Re: [bfcpbis] TBD issues #4 and #5: How to pick next sequence number
Tom Kristensen <tomkrist@cisco.com> Wed, 19 December 2012 08:54 UTC
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Date: Wed, 19 Dec 2012 09:54:04 +0100
From: Tom Kristensen <tomkrist@cisco.com>
Organization: Cisco
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To: Gonzalo Camarillo <Gonzalo.Camarillo@ericsson.com>
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Cc: BFCPbis WG <bfcpbis@ietf.org>, 'Tom Kristensen' <2mkristensen@gmail.com>
Subject: Re: [bfcpbis] TBD issues #4 and #5: How to pick next sequence number
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In order not to duplicate the normative description on how to pick Transaction ID values for unreliable transport, I move the text from 6.2 to Section 8 where the BFCP transaction semantics are defined. New section 8 enclosed. -- Tom On 11/13/2012 09:55 AM, Gonzalo Camarillo wrote: > Hi, > > the text should explain the requirement (i.e., to avoid getting confused > with retransmissions that arrive late). With respect to the mechanism, > you can recommend (i.e., at SHOULD level) to use monotonically > increasing sequence numbers and let implementations choose any other > method that would meet the requirement as well. > > Cheers, > > Gonzalo > > On 13/11/2012 10:43 AM, Tom Kristensen wrote: > >> Issue with picking sequence numbers and making sure it doesn't cause any >> issues with (late) arriving retransmissions. This may happen, even >> though UDP/BFCP requires just one outstanding transaction at the time. >> >> Gonzalo: >> >>> Section 6.2 >>> >> [...] >> >>> The document says: " Transaction ID values are non-sequential and >>> entities are at liberty to select values at random." >>> >>> The document needs to make it clear what is the requirement and the >>> level of randomness required. For example, what happens if the same >>> transaction ID is reused and a retransmission of the message that >>> first used that transaction ID arrives? >>> >> Tom: >> | No real random numbers are needed. This issue is solved in the >> | upcoming version of the draft. >> | However, we need a safe scheme to pick sequence numbers to avoid >> | late arriving retransmissions cause confusion). >> >> >> Gonzalo: >> >>> Section 8.1 >>> >>> "The client MUST set the Transaction ID value in the common header to >>> a number that is different from 0 and that MUST NOT be reused in >>> another message from the client until a response from the server is >>> received for the transaction." >>> >>> See my comment above about reusing transaction ID values and the risk >>> of receiving retransmissions of the original message. >>> >> Tom: >> | OK. This is kept from RFC 4582 using TCP (and no retransmissions). So, >> the >> | idea should be to keep the RFC 4582 semantics for TCP transport and >> | introduce some sort of "sliding window sequence number" reuse or simply >> | add a sequential/wrapping sequence numbering for UDP as transport. >> >> >> So, what do people think? >> >> I feel the simplest here is to at least requring the BFCP entities to >> select >> Transaction ID values in increasing order and a wrap around of the 16 >> bit Transaction ID value. (This will require saying something about >> picking the intial value at random, not to close to wrapping and so on - >> I guess. Similar to RTP sequence numbers). >> >> Or is it sufficient to mandate not reusing the X last Transaction IDs >> used by the BFCP entity? >> Where X == what? >> >> -- Tom >> >
- [bfcpbis] TBD issues #4 and #5: How to pick next … Tom Kristensen
- Re: [bfcpbis] TBD issues #4 and #5: How to pick n… Gonzalo Camarillo
- Re: [bfcpbis] TBD issues #4 and #5: How to pick n… Tom Kristensen