Re: IPCOMP and Tunnel Mode
Avram Shacham <shacham@cisco.com> Thu, 20 August 1998 22:20 UTC
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Date: Wed, 19 Aug 1998 19:32:20 -0700
To: Tim Jenkins <tjenkins@TimeStep.com>
From: Avram Shacham <shacham@cisco.com>
Subject: Re: IPCOMP and Tunnel Mode
Cc: ipsec@tis.com, ippcp@external.cisco.com
In-Reply-To: <319A1C5F94C8D11192DE00805FBBADDF32A36F@exchange.timestep.c om>
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Tim, At 04:03 PM 8/19/98 -0400, Tim Jenkins wrote: >>>> <excerpt> <fontfamily><param>Arial</param><smaller>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. </smaller></fontfamily> <fontfamily><param>Arial</param><smaller>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. </smaller></fontfamily> <fontfamily><param>Arial</param><smaller>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... </smaller></fontfamily> <fontfamily><param>Arial</param><smaller>Comments encouraged (although I doubt most of you need that...) :-)</smaller></fontfamily> </excerpt> 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
- RE: IPCOMP and Tunnel Mode Tim Jenkins
- RE: IPCOMP and Tunnel Mode Bob Monsour
- Re: IPCOMP and Tunnel Mode Avram Shacham
- RE: IPCOMP and Tunnel Mode Paul Koning