Johannes Berg <johannes@xxxxxxxxxxxxxxxx> writes: > On Fri, 2009-12-04 at 16:02 +0200, Kalle Valo wrote: > >> Method 3 (IPv4 DSCP field) feels most portable to us, at least most, >> if not all, wifi drivers should use it. And, in theory, the receiver >> should also benefit from the classification, unless ISPs modify it of >> course. But the standardisation for IPv4 QoS bits is a mess and I >> don't really understand where the use of DSCP bits (as used in WMM >> implementations) is specified. > > http://tools.ietf.org/rfc/rfc2474.txt ? This wasn't clear to me, so I investigated this a bit. This is what I found: The user priorities (0-7) are defined in IEEE 802.1D-2004 Annex G. I believe this the output from task group 802.1p. The mapping from user priorities (UP) to 802.11 queues is both in the WMM spec and in IEEE 802.11-2007 Table 9-1. Like you said, DSCP is defined in RFC 2474 "Definition of the Differentiated Services Field (DS Field) in the IPv4 and IPv6 Headers". But the definition is IMHO very vague: 4.3 Summary "This document defines codepoints 'xxx000' as the Class Selector codepoints, where PHBs selected by these codepoints MUST meet the Class Selector PHB Requirements described in Sec. 4.2.2.2. This is done to preserve a useful level of backward compatibility with current uses of the IP Precedence field in the Internet without unduly limiting future flexibility. In addition, codepoint '000000' is used as the Default PHB value for the Internet and, as such, is not configurable. The remaining seven non-zero Class Selector codepoints are configurable only to the extent that they map to PHBs that meet the requirements in Sec. 4.2.2.2." 4.2.2.2 The Class Selector PHB Requirements "We refer to a Class Selector Codepoint with a larger numerical value than another Class Selector Codepoint as having a higher relative order while a Class Selector Codepoint with a smaller numerical value than another Class Selector Codepoint is said to have a lower relative order. The set of PHBs mapped to by the eight Class Selector Codepoints MUST yield at least two independently forwarded classes of traffic, and PHBs selected by a Class Selector Codepoint SHOULD give packets a probability of timely forwarding that is not lower than that given to packets marked with a Class Selector codepoint of lower relative order, under reasonable operating conditions and traffic loads." >From the text above you can, somehow, make the assumption that it's ok to use priorities 0-7 in DSCP bits 0-3. There's also a requirement for backwards support of IP Precedence bit: "PHBs selected by codepoints '11x000' MUST give packets a preferential forwarding treatment by comparison to the PHB selected by codepoint '000000' to preserve the common usage of IP Precedence values '110' and '111' for routing traffic." Because '110' is mapped to UP 6 and '111' is UP 7, that requirement is also fulfilled. But I didn't find anywhere explicit mappings between DSCP and 802.1d priorities. Being an 802 standard 802.1d only talks about MAC level, just as it should. Also from RFCs I didn't find anything more concrete than above. So from this I guess I can conclude that it is acceptable to use 802.1d user priorities in DSCP. At least I hope so :) >> Also I was told that root privileges >> are needed to set this and that's somewhat cumbersome from application >> developer's point of view. > > I don't see that IP_TOS, which will end up setting IP TOS/DSCP, requires > any elevated privileges: [...] I tested with a small python script and you are correct, as usual. No extra priviliges were needed. > In any case, I think this topic would benefit of cross-posting to > netdev :) I will definitely do that. I just wanted to hear opinions from the wireless crowd first. I'll create a wiki page summarising this and then ask from netdev. -- Kalle Valo -- To unsubscribe from this list: send the line "unsubscribe linux-wireless" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
