Re: [dtn] working group last call on draft-ietf-dtn-bpbis

["Burleigh, Scott C (312B)" <[e-mail]>]

Hi.  As Rick suggested, I'm posting my thoughts on Stephen's comments to the list in addition to noting them on Ed's spreadsheet - mainly transcribing them but amplifying in a few instances.  In-line below.

Scott

-----Original Message-----
From: dtn [mailto:[e-mail]] On Behalf Of Stephen Farrell
Sent: Monday, January 23, 2017 7:33 PM
To: Marc Blanchet <[e-mail]>; dtn <[e-mail]>
Subject: Re: [dtn] working group last call on draft-ietf-dtn-bpbis


Hiya,

I've done a review of bpbis-06 and have many comments. (Sorry:-)

Overall I don't think this is ready, and some more discussion of some of the issues is needed. Since I've not followed the list as closely as I'd have liked I may have missed some such discussion in which case pointing me at the relevant bit(s) of the archive would be a fine response.

I've tried to separate stuff into things that'd cause me to ballot DISCUSS in IESG evaluation (*), things that might not, and nitty things. I hope that helps, but don't take that categorisation too seriously:-)

Cheers,
S.

(*) Note that I'll likely not still be on the IESG when this gets there (I escape in March) so the fact that I would ballot DISCUSS is not that relevant to what other ADs might do.


Possibly major issues (DISCUSS like)
------------------------------------

(A) intro: The last bullet list of the things that are not specified here seems problematic for a PS and I think needs more discussion/work. I'm not sure if it's only the text that needs work, or if the missing specification is required now.
Taking the bullets one at at time (numbers are in order of
presentation): 1) This isn't clear enough, I'm not sure what's being omitted, 2) Omitting routing I think is fair, 3) It's also fair to omot RIB/FIB issues, 4) it's not ok to omit security mechanism definition, (making [BPSEC] normative and waiting on that in the RFC editor queue would fix this, and is IMO needed), 5) I'm not sure what's right here.  I think it'd be good to have some list discussion about this, as it'll certainly come up in IETF LC and IESG review and having list traffic at which to point will help backup whatever conclusions are reached.  In particular in 4.3, I don't think it's acceptable for the BIB and BCB to not have a normative reference. Similarly, the "TBD" for the other extension block types are not appropritate. (But those can likely be informative refs.)

	-	I'm fine with bringing bpsec along at the same time as bp(bis) and tcpcl.

(B) The spec is overly prescriptive in many places and ought be loosened up wherever possible. All we need is interop and not the kind of conformance at which this spec seems to aim (but maybe miss). For exmple the "retention constraint" stuff has absoluely no reason to be a MUST. As another, I think section
5.4 only needs a MUST in step 4 and all the rest are bogus and a bad plan.  Also in 3.1, the text here is often far too prescritpive and I suspect based on only a couple of implementation strategies.  There are many more examples.  I think it'd be a good plan to do an editing pass to get rid of as many of the extraneous and unnecessary constraints that are here.  Examples feature in other comments, but I've not tried to be exhaustive in spotting all instances of this.

	-	I disagree.  In general, I think this language needs to be normative in order to ensure coherent behavior among nodes of the network.

(C) Many of the flags relaed to reporting provide ways in which the BP, if it became widely deployed (even if not planned to be widely used), could be a significant (D)DoS accelerator. Has anyone figured out the scale factors involved, (e.g. if N bogus blocks say report if this can't be processed) whether those might be significant and if so what potential countermeasures might apply? Absent such an analysis, or fixing the problem, I'd argue it'd seem irresponsible to standardise the BP.  I'd say for a PS, the minimum is that BPAs MUST default to not sending all these new bundles except when specifically configured to be so verbose.  This also affects 5.1 and maybe elsewhere which says an agent MUST emit admin bundles if asked.
In 5.6, step 2: again the SHOULD needs to be qualified in order to not have the BP be a fine DoS accelerator (given non-singletons). Step 3's SHOULD for this is even worse as a bad actor could include many such blocks.  In 5.13 - I think that is too many custody signals. If one envisages DTNs with custodians located at links that are particularly subject to disruption, then those may be few in number and having all other nodes/routers emit custody signals for each bundle not taken into custody seems hugely inefficient and unnecessary.
There may be more examples.  FWIW, my guess is that if all the current reporting is kept, then the IESG will require some kind of applicability statement about the kind of network in which the BP can safely be deployed. For me, fixing the problem is a better approach than constraining it via an applicability statement.

	-	I agree with making all status report generation "SHOULD" rather than "MUST".  But if it is possible to generate status reports (and I think it has to be, for network troubleshooting purposes) then it is always possible for a badly engineered implementation to generate them more frequently than might be optimal in a given network.  I'm unclear on how we can legislate against that.  Strong language warning the implementer of the possible dangers would be fine with me; is that enough?  Something like "Implementations MUST limit the generation of status reports so as to prevent excessive network traffic.  Strategies for limiting status report generation are beyond the scope of this specification."

A bit less major (maybe not DISCUSS-worthy)
-------------------------------------------

(1) 3.1's definition of a bundle correctly says that bundles are better when they include all the meta-data that might be useful. If considered naively, that conflicts with modern approaches to privacy where we want to ensure that meta-data is only seen by those (nodes) that need to see meta-data, as one form of data minimisation. OTOH, one could argue that such bundles will ensure that meta-data and payloads enjoy the same security services, which is a good thing. In any case, I think it'd be useful to have a discussion about the privacy aspects of the BP, esp the ways in which those may be different from other protocols. For example, would we expect report-to URIs to commonly allow re-identification of a person? I don't recall we've ever really discussed such issues.

	-	I think having that discussion would be fine.  I don't think that discussion has to happen in this specification, nor that it has to happen before this specification is published.

(2) 3.1, destination: I think this ought be clear that delivery to some node in the endpoint represents success, i.e.  that the BP does not force successful delivery to all or failure as a binary choice.

	-	The spec doesn't define "success" at all, and I don't know why it would need to.

(3) What bad things would follow if 3.2 was deleted?  It may be that I'm too familiar with DTN (and hence not a good judge if this section is useful or not) but I didn't find it useful.
Also - is 3.2 normative? If not, I'm even happier to see it go.
If it is, then I gotta wonder if it conflicts with other text later. And I see there are a few 2119 MUSTs in there so I guess you do mean it to be normative or did they sneak in in by accident whilst editing? (As can happen.) If not deleting this section, I'd argue to find all the bits of text in it that are needed and move them all elsewhere and then delete the section.

	-	I think 3.2 is essential to the specification.  I'm fine with moving the 2119 language to another section, if that helps.

(4) 3.2: The idea that the EID fully determines the MRG seems just wrong to me. While that might be a nice theory, I figure it's way more likely that the routing scheme determines how many copies of a bundle are rx'd at how many instances of the destination EID. What'd be bad about losing that concept and letting (the determinants of) the MRG be unspecified here?

	-	I think that would make the behavior of nodes so indeterminate that it can't be tested.

(5) 3.2, "Custody of a bundle MAY be taken only if the destination of the bundle is a singleton endpoint." That's plain wrong. Not all custodians can know about the desitnation being a singleton or not. And before you say it, I don't believe in the flag in 4.1.3 that allows an origin to specify this - I've never seen a real example of when that's useful - the only nearby case I recall was where the developer (me:-) knew we wanted distibution to all nodes in a multi-member endpoint but with best effort in terms of getting to them all and with custody and less frequent application layer re-tx's to ensure we got to as many as possible.  This also affects 5.2.

	-	"Singleton" doesn't say how many nodes are in the endpoint.  It says what the maximum number of nodes in the endpoint is.  The source node should know this.

(6) 3.3, I'm not sure this is useful either. What'd break if it were deleted? (But then I never liked those bits of DTNRG's work either;-)

	-	Sure, don't care.

(7) Including some examples and an RFC 7942 implementation status section would be a good thing, if easily done. That would help progression and increase confidence in the correctness of the spec.

	-	I don't think this is really necessary, but I don't mind adding an implementation status section.

(8) 4.1.6: Was sub-second timing discussed by the WG?  I'm not terribly pushed on that myself, but it'd be a shame to do an interop breaking change in the BP without discussing that topic. A reason to think about this is that there may be inter-VM (or intra-data-centre) reasons to consider the BP with sub-second timing as interesting.  It'd be a shame to make that impossible just to make it slightly simpler to represent time.

	-	It was discussed by the WG, and I believe we concluded that sub-second time representation wasn't needed here.

(9) 4.2.2, creation time rules: I don't see why it'd be a problem if node-id=X, creation-time=0,counter=0,lifetime=2s is used in two bundles emitted 3 seconds apart. Why does that justify a MUST NOT in the spec?

	-	I agree with Ed on this.  There may be purposes for which a log of bundles may be needed; that's impossible if bundles are not uniquely identified.

(10) 4.2.2: The "30 seconds" rule also seems wrong to me, as is the "MUST NEVER" (not a 2119 term btw) for re-use of the seq no, which is unrealistic.  As an example of that last: what do you expect to happen with a node that usually knows the wall clock time, but, at this moment, knows that it does not? E.g.
previous logs have some real dates, but current clock is
1970-01-01 or whatever. I think this is fixable but the current language is too prescriptive. Best might be to weaken the language here and to see what implementations do in the real world.

	-	Sure.

(11) 4.3.1: Is the SHALL here right? I would have thought a SHOULD is better to allow for legacy interop with 5050 via gateways, in which case there may be no node ID. That might be better off handled in some generic fashion though, and not piecemeal with each mention of node ID.

	-	There are node IDs in 5050 also, they're just not called by that term.  I think SHALL is correct here.

(12) 4.3.2: I don't believe it is correct to drop a bundle due to the lack of a previous node block, which is what sems to be implied here. Not all routing schemes need this and so it ought not be a MUST. Maybe a SHOULD is enough, but even if you say "MUST insert this" then I would like to argue that "receivers can decide to not care" be stated explicitly e.g.  by saying that bundles MUST NOT be rejected solely due to the lack of this EB.

	-	I think this will be a source of headaches eventually, but sure, we can relax this.

(13) I'm not sure if you have all the right "watch out for the null EID node ID" text needed. (I didn't go back over everything, but it'd seem wrong e.g. in an current custodian AR.

	-	I don't think there's an issue here, but sure, let's re-read with this in mind.

(14) 5.4: "The bundle protocol agent MUST determine which
node(s) to forward the bundle to." That's ungrammatical and close to BS - what if I want to multicast or broadcast the bundle or use some other opportunistic CLA? Or a sneakernet where nobody knows who'll be next hop.

	-	Of course the BPA must determine which node(s) to forward the bundle to.  What else is going to make that determination?  Nothing in the spec says it has to make that determination right now; maybe it sets the bundle aside for a while until a data mule comes along - and then it determines that this data mule is one of the nodes to forward the bundle to.  The text is correct.

(15) 5.4: The text about the flow label should be deleted as it says nothing. If includng this, then the flow label spec may need to be a normative ref (arguably).

	-	Okay.

(16) 5.4 - I think this is badly misleading. There will be many cases where a bundle cannot be forwarded now but may be forwarded later. Am I wrong in reading this section as precluding that?

	-	You are wrong in reading this section as precluding deferred transmission.

(17) 5.6, step 4 says one MUST handle "custody transfer redundancy" but that term seems undefined.

	-	Sure, let's add a clause saying "this condition is termed 'custody transfer redundancy'".

(18) 5.6 (step 5) points back to 5.3 which points to 5.7 or 5.4. I don't think such GOTOs are a good idea really.  I suggest removing lots of this and adding in some informative (i.e. non-normative) pseudo code (or real code) as an appendix.

	-	I disagree.

(19) I think some rules related to custody and fragmentation may be missing. For example, if bundle A is multicast and reaches two nodes on different paths who take custody (custodians C1/C2) and who both fragment but differently (into
F11/F12 and F21/F22 resp) with eventually a custody ack for F21 reaching C1.  Assume F21 is longer than F11, what is C1 to do with F11 when a custody timer expires?  Ought it re-transmit or consider that the custody ack for F21 matched F11 sufficiently well?  I'm not sure what'd be right here, if such cases can happen.  I'd be fine with the spec admitting that some such corner cases exist, or maybe it's easy enough to figure out, not sure.

	-	I don't think there is a problem.  Custody transfer is undefined if the destination endpoint is not singleton.

(20) 5.10.1, I've always wondered why custody timer expiry is covered here, and not really considered a part of DTN routing.
It seems to me to make more sense to couple custody timing and routing. If that resonated with folks, I think the change would be to make the timer-related text here into an illustrative example and to say that such things are better considered together with routing and/or by chunks of code that are somewhat more topology/disruption-aware of the situation in the particular DTN.

	-	Custodial retransmission really has nothing to do with routing.

(21) section 9 seems woefully incomplete - why is it ok to say "will be required" at WGLC? Surely the WG should at least have discussed the set of registries needed and the registration rules for those? E.g. do we sill want CCSDS to be able to add entries to some of those registries as we did with 5050?  And has the WG considered how do all the things in this draft relate to the set of IANA registries related to the BP? [1,2] (In the case of [3], section 4.1.5.1 really probably does need to say something.)

	-	Sure, let's review what other registries are needed here.

  [1] https://www.iana.org/assignments/bundle/bundle.xhtml
  [2]
https://www.iana.org/assignments/uri-schemes/uri-schemes.xhtml#uri-schemes-1

Seemingly more minor or nitty things
------------------------------------

- abstract: "This Internet Draft" is no longer appropriate language.

	-	Sure.

- abstract: I think this ought capture the fact that this version is not interoperable with 5050. That's not a bad thing, but worth noting here.

	-	Worth saying, but not in the Abstract.

- intro: I don't think the "sales" language is needed or appropriate in the first couple of paras. It should be entirely ok to say "we've learned stuff and fixed stuff."

	-	Sure, whatever.

- intro: "Custodial forwarding" is too terse at this point but also hard to explain briefly, is really a mechanism and not a capabililty, and maybe not such a highlight, so I'd delete that bullet

	-	I disagree.  Let's turn it into a sentence and keep it.

- intro: "[TUT]" is quite outdated and using dtnrg.org for the reference isn't wise (particularly at the moment when we're seeing SERVFAIL from the relevant NS;-)

	-	Sure.

- intro: this is a bit self-serving, but maybe a reference to the architectural retrospective [3] that Kevin and I wrote might be useful here, though I've not checked if it touches on enough of the issues behind the differences between 5050 and this.

	-	I don't think this is needed.

   [3] https://ieeexplore.ieee.org/document/4530739

- Figure 1: I wonder if it'd be worth pointing out that the BP does not have to run over a layer 4 that runs over a layer 3 etc. The figure and this text does give that impression that a "proper" transport is needed, which isn't the case.
(Tactically, I'm not sure if the text as-is, or something more correct, would make getting a new RFC easier or harder - I guess it'll depend on the reader;-)

	-	No, the spec already says "BP uses underlying "native" transport and/or network protocols".

- Figure 2: Few if any of the applications I've used with the BP had an administrative element. That's maybe down to the experimental nature of the work we've done but I don't think it's correct to imply that all applications using the BP need to be able to handle admin records, if that's what you're implying. (I'm not sure.) I'd say indicating that that's an optional thing would be right.

	-	I disagree.  The application agent -- not the application -- is the thing that has an administrative element.  We could say that a given BPA's application agent might lack an administrative element, in which case the node could not take custody of a bundle.  I don't see much advantage in that, but I don't mind saying it.

- 3.1, singleton: not sure if it's clear enough that all endpoints are sets, so this may puzzle folks. Maybe add e.g.
"remember that endpoints are sets," not sure.

	-	Let's assume that people reading this specification are able to read.

- 3.1, forwarding: the text is odd - "sustained effort" is not mandatory, and what "that node" is meant here?

	-	Nothing says how long "sustained" is, but okay, we could delete the word.  But I have no idea how to make this sentence any clearer.  There is exactly one possible antecedent for "that node".

- 4: The first two SHALL statements are odd in that there's no way in which one could implement this spec and not conform to those I think. In cases like that it's fine to avoid 2119 language. Not a big deal though, as the current IESG don't get anal about that, though some ADs in the past have done;-)

	-	I think those statements are needed and are not obvious.

- 4: last item MUST be break stop code. Is a decoder supposed to barf a bundle if this is not true? More generally, same question applies for all MUSTs stated only in terms of what the encoding must match.

	-	Let's add a general statement, somewhere, to the effect that the bundle protocol agent MAY discard any malformed bundle it receives.

- 4.1.1: why >1 CRC type? That seems bogus. None or strong seems better to me. (And I'd go for a crypto hash for strong.) I assume the WG discussed this and found that there are real use-cases for each of those specified. While those don't need to be in the spec, can someone tell me what they are as I'm not at all sure, e.g. why a 16 bit CRC is useful as an option.

	-	The WG discussed this and settled on these options.  See email list traffic starting on 18 January 2016.

- 4.1.3: "enables anonymous bundle transmission" - that's overstated, chances are that something in the CLA will be identifying, or allow re-identification, so I think what you want to say is that omitting the source EID helps with, but does not ensure, nymity.

	-	"Anonymous" doesn't mean you can't figure out who did it.  "Anonymous" means the identity of whoever did it was not attached to it.  The text is correct as written.

- 4.1.5.1: RFC3986 is the correct reference here, so the spec text is correct as-is. It may however be worth taking a look at the whatwg web page that has sometimes claimed to supercede
3986 for the browser-related things in which whatwg have an interest.  That's just in case there're some useful error handling considerations on the whatwg web page, (on the day you look at it;-). It's also the case that since BP EIDs are URIs, it's possible that strings that comply with today's or yesterday's whatwg web page may end up in the BP, so it'd be good to know if any of those (that are not valid according to
3986) might cause a problem with the CBOR encoding.

	-	If the text is correct, let's leave it.  If someone discovers that it is in error for one or more of the reasons proposed, then let's fix it.

- 4.1.5.2: Danger, metaphysics! "Every node MUST be a member of at least one singleton endpoint." This entire section is over-thought.  I think all you need to say is that nodes the emit bundles need to have an EID they can use as a source EID for as long as necessary.

	-	I disagree.  The section is exactly enough-thought.

- 4.2.1: this entire section is duplicative. That's a bad idea.

	-	If the text that it duplicates can be identified then we should remove the duplication.

- 4.2.2: 2nd para is badly written - that'd encourage coders to use the values 8,9,10 and 11 in ways that might be unwise.

	-	The text says nothing about using any values in any manner, for good or ill.   It only says how many elements are in the array.

- 4.2.2: wrt "anonymous" see earlier comment

	-	Correct as written, as noted above.

- 4.2.2: description of creation time is duplicative, except the earlier text didn't cover relative time.

	-	This section is not duplicative, it is expansive.

- 4.3.3: Is "Bundle Age Block" a good name? BAB used to mean another type of block, so that could confuse maybe. (That said, I forget how long we're had this name.)

	-	There are no longer any Bundle Authentication Blocks.  "Bundle Age Block" is a good name.

- 4.3.4: Do you need to say that the hop limit MUST NOT be changed, once a hop count EB is added. Also, can any node add one of these, if one was not prevsiously present?

	-	I agree, this needs to be clarified.

- 5: It's not necessary to say that new RFCs can supercede this.  That's just standard IEFF process.

	-	I don't see what harm this does.  For someone who is not steeped in standard IETF process, but wants to read the specification anyway, maybe it would be useful information.

- 5.2: mentions "dispatch pending" as if I should know what that is - is all the retention constraint stuff sufficiently explained I wonder? (Personally I don't think you need to mandate all this stuff and you cannot tell if an implementation has done it or not so I'd not bother trying to be so
prescriptive.)

	-	All of the retention constraint stuff is explained in detail.  I think it's necessary in order to ensure coherent behavior among the nodes of the network.

- 5.4: "at the last possible moment... MUST..." that's a bit silly as it seems to require BP code inside a NIC which is not how this'll usually be implemented.

	-	I don't understand the objection.  "Last possible moment" is very clearly in the scope of the operation of the CLA, which is BP code.  If that happens to be embedded in a NIC, fine, but that's not relevant.

- 5.5: I'm not convinced that the MUSTs here are right for all DTNs. I reckon that 5.5 could just as well say "MAY delete" and the BP would be fine. That might also provide some additional flexibility for some rounting schemes. That said, I won't press on this - if this doesn't resonate with folks now, and later turns out to be useful, I don't think we'd have such a hard time modifying BPAs where needed.

	-	Here again I think the normative language is necessary in order to ensure coherent behavior among the nodes of the network.

- 5.6: Again, this is overly prescriptive.

	-	I disagree, again because the normative language is necessary in order to ensure coherent behavior among the nodes of the network.

- 5.6, step 4: I wonder if an implementer will get all this right.

	-	It has been implemented.  The implementation works fine.

- 5.9: Badly implemented, re-assembly can create a memory consumption DoS vector, perhaps esp. if attempted on a non-destination node. It'd be better to warn about that. And maybe change from MAY for in-path reassembly to SHOULD NOT.

	-	I am doubtful that this specification should be a compendium of implementation tips.

- 5.11: does this mean that a custodian MUST ignore a custody signal destined for some other custodian?

	-	This text does not imply that the receiving node must ignore a custody signal destined for another custodian.  It means exactly what it says, and no more.

- Figure 6: I don't get when reason codes 5 to 8 would really be used. Are they in fact needed?  (They seem a bit implementation specific to me, but I've not gone looking.)

	-	DTNRG thought these codes would be needed.  Let's get more deployment experience before deciding that they are not.

- section 8: First sentence is bogus.

	-	Not bogus, but not necessary.  Sure, let's delete it.

- section 8: [SECO] isn't a good reference. It's outdated and I doubt will be picked up.

	-	Okay.