Re: Status of SAP
Ross Finlayson <finlayson@live.com> Mon, 19 April 1999 00:37 UTC
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Date: Sun, 18 Apr 1999 16:21:30 -0000
To: Colin Perkins <C.Perkins@cs.ucl.ac.uk>
From: Ross Finlayson <finlayson@live.com>
Subject: Re: Status of SAP
Cc: confctrl@ISI.EDU
In-Reply-To: <1322.924448249@cs.ucl.ac.uk>
References: <Your message of "Fri, 16 Apr 1999 11:01:17 PDT." <199904161801.OAA12810@windsor.research.att.com>
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At 04:10 PM 4/18/99 +0100, Colin Perkins wrote: >I volunteered to act as editor at the last IETF, and hope to get a revised >draft out before the next meeting. ----- Colin (cc. "confctrl"), As you're updating the SAP spec, please note also some comments/suggestions of my own. (I first posted this around the end of February, but thought I'd repeat this, in case it fell through the cracks.) Ross. ------- my earlier msg ------------ I'm not sure what the status of the SAP spec is these days, or who's working on it, but from reading the version at <ftp://ftp.isi.edu/confctrl/docs/draft-ietf-mmusic-sap-00.txt> - the only version I could find - I noticed some unclear wording that should be fixed before the spec advances further. The issue concerns the method that a SAP announcer uses to compute the delay between successive announcements for an advertisement. The spec curretly says: "The time period between one announcement and its repetition is dependent on two factors - the scope (TTL) of the session, and the number of other sessions currently being announced by other session directory instances." [This is OK] Then later, "Session announcers in the same scope band can normally be expected to hear your announcements, and reduce their data rates accordingly. Thus you should calculate the available bandwidth for your session's scope band by dividing the appropriate limit above by the number of other ^^^^^ announcers in your scope band. This gives you your bandwidth alloca- ^^^^^^^^^^ tion, which, given the size of your data packets, can be used to derive the base interval for announcements." The problem is the phrase "other announcers". Instead, it should be "different advertisements (including your own)". Note that the formula following this: interval =MAX(300, (8*no_of_ads*ad_size)/limit) is correct, because it uses a variable named "no_of_ads". It would be clearer, however, if the formula referred directly to the "available bandwidth" (say, B) that was defined above; this would make the formula: interval =MAX(300, (8**ad_size)/B) A second, related issue that needs to be clarified in the spec concerns how to handle the case where the same announcement is being advertised by more than one party. This can be useful, for instance, if the advertisement is for a multi-source session, where the sources are distributed, and you want to increase the likelihood of everyone seeing the announcement. (In this case, each source might wish to participate in the advertising of the session.) To address this case, the spec should make it clear that the available bandwidth for announcing this session is shared by all of its announcers - not allocated to each announcer individually. So, with this in mind, the available bandwidth *per announcer* - to be used for calculating the delay interval - should be defined as: limit B = ----------------------------------------------------------------------- (# of different advertisements)*(# of announcers for our advertisement) Ross.
- Re: Status of SAP Ross Finlayson