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[RRG] draft-farinacci-lisp-05



A few comments:

There is no version field in the LISP header. There should be. At least a few "set to zero on send, ignore on receive" bits that can be used for extensions without bumping the version number is also a good idea.

"In order to eliminate the need for a mapping lookup in the reverse direction, the ETR gleans RLOC information from the LISP header."

I find this undesirable because this way, the behavior of the system can be different depending on which end initiated the communication. Trusting information supplied directly by the other end is also problematic security-wise. I would much prefer it if both ends did an independent mapping lookup.

"LISP Locator Reach Bits: in the LISP header are set by an ITR to indicate to an ETR the reachability of the Locators in the source site."

I wonder how useful this is in practice. First of all, having 32 RLOCs is way too many. Second, depending on the way xTRs are deployed (see my next message), it could be quite hard for one xTR to know the status of any others for a particular EID. But more fundamentally, this assumes that reachability is a binary property: something is reachable or it isn't. In reality, it's more complex: a certain location may be reachable from some parts of the network and not from others. Even if it is reachable, it may be preferable to use a different RLOC. As such, I think it makes much more sense to determine which RLOC is going to be used by a given ITR for a given EID on a case-by-case basis rather than broadcast some reachability bits.

There are also potential issues with synchronizing the numbering of the RLOCs in the mapping system and in this field.

"Record TTL: The time in minutes the recipient of the Map-Reply will store the mapping. If the TTL is 0, the entry should be removed from the cache immediately. If the value is 0xffffffff, the recipient can decide locally how long to store the mapping."

Don't think this is a good idea. Experience with the DNS TTL has shown that people may be tempted to set both unreasonably low TTLs or ignore TTLs. Mandated minimum and maximum caching times would make the protocol more deterministic and remove an opportunity for people to shoot themselves in the foot.

" Priority: each RLOC is assigned a priority. Lower values are more preferable. When multiple RLOCs have the same priority, they are used in a load-split fashion. A value of 255 means the RLOC MUST NOT be used."

Isn't it more natural to use 0 as the special case value?

"Note that the destination RLOC address MAY be an anycast address if the tunnel egress point may be via more than one physical device."

This makes me rather uncomfortable, as anycasting could get in the way of reachability testing. We have room for a number of RLOCs, why not put in RLOCs for all physical ETRs rather than anycast them?

Is the "Loc-AFI" field the AFI for an RLOC? Why is this field only 8 bits while other AFI fields are 16 bits?

"2. Locator unreachability is determined by an ITR by receiving ICMP Network or Host Unreachable messages."

These can be spoofed or filtered/not generated, so both their presence and their absence don't necessarily mean anything. I suggest the only action triggered by such a message should be a one-time reduction in the time until a new mapping request is done. (I.e., if this happens every 300 seconds, reduce by 240 seconds for the first ICMP message but not subsequent ones, which means an immediate request 80% of the time but a maximum of 1 request per 60 seconds.)

"3. ETR unreachability is determined when a host sends an ICMP Port Unreachable message."

Which host? The one holding the EID? Or the ETR? The former won't happen if the ETR is unreachable, and an ETR being connected to the network but not being able to function as an ETR seems like a corner case to me.

So basically the use of mapping requests/replies is the only reliable (implicit) reachability detection mechanism. However, it is largely unspecified how ITRs should use this mechanism to determine reachability.

"perform a route-returnability check"

Return routability check?

"the use of a 6-byte Nonce field in the LISP encapsulation header"

The nonce field is only 32 bits in the LISP header earlier in the document.

" In practice, this is not really a problem. Hosts typically do not originate IP packets larger than 1500 bytes. And second, an informal survey of ISPs has been taken where nearly all ISP link MTUs are either 4470 bytes or support Ethernet jumbo frames of 9180 bytes. Therefore, we don't anticipate any problems with prepending additional headers."

Even if we assume that all the links within a transit AS support sufficiently large MTUs, this doesn't address the situation where ISPs interconnect over a shared ethernet infrastructure (i.e., an internet exchange). Also, the placement of ITRs/ETRs is left fairly open, with the suggestion that ETRs can be placed in end-user networks. In that case, it's very likely that there is at least one hop in the path that is limited to a 1500-byte MTU. I think it would be helpful to explicitly signal the MTU and possibly the maximum size of packets that can be fragmented that an ETR supports back to ITRs.

In my opinion, it would be beneficial to remove pretty much all of the text that doesn't pertain to actual LISP operation from this draft, and move that which is still useful (some of it is a bit stale after five iterations) to a new "LISP architecture" document.

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