RE: [AVT] Comments on draft-ietf-avt-hc-mpls-reqs-00.txt
"Ash, Gerald R \(Jerry\), ALABS" <gash@att.com> Fri, 21 May 2004 21:17 UTC
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Subject: RE: [AVT] Comments on draft-ietf-avt-hc-mpls-reqs-00.txt
Date: Fri, 21 May 2004 15:13:46 -0500
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Thread-Topic: [AVT] Comments on draft-ietf-avt-hc-mpls-reqs-00.txt
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From: "Ash, Gerald R (Jerry), ALABS" <gash@att.com>
To: Kristofer Sandlund <kristofer.sandlund@effnet.com>, avt@ietf.org
Cc: "Ash, Gerald R (Jerry), ALABS" <gash@att.com>
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Kristofer, Thanks for your comments and suggestions. > Comment 1: > ********** > When reordering is discussed in section 3, it is not made clear how the HC > scheme is supposed to handle reordering. Is the requirement that the > decompressor should be able to correctly decompress all reordered packets, or is > it enough that it can ensure no incorrectly decompressed packets will be > forwarded, i.e. avoid context damage? I assume that correct decompression is the > intention, but it could be made more clear. I agree with Colin and Lars-Erik that the requirement is that packet reordering not cause incorrectly decompressed packets to be forwarded on from the decompressor. We'll clarify that and add the requirement. > Comment 2: > ********** > * Section 4: > >[ECRTP] minimizes feedback based error recovery using CONTEXT_STATE > >packets to make cRTP more tolerant of packet loss, <snip> > Unless I misread this, doesn't this say that sending less feedback will make > cRTP more tolerant to packet loss? > > Is the intention to say: > "ECRTP uses mechanisms that make cRTP more tolerant to packet loss and it > thereby helps to minimize the use of feedback based error recovery > (CONTEXT_STATE packets)." Yes, this is the intention, thanks for the suggested rewording. > Comment 3: > ********** > * Section 4: > > ECRTP also is able to accommodate out-of-sequence packets. > > I am a little worried about this unconditional belief in eCRTPs ability to > handle reordered packets. RFC3545 does not explicitly say HOW an eCRTP > implementation should handle reordered packets and an aggressive implementation > of eCRTP will not have much higher protection against reordered packets than > CRTP. Only if you adapt eCRTP to send absolute values more often will it handle > reordering well (which is only hinted at the end of section 2.1 of RFC3545, but > does not say explicitly that it is for reordering). > > Therefore, a HC-over-MPLS solution using eCRTP would probably have to contain > some eCRTP implementation guidelines, and it is not clear to me why such > implementation concerns for eCRTP should differ much from those for ROHC (see > below). Fair comment. We'll reword the discussion of ECRTP reordering to reflect your comments. > Comment 4: > ********** > * Section 4: > > However, ROHC currently does not accommodate packet reordering > > needed to protect against out-of-sequence packets that can occur on MPLS > > LSPs, and would need to be extended to do so. > What 3095 (ROHC) says is that the spec is not written with reordering in mind > but it does not say if ROHC can handle some reordering anyway or not. Even a > default implementation of ROHC RTP will be able to handle a small degree of > reordering. Also, a ROHC implementation can be adapted in a similar way > as eCRTP (i.e. sending more bits) so that it handles reordering better. Exactly > _how_ to do this is done needs to be explained though, but the same must be done > for eCRTP. > > Therefore, my conclusion is that from the specs (3095 and 3545), neither of the > schemes is very good at handling reordering in their default configurations. > But, with implementation adaptations (without modifying any of the specs) both > can be made to handle reordering in a much better way. I thus think the text > regarding the two compression schemes should be more similar to each other. > > For example, the following text could be used in both: > > "A default [eCRTP/ROHC] implementation will probably not be able to meet the > requirements set up for reordering in this draft, and therefore some > implementation adaptations to address this would have to be presented in a > solution for HC over MPLS." OK, I agree with Colin, we'll reword the discussion to avoid judging the suitability of either ECRTP or ROHC where reordering can occur. Thanks, Jerry _______________________________________________ Audio/Video Transport Working Group avt@ietf.org https://www1.ietf.org/mailman/listinfo/avt
- [AVT] Comments on draft-ietf-avt-hc-mpls-reqs-00.… Kristofer Sandlund
- Re: [AVT] Comments on draft-ietf-avt-hc-mpls-reqs… Colin Perkins
- RE: [AVT] Comments on draft-ietf-avt-hc-mpls-reqs… Lars-Erik Jonsson (LU/EAB)
- Re: [AVT] Comments on draft-ietf-avt-hc-mpls-reqs… Colin Perkins
- RE: [AVT] Comments on draft-ietf-avt-hc-mpls-reqs… Ash, Gerald R (Jerry), ALABS
- RE: [AVT] Comments on draft-ietf-avt-hc-mpls-reqs… Ash, Gerald R (Jerry), ALABS