RE: [rohc] query: encoding of IP-ID for RND=0 in extn3
Remi Pelland <remi.pelland@octasic.com> Tue, 02 September 2003 13:35 UTC
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From: Remi Pelland <remi.pelland@octasic.com>
To: 'Alan Kennington' <ak1.rohc@topology.org>, ROHC mailing list <rohc@ietf.org>
Subject: RE: [rohc] query: encoding of IP-ID for RND=0 in extn3
Date: Tue, 02 Sep 2003 09:30:37 -0400
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Here's my understanding on this: The IP-ID field in extension 3 follows the same rules as the IP-ID fields in base headers: it is encoded based on its offset to the RTP SN and based on its associated NBO setting. When the base header conveying the extension 3 contains an IP-ID field (i.e. UO-1-ID in your example), then the IP-ID bits in the base header become irrelevant since all 16-bits are already available in the extension (based on the rule defined in RFC 3095, section 4.5.7). Note also that IP-ID can either be the inner or the outer IP Identification field. This is based on the version of the inner and outer (if present) IP headers and on their respective RND settings. Anyone, is this correct? Salut, Remi. -----Original Message----- From: Alan Kennington [mailto:ak1.rohc@topology.org] Sent: Saturday, August 30, 2003 6:56 AM To: ROHC mailing list Subject: [rohc] query: encoding of IP-ID for RND=0 in extn3 I am having some difficulty in interpreting RFC 3095, 5.7, page 75. ============================================================ IP-ID: A compressed IP-ID field. IP-ID fields in compressed base headers carry the compressed IP-ID of the innermost IPv4 header whose corresponding RND flag is not 1. The rules below assume that the IP-ID is for the innermost IP header. If it is for an outer IP header, the RND2 and NBO2 flags are used instead of RND and NBO. If value(RND) = 0, hdr(IP-ID) is compressed using Offset IP-ID encoding (see section 4.5.5) using p = 0 and default-slope(IP-ID offset) = 0. If value(RND) = 1, IP-ID is the uncompressed hdr(IP-ID). IP-ID is then passed as additional octets at the end of the compressed header, after any extensions. If value(NBO) = 0, the octets of hdr(IP-ID) are swapped before compression and after decompression. The value of NBO is ignored when value(RND) = 1. ============================================================= Suppose I have RND=0 and I send a UO-1-ID packet with extension 3. Suppose also that I include the 16-bit IP Identification field in the extension 3. This is in 5.7.5, p.85. Then the IP-ID field is clearly not compressed. But RND = 0. But sometimes the IP-ID field in the IP packets does not follow the established pattern. Therefore I may have to send all 16 bits even when RND = 0. The above rules tell me what happens when RND = 0 and the IP-ID is compressed. But here the IP-ID is _not_ compressed. Question: Now should I swap the bytes of IP-ID according to NBO? Should I subtract SN from the IP Identification field before I write it into the type 3 extension? I think that it is equally credible that the IP-ID is encoded (1) without SN offset and bytes swap (i.e. verbatim) or (2) with SN offset and bytes swap. Can anyone tell me what the majority view on this is? Cheers, Alan Kennington. PS. The ROHC implementer's guide, version 3, 4.8, page 7, says that the 16-bit IP Identification field in an IR/IR-DYN packet must be sent verbatim because it is called "Identification", not IP-ID. This tends to make me think that the extn 3 IP-ID _may_ be the swapped/offset version of IP-ID. But it is still unclear to me. _______________________________________________ Rohc mailing list Rohc@ietf.org https://www1.ietf.org/mailman/listinfo/rohc _______________________________________________ Rohc mailing list Rohc@ietf.org https://www1.ietf.org/mailman/listinfo/rohc
- [rohc] query: encoding of IP-ID for RND=0 in extn3 Alan Kennington
- RE: [rohc] query: encoding of IP-ID for RND=0 in … Remi Pelland
- RE: [rohc] query: encoding of IP-ID for RND=0 in … Lars-Erik Jonsson (LU/EAB)
- RE: [rohc] query: encoding of IP-ID for RND=0 in … Remi Pelland
- Re: [rohc] query: encoding of IP-ID for RND=0 in … Tommy Lundemo