
=========
The former can be accomplished by associating a flag with the
path;
when a packet is sent on a path with this flag set, the IP layer
does
not send packets larger than the IPv6 minimum link MTU.
SB> We do not normally give this level of implementation advice
I think it varies a lot.
========================
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