Some small remarks for REAP

[Matthijs Mekking <matthijs@NLnetLabs.nl>]

Hello,

I looked at draft-ietf-shim6-failure-detection-07, and want to make two 
remarks.

1.
The draft states that "FBD works by generating REAP keepalives if the 
node is receiving packets from its peer but not sending any of its own."
Suppose we have two hosts A and B, and A is sending payload packets to B 
(unidirectional). B will respond with REAP keepalive messages. But 
suppose a link failure occurs: A cannot receive any messages, but is 
still able to send. A will continue sending payload packets and might 
receive (delayed) keepalive messages that were triggered from payload 
data before the link failure. Now the payload sent after the link 
failure is lost without being noticed. This occurs because keepalives 
cannot be related to the transmitted payload.

This will not be a problem if you run a reliable protocol like TCP on 
top of it, because in that case TCP will fetch this problem. But if UDP 
is used, this remains an issue I think.

Example scenario: Unidirectional traffic from A to B using address pair 
(A1, B1):
----------------------------------------------------------------------------------------------------------------------------------------
(1) A --> B: Payload1 (A1, B1)
(2) B --> A: Keepalive1 (B1, A1)
(3) B --> A: Keepalive1 (B1, A1)

Link failure (A1, B1)

(1) A --> B: Payload2 (A1, B1) (will fail)
(1) A --> B: Keepalive1 (B1, A1)
----------------------------------------------------------------------------------------------------------------------------------------


2.
How much locators will a host use? Because the Shim6 context will be 
garbage collected after 5 minutes (according to 
draft-ietf-shim6-proto-08) and with the proposed constant values, only a 
maximum of fifteen different address pairs can be probed. In reality, 
this number will be smaller, since the ULP will probably time out after 
two minutes. So, if only two or three locators per host are going to be 
used, these constants seem to work fine. With more locators, the 
protocol will probably also work fine, but a host will maintain locators 
that are probably not going to be probed. That seems a bit inefficient.

What is your opinion about these two subjects?



Regards,

Matthijs Mekking