RE: IPCOMP and Tunnel Mode

[Tim Jenkins <[e-mail]>]

The attempt to avoid possible fragmentation is a good one (your wording
"will" is too strong), however:
1) No compression occurs for small packet sizes (below the threshold of the
particular algorithm)
2) We're talking IPSec tunnel mode, which has to add another IP header, so
the packet's going to grow anyway
3) The IPComp header is 4 bytes.
 
Thus, the net increase is 24 bytes. If we assume that the smallest MTU in
the network is 576 bytes, that means that packets that don't get compressed
that are 552 bytes to 556 bytes in size will cause fragmentation under these
conditions. What is the probability that there will be no compression of
packets of that size?
 
Does someone have any statistics on the probability of larger packets
expanding during compression, thus not being compressed, so that this can be
used as part of this discussion?
 
On the issue of the flags: The bits are there. If they can be made to
provide some useful purpose, they should. I note that this document
(draft-ietf-ippcp-protocol-06.txt) was *not* one of those advanced to
Proposed Standard. The document also states that the flag bits are reserved
for future use. Perhaps this could be a future use.
 
This discussion is based on the trade-offs between performance at the
network level (possible fragmentation) and performance at the computational
level (inconsistent implementation). Given equal probability of impacting
both, I would design in favour of the network. However, in this case, I
don't know that the probabilities favour the network.

--- 
Tim Jenkins                       TimeStep Corporation 
[e-mail]          http://www.timestep.com
<http://www.timestep.com/>  
[phone] x4304               Fax: [phone] 

 

-----Original Message-----
From: Avram Shacham [mailto:[e-mail]]
Sent: Wednesday, August 19, 1998 10:32 PM
To: Tim Jenkins
Cc: [e-mail]; [e-mail]
Subject: Re: IPCOMP and Tunnel Mode



Tim, 


At 04:03 PM 8/19/98 -0400, Tim Jenkins wrote: 

>>>> 


I have some concerns about one of the requirements of the IPCOMP draft. It
states that if no compression is actually done, no IPCOMP header should be
added. While this may be fine in transport mode, it leads to the appearance
of an IP-in-IP packet in tunnel mode. 


This concerns me, since it seems that the only way to be sure that the
inbound IPCOMP SA should handle packet is to perform an SA lookup to see if
it should have been compressed. (Issues of policy verification on inbound
packets are intentionally left out of this discussion.) This leads to
inconsistent processing of inbound SAs. 


As an alternative, I implemented using one of the flag bits to indicate that
there was no compression and left the IPCOMP header in. This allowed a
consistent lookup on inbound processing for an SA based on SPI (or the
IPCOMP equivalent). I have also implemented the policy lookup method, and
the full-time use of the IPCOMP header was much cleaner... 


Comments encouraged (although I doubt most of you need that...) :-) 


The draft (rfc?) (sorry Dan, I could not avoid following your style :),
while defining the non-expansion policy, explains the reason for not adding
the IPCOMP header in that scenario (see the marked lines): 


2.2. Non-Expansion Policy 


If the total size of a compressed ULP payload and the IPComp header, 

as defined in section 3, is not smaller than the size of the original 

ULP payload, the IP datagram MUST be sent in the original 

non-compressed form. To clarify: If an IP datagram is sent 

non-compressed, no IPComp header is added to the datagram. This 

| policy ensures saving the decompression processing cycles and 

| avoiding incurring IP datagram fragmentation when the expanded 

| datagram is larger than MTU. 


In other words, when the size of a non-compressible packet is MTU, your
suggestion to add the IPCOMP header will cause packet fragmentation. 


The wg debated having always an IPCOMP header, even when the packet in sent
without compression. As such policy is actually equivalent to lowering the
MTU by four octets, the wg decided to reject this proposal. 


In addition, your implementation does not comply with the requirement to set
the flags field to zero: 


3.3. IPComp Header Structure 


[snip] 


Flags 


8-bit field. Reserved for future use. MUST be set to zero. 

MUST be ignored by the receiving node. 


As for the implementation issues that you raised, there were several
interoperable stacks with IPComp in the bake-off last March, so working
draft-compliant solutions do exist. 


Regards, 

avram