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Re: review of draft-ietf-shim6-failure-detection-03.txt

On 20-jun-2006, at 15:45, Jari Arkko wrote:


I'm fairly critical of the current version but I don't want to  waste
time with an in-depth review if there's going to be a new  version
within a week anyway.



Don't hold back - fire away if you have known problems. You saw
the discussion with John; I'm addressing those comments but will
not rewrite other parts. So it could well be that your comments
are still valid even for -04.
Ok. First of all, have a look at my message from october 24th, some comments are still valid.
This draft works per-context exclusively. So if there are 2, 5 or 10 contexts between two hosts, this means 2, 5 or 10 times the amount of work is done.
I think it makes sense to see whether the mechanism can be made to be more general, so that it can be used for other applications as well, ranging from MIP (as indicated in the applicability draft) to tunnel maintenance and maybe even peer-to-peer applications. However, it's important that there is fate sharing between the reachability protocol and the user protocol (shim in our case). I think this can be solved by having the quick reachability verification stuff (= FBD) encapsulated in the user protocol, but let the full path exploration be a protocol of its own or live under ICMPv6 or some such. However, this introduces some issues with knowing whether the same correspondent lives at different addresses.
In any event, I think just ignoring the possibility that there are multiple contexts between two hosts isn't good enough. The issue of avoiding "reachability probe storms" is somewhat related to this, and also not addressed in the draft.
Another thing that's missing completely from this draft is a discussion of how to use address pair preference information. This makes it impossible to address traffic engineering needs.
I believe section 3 (related work) is at best unnecessary and may even confuse the reader. The same is true of much of section 4 (definitions). Implementers know which addresses are usable and which aren't, at least for the most part, so covering this in such detail is superfluous. Things like "Both types of operational pairs could be discovered and monitored through the following mechanisms:" have no place in a protocol specification document, in my opinion. These need to specify what IS, not what COULD be.
I suggest tightening the use of words like "operational", "work", "reachable". They're mostly used interchangably in the draft. 

4.5.  Miscellaneous

   Addresses can become deprecated [3].  When other operational
   addresses exist, nodes generally wish to move their communications
   away from the deprecated addresses.

   Similarly, IPv6 source address selection [6] may guide the selection
   of a particular source address - destination address pair.
This doesn't say what shim6 implementers should do. In my opinion: keep using deprecated addresses as the ULID/primary locator as long as possible, but prefer non-deprecated addresses when selecting alternative locators.
"A "path" is defined as" should probably be in the "definitions" section. 

"when link layer" -> when a link layer

   1.  The base timing unit for this mechanism is named Keepalive
       Timeout.  Until a negotiation mechanism to negotiate different
       values for this timer becomes available, the value (3 seconds)
       specified in Section 6.5 SHOULD be used.

Sending a keepalive every 3 seconds? That's way too often, IMO.

   2.  Whenever outgoing data packets are generated

Data packets as opposed to what other types of packets?

   4.  The reception of a REAP keepalive packet leads to stopping the
       timer associated with the return traffic from the peer.
So when we receive a keepalive from the other side, _we_ stop sending keepalives? This may be the right thing to do, but it's not obvious to me why. Some explanation would help.
What if an attacker generates spurious keepalives in order to get us to stop sending keepalives on our side? 

   6.  Send Timeout seconds (10 s; see Section 6.5) after the
       transmission of a data packet with no return traffic on this
       context, a full reachability exploration is started.  This
       timeout period is larger than the Keepalive Timeout to
       accommodate for lost keepalives and regular variations in round
       trip times.
The keepalives are sent at an interval of 3 seconds (or shorter, I imagine that an implementation isn't going to keep an exact timer for each context, any rounding must obviously be in the down direction) and the timeout is 10 seconds. In these 10 seconds you'd normally receive 3 keepalives, while 1 is enough to indicate that the other side is still alive. The other 2 are only there in case of packet loss. I think that's excessive. Starting the full reachability exploration because of incidental packet loss isn't such a big deal that it warrants sending three times as many packets as necessary. (Note that this assumes the worst case of a unidirectional transport protocol which isn't exactly the common case.)
I find it jarring that section 5.5 immediately jumps to examples, before the protocol is fleshed out. Although the examples are referred by using a number, they aren't actually numbered. 

Why would a keepalive need an id field?
In the first long example B times out once, but then doesn't send more probes until it sees return traffic from A. Why does B stop sending probes?
I believe that since the id of the last received probe is included, the iseeyou flag is unnecessary.
Although copying back the last seen id seems to do the job, I can't help but feel that it would be preferable to add timers to reach round trip times and copy back more received ids and also sent ids. This allows the receiver of a probe to determine which of the probes that made it to the other side did so faster, so it can select the address pair with the shortest round trip time.
Including sent ids along with the addresses the probe with that id was sent to helps the receiver determine that some probes didn't make it (yet). If a probe didn't work in one direction of an address pair, it's reasonable to assume that it may also not work in the other direction and try other pairs first.
I believe the next example (the fifth, I think?) is flawed because A doesn't send anything after it has sent a data packet, even though it never receives a return packet. The same is true for the example after that.
When does address pair exploration conclude, both in the cases where there is alternative reachability, and in the case when there appears to be no reachability at all? The exponential backoff isn't described.
The keepalive is a fairly long packet. I think just a shim header as would be used for data packets but with no ULP following the shim header would be sufficient.
Requiring random numbers in packets that are sent rather frequently is a bad idea, because it depletes the typically limited amount of entropy that's available for strong random number generation rather quickly and semi-random number generation may be somewhat expensive (and not that good). And I don't see what good an id does in a keepalive anyway... Also, there may be reasons to have non-random numbers, such as ease of lookup.
The description of the different messages repeats text that is the same for different messages, which is very tedious and makes it likely that people will miss the things that are actually different. (That's why programmer's keyboards don't have ctrl-c/ctrl-v keys: reuse code, don't copy it.) 

The use of options for mandatory fields is awkward.

"   The node maintains also the Send and Keepalive timers."
These timers were previously unnamed, it would be better to use their names as soon as they're introduced. 

"   Upon the reception of a payload packet in the Operational state, the
   node starts the Keepalive timer if it is not yet running, and stops
   the Send timer if it was running.  If the node is in the Exploring
   state it transitions to the ExploringOK state, sends a Probe message
   with the I See You flag set to 1 (Yes), and starts the Send timer.
   In the ExploringOK state the node stops the Send timer if it was
   running, but does not do anything else."
I don't have a good feeling about this... It's too hard to determine what should be happening. Maybe it would be better rather than go down the list of packets that are sent/received and describe the behavior in each state, to take one state at a time and describe what happens with packets in that state. 

   Upon a timeout on the Keepalive timer the node sends a Keepalive
   message.  This can only happen in the Operational state.

Why?
Why are there different Exploring and ExploringOK states? In both cases, the host needs to continue trying different addresses, it's mostly/only the other side that needs to behave differently when there is successful reachability from them to us but not (yet) from us to them. 

   Garbage collection of SHIM6 contexts terminates contexts that are
   either unused or have failed due to the inability of the exploration
   process to find a working pair.

How is the latter determined?
This is an important issue. For instance, once that a shim context is rehomed, will it ever return to using the primary locators? 

   In the PDF version of this specification, an informational drawing
   illustrates the state machine.  Where the text and the drawing
   differ, the text takes precedence.

I'm not reviewing the PDF state machine here as the text is normative.

   A tabular representation of the state machine is shown below.  Like
   the drawing, this representation is only informational.

Then I'm ignoring this too.
But I would be happier if they'd be removed, because either they're superfluous as they're not normative, or they're actually necessary to understand the protocol, which is even worse because they're not part of the normative text.