Native NAT Traversal Mode for the Host Identity Protocol
Summary: Has 2 DISCUSSes. Needs 2 more YES or NO OBJECTION positions to pass.
Mirja Kühlewind Discuss
1) This document should also update the IANA port registry to add a reference to this RFC-to-be to the existing entry for port 10500 (eventually even with note that this RFC-to-be discusses how to distinguish the services using NAT_TRAVERSAL_MODE). 2) Sec 4.4: "Hosts SHOULD NOT use values smaller than 5 ms for the minimum Ta,..." In rfc5245bis this is a MUST. Why is this a SHOULD here? Also in sec 4.6.2.: "If neither one of the hosts announced a minimum pacing value, a value of 50 ms SHOULD be used." This must be a MUST to be inline with sec 4.4. 3) Appendix A: "Ta value so that only two connectivity check messages are sent on every RTT." Why two? RFC8085 recommends (SHOULD) at may one packet per RTT for non-congestion control transmissions
I agree with other ADs that it is not clear to me why this mechanism is needed in addition RFC5770. This is a use case for ICE and I would think that re-using existing code and library would make implementation easier, fast and less-error-prone. I especially agree to the comments from Adam! Other comments/nits: 1) sec 4.6.2: "Upon successful nomination an address pair, a host MAY immediately stop sending such retransmissions." Not sure I understand this sentence; why MAY? 2) sec 4.1: The registration to the Control Relay Server can be achieved using RELAY_UDP_ESP parameter as explained later in this section." I guess that should be RELAY_UDP_HIP? 3) sec 4.1: "It is RECOMMENDED that the Relay Client select a random port number..." Please say source port -> s/random port number/random source port number/ 4) sec 4.8: "When a host does not receive acknowledgments, e.g., to an UPDATE or CLOSE packet after a timeout based on local policies, a host SHOULD resend the packet through the associated Data Relay Server of the peer (if the peer listed it in its LOCATOR_SET parameter in the base exchange." I did not really find anything about this in section 5.10 of RFC5770. In think the timeout needs to be further specified. 5) sec 5.3: "It is worth noting that sending of such a HIP NOTIFY message MAY be omitted if the host is actively (or passively) sending some other traffic (HIP or ESP) " This should really be a SHOULD (at least). 6) Appendix A: "One way to estimate the RTT is to use the time that it takes for the Control Relay Server registration exchange to complete;" That does estimate the RTT between the endhost... I not aware of a good way to estimate that, so I'm actually not convinced that the recommendation in appendix A is that useful at all.
Eric Rescorla Discuss
Rich version of this review at: https://mozphab-ietf.devsvcdev.mozaws.net/D3099 I am very familiar with ICE and yet I found this document extremely hard to follow. The problem is that it cherry-picks pieces of ICE and I'm just not sure that it's a complete specification when put all together. I have noted a number of places where I actually am not sure how to implement something, and fixing those will resolve this DISCUSS, but IMO you really should totally rewrite this document either (a) as a variant of ICE or (b) as an entirely new document not with a pile of new text and then references out to ICE sections. DETAIL S 4.2. > request type SHOULD NOT create any state at the Control Relay Server. > > ICE guidelines [I-D.ietf-ice-rfc5245bis] for candidate gathering are > followed here. A number of host candidates (loopback, anycast and > others) should be excluded as described in the ICE specification > [I-D.ietf-ice-rfc5245bis]. Relayed candidates SHOULD be gathered in If you're going to normatively cherry-pick ICE, you need to note specific sections, I think. S 4.6.2. > > A host may receive a connectivity check before it has received the > candidates from its peer. In such a case, the host MUST immediately > generate a response, and then continue waiting for the candidates. A > host MUST NOT select a candidate pair until it has verified the pair > using a connectivity check as defined in Section 4.6.1. Are you supposed to put this on a TODO check list as with ICE? S 5.8. > > 5.8. RELAY_HMAC Parameter > > As specified in Legacy ICE-HIP [RFC5770], the RELAY_HMAC parameter > value has the TLV type 65520. It has the same semantics as RVS_HMAC > [RFC8004]. What key is used for the HMAC?
S 2. > Server reflexive candidate: > A translated transport address of a host as observed by a Control > or Data Relay Server. > > Peer reflexive candidate: > A translated transport address of a host as observed by its peer. You should indicate which definitions are the same as ICE/STUN S 3. > connected to the public Internet. To be contacted from behind a NAT, > at least the Responder must be registered with a Control Relay Server > reachable on the public Internet. The Responder may have also > registered to a Data Relay Server that can forward the data plane in > case NAT traversal fails. While, strictly speaking, the Initiator > does not need any Relay Servers, it may act in the other role with Why doesn't it need Relay Servers? This isn't true for ordinary ICE. S 4.1. > for that host and close the corresponding UDP port (unless other Data > Relay Clients are still using it). > > The Data Relay Server SHOULD offer a different relayed address and > port for each Data Relay Client because this can cause problems with > stateful firewalls (see Section 6.5). by "this" I think you mean "not doing so" S 4.2. > parameter discovered during the Control Relay Server registration as > a server reflexive candidate. In this case, no further candidate > gathering is needed. > > A Data Relay Client MAY register only a single relayed candidate to > be used with multiple other peers. However, it is RECOMMENDED that a Nit: this would be clearer if you said "that it uses with" S 4.2. > gathering is needed. > > A Data Relay Client MAY register only a single relayed candidate to > be used with multiple other peers. However, it is RECOMMENDED that a > Data Relay Client registers a new server reflexive candidate for each > of its peer for the reasons described in Section 4.12.3. The This sentence feels like a bit of a non-sequiter. How do you "register" a srflx? Or should this say "relayed" S 4.2. > deployments in order to enable it by software configuration update if > needed at some point. A host SHOULD employ only a single server for > gathering the candidates for a single HIP association; either one > server providing both Control and Data Relay Server functionality, or > one Control Relay Server and also Data Relay Server if the > functionality is offered by another server. When the relay service How does this interact with mult-layered NAT?> S 4.3. > Servers (e.g. in the case of a single server, it would be 1). A > smaller value than 500 ms for the RTO MUST NOT be used. > > 4.3. NAT Traversal Mode Negotiation > > This section describes the usage of a new non-critical parameter Can you name the type here please? S 5.6. > Protocol IANA assigned, Internet Protocol number. > 17 for UDP, 0 for plain IP > Reserved reserved for future use; zero when sent, ignored > when received > Address an IPv6 address or an IPv4 address in "IPv4-Mapped > IPv6 address" format Is there a concern about NATs rewriting these, as we had for XOR- MAPPED-ADDRESS S 5.7. > | Reserved | 0 | Reserved for future extensions | > | Preferred | 0 or 1 | Set to 1 for a Locator in R1 if the | > | (P) bit | | Responder can use it for the rest of the | > | | | base exchange, otherwise set to zero | > | Locator | Variable | Locator lifetime in seconds | > | Lifetime | | | What is the purpose of this? It's not an ICE parameter. S 5.7. > | Port | | | > | Transport | Variable | IANA assigned, transport layer Internet | > | Protocol | | Protocol number. Currently only UDP (17) | > | | | is supported. | > | Kind | Variable | 0 for host, 1 for server reflexive, 2 for | > | | | peer reflexive or 3 for relayed address | Why do you need to send prflx candidates? S 10.2. > o If the agent is using Diffserv Codepoint markings [RFC2475] in its > media packets, it SHOULD apply those same markings to its > connectivity checks. > > o Unlike in ICE, the addresses are not XOR-ed in Native ICE-HIP > protocol in order to avoid middlebox tampering. I noted this earlier. Why?
Terry Manderson Yes
Deborah Brungard No Objection
Alissa Cooper No Objection
I admit to not having much familiarity with HIP, so apologies if some of these questions seem off-base. Why is this document on the standards track when RFC 5770 was experimental? Section 6.1 says: "The locators are in plain text format in favor of inspection at HIP- aware middleboxes in the future. The current document does not specify encrypted versions of LOCATOR_SETs, even though it could be beneficial for privacy reasons to avoid disclosing them to middleboxes." This seems to cut in the opposite direction of some of the other work we have going on in the IETF, where the justification for maintaining header information in the clear is for backwards-compatability with existing middleboxes, not to facilitate some to-be-developed middlebox behavior. Why is this justified for HIP? Section 6.1 also says "an end-host may exclude certain host addresses from its LOCATOR_SET parameter," but I don't think this is totally clear in Section 4.5 where it talks about "all the HIP candidates." I also wonder if it would be possible to provide some guidance about the circumstances under which an initiator might choose to exclude certain addresses, e.g. if there is a common deployment scenario where it's clear that certain candidates are meant to remain private. Nits: = Section 1 = " As one solution, the HIP experiment report [RFC6538] mentions that Teredo based NAT traversal for HIP and related ESP traffic (with double tunneling overhead)." This is a sentence fragment. = Section 2 = The paragraph about RFC2119 should also reference RFC8174.
Spencer Dawkins No Objection
I'm balloting No Objection, but I'm watching the discussion in Eric's ballot thread about reusing pieces of ICE, and I look forward to some discussion about the provisions being made for middleboxes in this draft - I'm not denying that such things exist, only that it would be best if we understood why middleboxes are needed for this usage.
Benjamin Kaduk No Objection
This document does a poor job at convincing me that there is a need to re-specify ICE for use in HIP contexts as opposed to just using ICE directly, up until Appendix B. I'd suggest moving some of the key points into at least the Introduction and arguably the Abstract as well, to make it clear that this is not just needless duplication for ideological purity. I think there's some lingering terminology confusion (as a result of needing to align the new bits in Native HIP-ICE with those retained from RFC 5077, along with the move from 5245 to 5245bis. Specifically, while it's fine for this document to refer to "ICE offer" and "ICE answer", 5245bis itself does not talk of "offer" and "answer", which are now concepts only in SDP, IIUC. Things also seem a bit hazy about server reflexive vs. server relay candidates (though maybe here the confusion is just on my end) -- in regular ICE, a server reflexive candidate is obtained by a STUN client making a one-shot request of a STUN server to find out what address is being used on the other side of a NAT, and doesn't require any state on the STUN server. But in this document we have a SERVER_REFLEXIVE_CANDIDATE_ALLOCATION_FAILED Notify/error packet that implies that state is needed on the Data Relay Server for a reflexive candidate, text about "when the relay service is split between hosts, the server reflexive candidate [from Control SHOULD be used over the one from Data]", and also other discussion about needing to register new reflexive candidates to avoid collisions or in potential multihoming future work. Some section-by-section comments follow. Section 2 Responder: The Responder is the host that receives the I1 packet from the Initiator. Does this still hold if the message is misdelivered or an attacker is in the network? Section 3 [...] At this point, also the HIP signaling can be sent over the same address/port pair, and is demultiplexed from IPsec as described in the UDP encapsulation standard for IPsec [RFC3948]. "demultiplex" does not appear anywhere in RFC 3948; it might be worth using a few more words here to clarify what is going on. Section 4.1 A Control Relay Server MUST silently drop packets to a Control Relay Client that has not previously registered with the HIP relay. How does the relay know where they are targetted without the registration information? This applies both renewals of service registration but also to host movement, where especially the latter requires the Control Relay Client to learn its new server reflexive address candidate. This is kind of awkward wording; maybe: This applies to both renewals of service registration and to host movement. It is especially important for the case of host movement, as this is the mechanism that allows the Control Relay Client to learn its new server reflexive address candidate. Section 4.2 [...] A host SHOULD employ only a single server for gathering the candidates for a single HIP association; either one server providing both Control and Data Relay Server functionality, or one Control Relay Server and also Data Relay Server if the functionality is offered by another server. The second half of this sentence seems to contradict the SHOULD, so probably some rewording is in order. [...] If a relayed candidate is identical to a host candidate, the relayed candidate must be discarded. NAT64 considerations in [I-D.ietf-ice-rfc5245bis] apply as well. I don't think this has enough detail of reference to ICE -- a relayed candidate being identical to a host candidate is, IIUC, quite unexpected. This is even noted in 5245bis, at the bottom of page 20 (of the -20). Unlike ICE, this protocol only creates a single UDP flow between the two communicating hosts, so only a single component exists. Hence, the component ID value MUST always be set to 1. ICE or SDP? Section 4.5 In step 2, the Control Relay Server receives the I1 packet. If the destination HIT belongs to a registered Responder, the Control Relay Server processes the packet. Do HIP participants register specifically as Responder/Initiator, or is this just that the entity that is Responder in this exchange, has registered at the Control Relay Server? [...] The RELAY_TO parameter is not integrity protected by the signature of the R1 to allow pre-created R1 packets at the Responder. This seems to allow an attacker to replay R1 packets and have the Relay transmit them to the Initiator. Are there cases where this presents an increase in attacker capabilities (e.g., when the Relay is allowed to send to the Initiator but the attacker is not)? (When would such pre-created R1 packets be useful?) Should step 7 explicitly duplicate the "validate REPLAY_HMAC" part of step 3? Section 4.6 The situation with the (non-)existence of aggressive nomination between 5245bis and MMUSIC-ICE probably merits further investigation. (That is, it may not be necessary to mention explicitly that regular nomination is used.) The Initiator MUST take the role of controlling host and the Responder acts as the controlled host. The roles MUST persist throughout the HIP associate lifetime (to be reused in the possibly mobility UPDATE procedures). In the case both communicating nodes are initiating the communications to each other using an I1 packet, the conflict is resolved as defined in section 6.7 in [RFC7401]: the host with the "larger" HIT changes to its Role to Responder. In such a case, the host changing its role to Responder MUST also switch to controlling role. The last clause seems to not match the earlier text about the Initiator being the controlling role. Section 4.6.1 In step 2, the Initiator sends a connectivity check, using the same address pair candidate as in the previous step, and the message traverses successfully the NAT boxes. The message includes the same parameters as in the previous step. It should be noted that the sequence identifier is locally assigned by the Responder, so it can be different than in the previous step. Step 2 is from Initiator to Responder, and the message in step 1 got dropped, so I'm quite confused at how the sequence identifier in step 2 could be assigned by the Responder as opposed to the Initiator. Step 4 could say whether the sequence number from step 1 is reused or a new one is allocated for the retransmission (even though it is clarified later as "SHOULD be sent with the same sequence identifier"). Section 4.7.2 [...] However, the Responder SHOULD NOT send any ESP to the Initiator's address before it has received data from the Initiator, as specified in Sections 4.4.3. and 6.9 of [RFC7401] and in Sections 3.2.9 and 5.4 of [RFC8046]. I don't see a Section 3.2.9 in RFC 8046. Since an I2 packet with UDP-ENCAPSULATION NAT traversal mode selected MUST NOT be sent via a Control Relay Server, the Responder SHOULD reject such I2 packets and reply with a NO_VALID_NAT_TRAVERSAL_MODE_PARAMETER NOTIFY packet (see Section 5.10). How does this mesh with a few paragraphs back, when the Responder MUST include the address it got by registering at a Control Relay Server in its R1 (only when it is including UDP-ENCAPSULATION NAT as one of its supported modes)? Section 4.7.3 [...] The Initiator may receive multiple R1 packets, with and without UDP encapsulation, from the Responder. However, after receiving a valid R1 and answering it with an I2, further R1 packets that are not retransmissions of the original R1 message MUST be ignored. We just said there may be multiple, so which one is "the" original R1 message? Should Section 4.8 repeat the earlier admonitions against a Relay Server forwarding traffic that does not involve a Client that has egistered with that Relay Server? Section 4.9 The relay still has to apply RELAY_HMAC; that's just not currently shown in the diagram, right? Section 5.6 The format of the REG_FROM, RELAY_FROM, and RELAY_TO parameters is shown in Figure 10. All parameters are identical except for the type. REG_FROM is the only parameter covered with the signature. I suggest "Of the three, only REG_FROM is covered by the signature." Section 6.1 The RFC 5770 text talks about TURN servers, but that's no longer applicable in this document (instead, Data Relay Servers are used to relay data in a similar usage). Also, the protection against DoS described in the last paragraph seems to only be partial protection, since incoming connections can still be affected by DoS, but outgoing ones are still possible. Section 6.2 This is the first mention of Opportunistic Mode at all in the document; it might be nice to refer to the section of 7401 where it's documented. Section 6.6 Is the invalid list of candidates sent *for* its peer or *to* its peer? Appendix B o Unlike in ICE, the addresses are not XOR-ed in Native ICE-HIP protocol in order to avoid middlebox tampering. This reads a bit oddly. Just to check: we don't need to do the XORing in Native ICE-HIP because the addresses are covered by an HMAC and the "tampering" wouldn't work? If so I'd suggest: o In ICE, addresses being conveyed across a NAT are XOR-ed to prevent middlebox tampering. Native ICE-HIP does not need to use XOR because such tampering is prevented at the HIP layer.
Suresh Krishnan No Objection
Alvaro Retana No Objection
Like Benjamin, I also found the relationship between this document and ICE somewhat confusing until the very end (Appendix B). The attempt to "follow ICE methodology but reuse HIP messaging format to achieve the same functionality" results in lack of clarity, so I also agree with Eric's opinion that a rewrite would help, and support his DISCUSS.
Martin Vigoureux No Objection
Ben Campbell Abstain
I support all points of Ekr's discuss and comment points. I think this either needs to use ICE mostly as is (maybe with some minor profiling) or it needs to be self-contained here. I understand the material in appendix B, but the current mix seems untenable for implementors. Therefore I am balloting "abstain". I will reconsider that position if there is a substantial reorganization. Substantive Comments: I share Alissa's question about why this is standard track when the previous work has been experimental. §1, second paragraph: The citation for the version of ICE used by "legacy ICE-HIP" should be RFC5245, not the bis version. §2: There are a number of lower-case keywords. Please use the RFC 8174 boilerplate. §4.2: - paragraph 5: Is everything in this paragraph from the ICE specification? I suspect not, but it's hard to tease out what is from ICE and what is new specification. It would be helpful to reference the ICE bits by section number. - paragraph 6: I'm confused in that I thought the previous text said that native ICE-HIP does not use STUN. §6: I am skeptical of the assertion that the security considerations for Native ICE-HIP are no different than those for Legacy ICE-HIP. Editorial Comments: §1, 2nd paragraph: - "responsible of NAT traversal": s/of/to - "responsible of end-host": s/of/to §4.3: "This section describes the usage of a new non-critical parameter type. ": Which is? §4.6, first paragraph: 2nd sentence is hard to parse.
Adam Roach Abstain
I share Ben's concerns about the disjointedness of this document's specification, and am likewise abstaining. My reasons for abstaining are deeper than Ben's, however. I recognize that the effort put into this document is substantial, and that the recommendations I make are unlikely to be taken at this point in time, but I believe that the HIP ecosystem would be far better served by an "RFC 5570 bis" approach rather than a modified form of ICE recast using HIP messages. Among other reasons: several companies already offer geographically-distributed hosted TURN solutions, largely due to the relative popularity of WebRTC. HIP will have to reach a similar level of popularity before HIP-specific relay nodes are similarly commercially available. Using ICE as-is would allow HIP to use those services that are available today. As a further concern, I worry that this pattern may end up replicated in other protocols. For example, although ICE was initially developed with RTP/RTCP in mind, it was not implemented as a series of extensions to RTP or RTCP; instead, it is its own protocol, intended to be re-used in other contexts. I would not like to see, e.g., ICE-like extensions to the QUIC protocol to enable its use in peer-to-peer situations; I would certainly hope that such an effort would use ICE as currently defined. Given that the headline rationale offered in this document is that "Implementing a full ICE/STUN/TURN protocol stack as specified in Legacy ICE-HIP results in a considerable amount of effort and code which could be avoided by re-using and extending HIP messages and state machines for the same purpose," this document puts forth an implication that all protocols could benefit from similar not-quite-ICE-but-almost-ICE extensions. I believe this implication is harmful. I also believe this analysis overlooks the availability of multiple existing, open-source, already-debugged, and "compatible with commercial use" ICE implementations. It is not clear that the four additional reasons offered in Appendix B are sufficient to justify the design. Taken in turn: > For example, ICE is meant for application-layer protocols, whereas HIP > operates at layer 3.5 between transport and network layers. This doesn't have practical effect: ICE is designed to work with generic UDP packet flows, subject only to the ability to demultiplex STUN from such packets. > This is particularly problematic because the implementations employ IPsec > ESP as their data plane: demultiplexing of incoming ESP, HIP and TURN > messages required capturing of all UDP packets destined to port 10500 to > the userspace, thus causing additional software complexity and an > unnecessary latency/throughput bottleneck for the dataplane performance. This doesn't seem like a foregone consequence. If you're using user-space HIP implementations, this user-space diversion is already necessary. If you're using kernel-space HIP implementations, it seems a modest step to add minimal STUN demultiplexing to the kernel implementation that is already performing ESP/HIP demultiplexing. It's possible that I'm misunderstanding some subtle aspect of the way these protocols interact with each other, but isn't this described performance impact simply the result of specific implementation design decisions rather than inherent to the design of RFC 5570's mechanism? > Also, relaying of ESP packets via TURN relays was not > considered that simple because TURN relays require adding and > removing extra TURN framing for the relayed packets. While it's been a while since I've looked at network kernel code, my recollection is that implementation of the POSIX "sendmsg()" system call generally maintains scatter/gather buffers all the way down the stack until such bytes are copied from system memory to the network hardware. Stripping such headers on receipt can be accomplished with simple pointer arithmetic. It's not clear what aspect of the system "simple" is intended to refer to, but both implementation and performance impacts should be immeasurably small when implemented in this way, unless I'm missing something. > Finally, the developers of the two Legacy ICE-HIP implementations concluded > that "effort needed for integrating an ICE library into a HIP > implementation turned out to be quite a bit higher that initially > estimated. Also, the amount of extra code (some 10 kLoC) needed for all > the new parsers, state machines, etc., is quite high and by re-using the > HIP code one should be able to do with much less. This should result in > smaller binary size, less bugs, and easier debugging.". Such size is not inherent in the implementation of ICE: for example, the ICE stack used by Firefox is 2.2 kLoC in size (if you ignore the ~1.2 kLoC of boilerplate copyright notice). Having seen the debugging of an ICE stack up-close-and-personal, I'm pretty comfortable saying that the effort to integrate a working stack has to be orders of magnitude less than implementing even the simplified ICE procedures defined in this document correctly. There are a lot of surprising corner cases that don't really turn up until you get into production. For the foregoing reasons, it is my conclusion that publication of this document is harmful for HIP and is harmful as a precedent that other protocols may mistakenly emulate. I believe that a restructuring of the document to more clearly explain why HIP chose this path while other protocols should not would limit some of these concerns. However, I do not believe that the fundamental flaw -- a partial respecification of ICE for the cited reasons -- can be fixed. I do not support publication of a document describing this mechanism, and encourage the working group to withdraw the document from consideration for publication. To be clear: despite the length and detail of my preceding objections, I recognize that I may be off in the weeds. I am happy to be corrected about any of the assertions I make above, up to and including corrections that make my conclusion incorrect. I will further note that this is not a blocking ballot position, and that, procedurally, the document can be published despite my misgivings. =========================================================================== I have included some additional comments below. --------------------------------------------------------------------------- §1: > As one solution, the HIP experiment report [RFC6538] mentions that > Teredo based NAT traversal for HIP and related ESP traffic (with > double tunneling overhead). This isn't a sentence. Perhaps remove "that"? Also: "Teredo-based" --------------------------------------------------------------------------- §4.6: > The connectivity checks follow the ICE methodology [MMUSIC-ICE], but > UDP encapsulated HIP control messages are used instead of ICE > messages. Only normal nomination MUST be used for the connectivity > checks, i.e., aggressive nomination MUST NOT be employed. The cited document does not describe aggressive nomination, and deprecates its use. Consider removing the mention of aggressive nomination in this document. --------------------------------------------------------------------------- §5.4: > The following NAT traversal mode IDs are defined: > > ID name Value > RESERVED 0 > UDP-ENCAPSULATION 1 > ICE-STUN-UDP 2 > ICE-HIP-UDP 3 This should probably point to the IANA table rather than replicating a snapshot of its contents. --------------------------------------------------------------------------- Appendix B: > o Unlike in ICE, the addresses are not XOR-ed in Native ICE-HIP > protocol in order to avoid middlebox tampering. This bullet should explain why such obfuscation is unnecessary.