Subject: [Rift] AD Review of draft-ietf-rift-rift-12 (Part 2a)

From: Alvaro Retana <[email-masked]>
Date: Fri, 5 Mar 2021 07:36:09 -0800

Dear authors:

This is Part 2a of my review of this document.  I wanted this second
installment to be longer, but given the IETF meeting next week, I am
sending this fragment out now.

Part 2a starts with §4.2 (Specification) and goes just through §4.2.2 -- I
just started reading the LIE FSM/§4.2.2.1.  I included a couple of comments
on the schema itself.

While reading this part of the document I've been learning thrift and
applying my programming and modeling experience (*).  In general, the model
seems straight forward (so far).  It is important to highlight somewhere at
the start of §4.2 that Appendix B is Normative.  The existing references
don't explicitly say so.

In this first part of the specification there are a number of assumptions
made that should be explained explicitly and not by reference.  For
example, it seems to be assumed that the reader knows what a three-way
handshake is -- while most readers of this specification will be familiar
with other routing protocols, this document is *the* RIFT spec and should
not rely on other protocol specifications, much less when the specification
is not (can't!) be the same -- this is what §4.2.2 says:

   It uses a three-way handshake mechanism which is a cleaned up version of
   [RFC5303].  Observe that for easier comprehension the terminology of
   one/two and three-way states does NOT align with OSPF or ISIS FSMs albeit
   they use roughly same mechanisms.

IOW, "similar to IS-IS, but not the same."   There are more specific
comments inline.

"Traditional" specifications usually present packet formats and describe
the fields before explaining their use.  §4.2.2 jumped right into talking
about a specific flag (v4_forwarding_capable) without introducing it or the
place where it is carried (LIEPacket).  It is important to describe the
different structures, and provide a way to make them simpler to find in
Appendix B.

In my comments I'm asking for a lot of pointers to things that have not
been discussed.  I don't know (at this point) whether the overall
organization of the document makes sense as is or if it might be worth
rethinking (i.e. shuffle things around).  Food for thought.

These comments/questions are for the Chairs/Shepherd:

- It looks like an early TSV review was requested, but I didn't see the
review come in.  Did I miss it?

- §8.1 includes a set of suggested UDP ports, but they are not reflected in
the registry.  Early allocation has not been requested -- you should
consider doing so.  See rfc7120.

Thanks!

Alvaro.

(*) I don't have any. ;-)

[Line numbers from idnits.]

...
1341 4.2.  Specification
...
1350   "On Entry" actions on FSM state are performed every time and right
1351   before the according state is entered, i.e. after any transitions
1352   from previous state.

[nit] Suggestion>
   "On Entry" actions at an FSM state are performed right before the
   corresponding state is entered, i.e. after any transitions from a
   previous state.

1354   "On Exit" actions are performed every time and immediately when a
1355   state is exited, i.e. before any transitions towards target state are
1356   performed.

[nit] Suggestion>
   "On Exit" actions are performed immediately when a state is exited,
   i.e. before a transition towards a target state is performed.

1358   Any attempt to transition from a state towards another on reception
1359   of an event where no action is specified MUST be considered an
1360   unrecoverable error.

[major] Any type of action, or are you referring to an "on exit" one?

[major] "MUST be considered an unrecoverable error"  What is the result of
that?  Should the adjacencies be reset, the rift process restarted, the
interfaces shut down, etc.??   There's no other place in the document that
mentions "unrecoverable".

1362   The FSMs and procedures are normative in the sense that an
1363   implementation MUST implement them either literally or an
1364   implementation MUST exhibit externally observable behavior that is
1365   identical to the execution of the specified FSMs.

[major] How can you tell the difference?  rfc2119 keywords "MUST only be
used where it is actually required for interoperation".  In this case there
are options, and there is no way to verify one or the other as long as the
implementations "exhibit externally observable behavior that is
identical".  IOW, I am not comfortable with using MUST if it is not
required.  Also, I believe the statement to generally be true for any
specification and not needed.

Suggestion (borrowing from rfc4271):
   The data structures and FSMs described in this document are conceptual
   and do not have to be implemented precisely as described here, as long
   as the implementations support the described functionality and exhibit
   the same externally visible behavior.

1367   Where a FSM representation is inconvenient, i.e. the amount of
1368   procedures and kept state exceeds the amount of transitions, we defer
1369   to a more procedural description on data structures.

[] This paragraph can also be eliminated.

1371 4.2.1.  Transport

1373   All packet formats are defined in Thrift [thrift] models in
1374   Appendix B.

[] Can we get an index/TOC or some type of guide in the appendix to make it
easier to find specific parts?  For example, finding where a LIE is defined
is not straight forward -- I happened to stumble on it in B.2.  Another
option would be to point in §4.2.2 to look for LIEPacket in B.2.

[major] It must be stated somewhere that the contents of Appendix B are
Normative.

1376   The serialized model is carried in an envelope within a UDP frame
1377   that provides security and allows validation/modification of several
1378   important fields without de-serialization for performance and
1379   security reasons.

[minor] This is a very short section -- please put references to other
places where these topics are explained more, if any.

1381 4.2.2.  Link (Neighbor) Discovery (LIE Exchange)

1383   RIFT LIE exchange auto-discovers neighbors, negotiates ZTP parameters
1384   and discovers miscablings.  It uses a three-way handshake mechanism
1385   which is a cleaned up version of [RFC5303].  Observe that for easier
1386   comprehension the terminology of one/two and three-way states does
1387   NOT align with OSPF or ISIS FSMs albeit they use roughly same
1388   mechanisms.  The formation progresses under normal conditions from
1389   one-way to two-way and then three-way state at which point it is
1390   ready to exchange TIEs per Section 4.2.3.

[minor] "...which is a cleaned up version of [RFC5303]."  There's no need
to hint at the fact that other implementations/protocols may not be as
good/clean as rift.  Please take this part of the sentence out.

[minor] "Observe that for easier comprehension the terminology of one/two
and three-way states does NOT align with OSPF or ISIS FSMs albeit they use
roughly same mechanisms."   I don't know how changing everything I know
makes comprehension easier. ;-)   Seriously: we don't need to tell the
reader what is not used.

1392   LIE exchange happens over well-known administratively locally scoped
1393   and configured or otherwise well-known IPv4 multicast address
1394   [RFC2365] and/or link-local multicast scope [RFC4291] for IPv6
1395   [RFC8200] using a configured or otherwise a well-known destination
1396   UDP port defined in Appendix C.1.  LIEs SHOULD be sent with an IPv4
1397   Time to Live (TTL) / IPv6 Hop Limit (HL) of 1 to prevent RIFT
1398   information reaching beyond a single L3 next-hop in the topology.
1399   LIEs SHOULD be sent with network control precedence.

[] The first sentence is long and convoluted.  I think I understood that
the assigned destination IP address and UDP port are used, but that they
also can be configured.  Is that right?

Suggestion>

   LIEs are exchanged over the well-known multicast addresses and UDP
   port, as summarized in Appendix C.1.  An implementation MAY allow
   for the local configuration of these parameters.

[] If configured, do both sides need to be configured beforehand or is
there a discovery mechanism?

[major] "LIEs SHOULD be sent with..."  When is it ok to not use these
values?  IOW, why is this action recommended and not required?  (The next
comment is related.)

[minor] "IPv4 Time to Live (TTL) / IPv6 Hop Limit (HL) of 1"  Was GTSM
(rfc5082) considered?  Several documents (for example, rfc8085 and
draft-ietf-opsec-v6) suggest its use.  You may be asked about it later.  It
would be good to preempt those questions by adding some text in the
Security Considerations about any risks...or using it.

[major] "LIEs SHOULD be sent with network control precedence."  When is it
ok to not do so?  IOW, when is this behavior recommended and not required.
BTW, please add a reference.

1401   Originating port of the LIE has no further significance other than
1402   identifying the origination point.  LIEs are exchanged over all links
1403   running RIFT.

[nit] s/Originating port/The originating port

1405   An implementation MAY listen and send LIEs on IPv4 and/or IPv6
1406   multicast addresses.  A node MUST NOT originate LIEs on an address
1407   family if it does not process received LIEs on that family.  LIEs on
1408   same link are considered part of the same negotiation independent of
1409   the address family they arrive on.  Observe further that the LIE
1410   source address may not identify the peer uniquely in unnumbered or
1411   link-local address cases so the response transmission MUST occur over
1412   the same interface the LIEs have been received on.  A node MAY use
1413   any of the adjacency's source addresses it saw in LIEs on the
1414   specific interface during adjacency formation to send TIEs.  That
1415   implies that an implementation MUST be ready to accept TIEs on all
1416   addresses it used as source of LIE frames.

[major] "An implementation MAY listen and send LIEs on IPv4 and/or IPv6
multicast addresses.  A node MUST NOT originate LIEs on an address family
if it does not process received LIEs on that family."

The first sentence says that it is optional to listen and send LIEs over
either IPv4/IPv6 (IOW, a router doesn't have to do either).  The second one
gives the impression that originating a LIE on a specific AF means that the
router is willing to only receive/process LIEs on it (e.g. if an IPv6 LIE
is not generated then there's no positive indication that the router can
receive/process IPv6 LIEs).  The combination results in a potential lock at
startup:

For example, if a router chooses to only listen on IPv4 (MAY = optional)
and it's neighbor on IPv6, then they will be sending LIEs and never talk to
each other.

I'm assuming that the intent is to allow for maximum flexibility: allow to
send/receive on either (or both).  But, what if only one AF is supported?
Should an implementation be capable to control which AF is used?  If I have
an IPv6-only deployment, should the router be expected to receive LIEs over
IPv4?

[nit] s/LIEs on an address family/LIEs using an address family

[nit] s/LIEs on same link/All LIEs on a link

[minor] "part of the same negotiation"  Negotiation of what?  The text at
the start of this section mentions that LIEs negotiate ZTP parameters -- is
that it?  Other actions don't seem to be negotiations: discovery, for
example.   I'm not sure if something like "session" would capture the
intended meaning.

[major] "MAY use any of the adjacency's source addresses it saw in
LIEs...to send TIEs."  This use of "MAY" makes using the source address
from a LIE optional.  §4.2.3.3 says that "TIEs...using the destination
address on which the LIE adjacency has been formed", which doesn't make the
use optional.

OLD>
   A node MAY use any of the adjacency's source addresses it saw in
   LIEs on the specific interface during adjacency formation to send
   TIEs.

NEW>
   A node may use any of the adjacency's source addresses from the
   LIEs received on the specific interface during adjacency formation
   to send TIEs (Section 4.2.3.3).

[See related question in 4.2.3.3.]

1418   A three-way adjacency over any address family implies support for
1419   IPv4 forwarding if the `v4_forwarding_capable` flag is set to true
1420   and a node can use [RFC5549] type of forwarding in such a situation.
1421   It is expected that the whole fabric supports the same type of
1422   forwarding of address families on all the links.  Operation of a
1423   fabric where only some of the links are supporting forwarding on an
1424   address family and others do not is outside the scope of this
1425   specification.

[minor] "three-way adjacency"   I know there's a definition in the
terminology section, but it is not a specification of what a three-way
adjacency is.  Please add a forward reference to where it is specified.

[major] "`v4_forwarding_capable` flag"   (This comment is not specific to
this flag.)

The specific LIE packet format has not been presented, introduced or even
referenced up to now.  Pointing to a flag, which happens to be optional in
an optional part of the LIEPacket, comes out of the blue and feels
completely out of place without prior explanation.

I can see that some of the fields are described in Appendix B, but there
should at least be a pointer to that.

I also expect a description somewhere of expectations and error conditions.
For example, see my comments in the appendix related to LinkCapabilities.

[major] "a node can use [RFC5549] type of forwarding"

First of all, rfc5549 is a BGP-specific RFC.  How do the BGP mechanisms
defined there (and in rfc8950) map to RIFT?  IOW, please specify how RIFT
works.

Because you listed rfc5549 as a Normative reference, I assume you want this
behavior to be normative.  s/can use/MUST/SHOULD/MAY ??

[major] "It is expected that the whole fabric supports the same type of
forwarding of address families on all the links.  Operation of a fabric
where only some of the links are supporting forwarding on an address family
and others do not is outside the scope of this specification."

Please remind me, is IPv4 optional or required?  I'm assuming that IPv6 is
required and IPv4 optional, but I can't find anything definitive in the
document.

Even if IPv4 is optional, I'm assuming that most deployments will support
both.  But as time goes on that may (slowly) change to IPv6-only.  Is that
a fair assumption?

Because the ability to support IPv4 is signaled per link
(v4_forwarding_capable), then the expectation that "the whole fabric
supports the same type of forwarding of address families" may be "easily"
broken if one link doesn't support IPv4 (including the case where a rogue
node may not advertise v4_forwarding_capable to mess things up).  What
should a node that detects an inconsistency (e.g. some links use
dual-stack, but others only one AF) do?  I'm assuming that it SHOULD/MUST
(?) go as far as disabling the adjacencies as described below, right?
Please be explicit and specific on the actions to be taken.

1427   The protocol does NOT support selective disabling of address
1428   families, disabling v4 forwarding capability or any local address
1429   changes in three-way state, i.e. if a link has entered three-way IPv4
1430   and/or IPv6 with a neighbor on an adjacency and it wants to stop
1431   supporting one of the families or change any of its local addresses
1432   or stop v4 forwarding, it has to tear down and rebuild the adjacency.
1433   It also has to remove any information it stored about the adjacency
1434   such as LIE source addresses seen.

1436   Unless ZTP as described in Section 4.2.7 is used, each node is
1437   provisioned with the level at which it is operating.  It MAY be also
1438   provisioned with its PoD.  If any of those values is undefined, then
1439   accordingly a default level and/or an "undefined" PoD are assumed.
1440   This means that leaves do not need to be configured at all if initial
1441   configuration values are all left at "undefined" value.  Nodes above
1442   ToP MUST remain at "any" PoD value which has the same value as
1443   "undefined" PoD.  This information is propagated in the LIEs
1444   exchanged.

[major] There are references made in this paragraph to things that are
defined elsewhere (""any" PoD"..."propagated in the LIEs").  Please put
pointers to where these things are defined.  As mentioned above, the LIE
format hasn't been presented yet.

The text above gives the impression that the type of the PoD is somehow
related to the level, so I went in search of that and found this in the
LIEPacket:

   struct LIEPacket {
  ...
       /** Node's PoD. */
     7: optional common.PodType            pod =
             common.default_pod;

...but then I can't find anything that corresponds to PodType matching the
values above: undefined, any...

...but I did find this:

   /** Common RIFT packet header. */
   struct PacketHeader {
...
     /** Level of the node sending the packet, required on everything
         except LIEs. Lack of presence on LIEs indicates UNDEFINED_LEVEL
         and is used in ZTP procedures.
      */
     4: optional common.LevelType            level;
   }

...and a couple of constants:

  const LevelType   top_of_fabric_level         = 24
  const LevelType   leaf_level                  = 0
  const LevelType   default_level               = leaf_level

If ZTP is not used, are there operational/deployment considerations for an
operator to provision the levels?  Should they simply start counting from
0, is incrementing by one good enough or should it be by 2?  Or maybe there
are considerations about the use of ZTP?  Please take a look at rfc5706.

[minor] "It MAY be also provisioned with its PoD."  What does this mean?
Does it mean that all the nodes in the PoD have the same level?  ...

[major] BTW, this reminds me, what are the manageability considerations
related to RIFT?  Among other things, it would be nice to have a summary of
parameters that are expected to be configured and their defaults.  Please
take a look at rfc5706.

1446   Further definitions of leaf flags are found in Section 4.2.7 given
1447   they have implications in terms of level and adjacency forming here.

[] "leaf flags"   What are those?  I think this is the first mention.

[] Suggestion>
   Further definitions of leaf flags are found in Section 4.2.7.

1449   A node tries to form a three-way adjacency if and only if

[nit] s/tries to form/starts to form

[major] "if and only if"   Can this be translated into a Normative
statement?  Are all these required (MUST) before the formation of a 3-way
adjacency started?

1451   1.  the node is in the same PoD or either the node or the neighbor
1452       advertises "undefined/any" PoD membership (PoD# = 0) AND

[minor] "the node is in the same PoD"  As what?   I guess you mean in the
same PoD as what a neighbor indicated in its LIE, right?

1454   2.  the neighboring node is running the same MAJOR schema version AND

[] The formats have not been introduced yet.

I peeked into Appendix B and found the PacketHeader.  As with #1, you're
assuming a LIE packet has been received from the neighbor, but that is not
mentioned.

[] The versioning system has not been introduced.  Please put a reference
to it.

[nit] This is the only place where "MAJOR" (all caps) is used.  Please be
consistent.

[minor] Are there any requirements related to the minor version?

1456   3.  the neighbor is not member of some PoD while the node has a
1457       northbound adjacency already joining another PoD AND

[nit] s/not member of some PoD/not a member of the same PoD

[major] "not [a] member of [the] some PoD"  #1 says that the "node is in
the same PoD" -- it looks like there's some context missing -- I guess
about the ability to join other PoDs.  Is this only a northbound
characteristic?

1459   4.  the neighboring node uses a valid System ID AND

[minor] "System ID" and "SystemID" are both used.  Please pick one.

[major] What is a "valid System ID"?  The only clue that I found is in
§4.2.7.2, which makes we wonder whether it is specific to ZTP or not (??):

   RIFT nodes require a 64 bit SystemID which SHOULD be derived as
   EUI-64 MA-L derive according to [EUI64].  The organizationally
   governed portion of this ID (24 bits) can be used to generate
   multiple IDs if required to indicate more than one RIFT instance."

If this is a recommendation (SHOULD), and assuming that it applies beyond
ZTP, what is a "valid System ID"?

I also only found one place that talks about an IllegalSystemID, but that
is probably not the same as an invalid one:

   /** 0 is illegal for SystemID */
   const SystemIDType IllegalSystemID        = 0

1461   5.  the neighboring node uses a different System ID than the node
1462       itself

[minor] Is there a way to resolve/address duplicate System IDs?  It seems
to me that a rogue node may reflect the sender's System ID and prevent an
adjacency from forming (along with a number of other values).

[minor] Also, I hope that the selection/assignment of the System ID is
discussed elsewhere.  A pointer might be nice.

1464   6.  the advertised MTUs match on both sides AND

[] Ahhh...this in in the LIEPacket too.

[major]
   struct LIEPacket {
...
     /** Layer 3 MTU, used to discover to mismatch. */
     4: optional common.MTUSizeType        link_mtu_size =
             common.default_mtu_size;

If MTUSizeType is not included, where is it specified that the
default_mtu_size must be considered?  Without that specification it is not
possible to satisfy this condition.

1466   7.  both nodes advertise defined level values AND

[major] The advertisement of LevelType in PacketHeader is optional.  That
means that even if there is a default value a node may not "*advertise*
defined level values".  Also, there are only 2 level values defined:

   const LevelType   top_of_fabric_level         = 24
   const LevelType   leaf_level                  = 0

What am I missing?

1468   8.  [

1470          i) the node is at level 0 and has no three way adjacencies
1471          already to nodes at Highest Adjacency Three-Way level (HAT as
1472          defined later in Section 4.2.7.1) with level different than
1473          the adjacent node OR

[nit] s/three way/three-way/g

...
1478          iii) both nodes are at level 0 AND both indicate support for
1479          Section 4.3.8 OR

[minor] "support for Section 4.3.8"  Is there a name for that?  How is it
indicated?  §4.3.8 says that a leaf can "advertise the LEAF_2_LEAF flag in
its node capabilities"; there is nothing called "LEAF_2_LEAF" in the
schema, but "leaf_only_and_leaf_2_leaf_procedures" does show up.  Please be
consistent in the terminology.  [Also, there are places where "leaf-2-leaf"
is used.]

...
1486   The rules checking PoD numbering MAY be optionally disregarded by a
1487   node if PoD detection is undesirable or has to be ignored.  This will
1488   not affect the correctness of the protocol except preventing
1489   detection of certain miscabling cases.

[nit] s/rules checking/rules for checking

[nit] s/MAY be optionally/MAY be    (redundant)

[minor] What are that the "rules [for] checking PoD numbering"?  Where are
they specified?

[major] "MAY be optionally disregarded by a node if PoD detection is
undesirable or has to be ignored."

This is the only place where "PoD detection" is used.  What is it?  Where
is it specified?  Why would be it undesirable?  When would it have to be
ignored?  Are there operational considerations for making that decision?

[major] "...except preventing detection of certain miscabling cases."
 Where is miscabling detection described?  Which cases may not be covered?
 §4.2.7.6 says that "internal ruleset flags a possible miscabling", but
that is the closest that I came to finding a description (and rulesets are
not mentioned anywhere else).

1491   A node configured with "undefined" PoD membership MUST, after
1492   building first northbound three way adjacencies to a node being in a
1493   defined PoD, advertise that PoD as part of its LIEs.  In case that
1494   adjacency is lost, from all available northbound three way
1495   adjacencies the node with the highest System ID and defined PoD is
1496   chosen.  That way the northmost defined PoD value (normally the ToP
1497   nodes) can diffuse southbound towards the leaves "forcing" the PoD
1498   value on any node with "undefined" PoD.

[nit] s/building first...adjacencies/building its first...adjacency

[nit] s/to a node being in a defined PoD/to a node in a defined PoD

[minor] "...defined PoD is chosen."  ...chosen as the PoD value to
advertise.  (or somehting along those lines)

[nit] s/northmost/northernmost

[] I'm missing how using the System ID to select will always result in the
north direction...

[minor] "...diffuse southbound towards the leaves "forcing" the PoD value
on any node with "undefined" PoD."  An example of this would be nice.

1500   LIEs arriving with IPv4 Time to Live (TTL) / IPv6 Hop Limit (HL)
1501   larger than 1 MUST be ignored.

[major] "MUST be ignored"  The specification of the sender (§4.2.2) says
that "LIEs SHOULD be sent with an IPv4 Time to Live (TTL) / IPv6 Hop Limit
(HL) of 1".  This is a Normative conflict because it is only recommended to
the sender, but required by the receiver.

1503   A node SHOULD NOT send out LIEs without defined level in the header
1504   but in certain scenarios it may be beneficial for trouble-shooting
1505   purposes.

[] See the related comments in #7 above.

[nit] s/trouble-shooting/troubleshooting

[major] #7 above makes advertising defined values a requirement to forming
a three-way adjacency, but it is only recommended here.  Unless there are
specific troubleshooting cases described, we should retain the
requirement.  I believe that a packet can be crafted in any way for
troubleshooting/testing, but that doesn't have to be indicated in a
specification which should deal with the correct operation of the
protocol.  IOW, I think we can do without this paragraph.

1507 4.2.2.1.  LIE FSM

1509   This section specifies the precise, normative LIE FSM and can be
1510   omitted unless the reader is pursuing an implementation of the
1511   protocol.

[nit] s/and can be omitted..../and can be omitted.

[] It would be very nice if there was an upfront description (at least a
list) of the different elements: states, events, etc.. to make reading the
diagrams easier.  Also, the terminology (PUSH, etc.) should also be upfront.

1513   Initial state is `OneWay`.

1515   Event `MultipleNeighbors` occurs normally when more than two nodes
1516   see each other on the same link or a remote node is quickly
1517   reconfigured or rebooted without regressing to `OneWay` first.  Each
1518   occurrence of the event SHOULD generate a clear, according
1519   notification to help operational deployments.

[] This event is the only one described.  Is there anything special about
it?  Is this the only one for which generating a notification is
recommended?  Is that why it is described here?

[minor] "occurs normally when..."   Are there other conditions (which are
not considered "normal")?

[major] "more than two nodes see each other on the same link"  The
description below is different (and I think more accurate): "more than one
neighbor seen on interface".  Also, it doesn't include the second part
about a "reconfigured or rebooted" node.  What is the characteristic of
that behavior?

[major] "SHOULD generate a clear, according notification to help
operational deployments."   Normatively, what is "clear"?  Is there
information that you have in mind that should be included to make it
"clear"?  I assume that a notification is a message logged on the node, is
that true?

Suggestion>
   ...SHOULD log a message including the System IDs of each node.

   (Anything else?)

[] This seems to be the only occurrence of a log resulting from a message
or other action.  I think I'm missing the significance of this event.

[Ok.  This is where I am in my reading.  There are more comments below.]

...
2069 4.2.3.3.  Flooding
...
2082   As described before, TIEs themselves are transported over UDP with
2083   the ports indicated in the LIE exchanges and using the destination
2084   address on which the LIE adjacency has been formed.  For unnumbered
2085   IPv4 interfaces same considerations apply as in equivalent OSPF case.

[major] (Related to a comment in 4.2.2.) "using the destination address on
which the LIE adjacency has been formed"  If multiple addresses are used
(from different AFs), which one is selected?  Is there a "primary" address
that is considered when the adjacency is formed?

...
6165 10.2.  Informative References
...
6175   [DOT]      Ellson, J. and L. Koutsofios, "Graphviz: open source graph
6176              drawing tools", Springer-Verlag , 2001.

[major] Is there an URI that can be used as a stable pointer?

...
6362 B.1.  common.thrift
...
6730 /** Link capabilities. */
6731 struct LinkCapabilities {
6732     /** Indicates that the link is supporting BFD. */
6733     1: optional bool                           bfd =
6734             common.bfd_default;
6735     /** Indicates whether the interface will support v4 forwarding.

6737         @note: This MUST be set to true when LIEs from a v4 address are
6738                sent and MAY be set to true in LIEs on v6 address. If v4
6739                and v6 LIEs indicate contradicting information the
6740                behavior is unspecified. */

6742     2: optional bool                           v4_forwarding_capable =
6743             true;
6744 }

[major] Not knowing thrift, and assuming that this structure is some sort
of enum/list, what should a receiver do if the undefined/unknown value 3 is
received?

[major] "contradicting information the behavior is unspecified."  How can
the behavior be unspecified if this *is* the specification?   More to the
point: if the indication is required in IPv4 LIEs, but only optional in
IPv6 LIEs, why wouldn't the information from the IPv4 LIEs be considered as
the truth?

[nit] s/interface will support v4 forwarding/interface supports IPv4
forwarding/g

[nit] s/v4/IPv4/g  s/v6/IPv6/g

[major] "Indicates that the link is supporting BFD."  I was going to ask if
setting bfd to TRUE means that the link can support BFD (is capable) or if
it BFS is actively enabled (supporting), but I found this somewhere else:

   /** default link being BFD capable */
   const bool            bfd_default                = true

...so I'm assuming that it just means that the link is capable, which makes
sense given that is it in LinkCapablities.

s/Indicates that the link is supporting BFD./Indicates that the link is
BFD-capable./g

Also, note that §4.3.5 says that "RIFT MAY incorporate BFD", which is not
completely aligned with the default indication of the link being
BFD-capable.  Using BFD is different than being BFD-capable, so it is ok
for its use to be optional (§4.3.5).  However, there is no stated
requirement for a link to be BFD-capable, as indicated in the default
setting.

[End of Review 2a]