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[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index] RE: iSCSI: Negotiation clarifications still neededfine - thanks - Julo
Julian, I think the text looks fine except for the word "responder" which is difficult because one is a "responder" or an "originator" per key and having received a partial key, one doesn't know whether one is a responder or an originator yet. It would be better to replace "responder" with "initiator or target" Pat -----Original Message----- From: Julian Satran [mailto:Julian_Satran@il.ibm.com] Sent: Friday, May 24, 2002 10:26 PM To: pat_thaler@agilent.com Cc: ips@ece.cmu.edu; mkrikis@yahoo.com Subject: RE: iSCSI: Negotiation clarifications still needed Pat your proposed 2b may be what we are looking for - i.e. a responder may not originate a key if it has an incomplete key text. The text we may want to add to section 4.2 is: Key=value pairs may span PDU boundaries. A responder having a received partial key text MUST refrain from originating any new key=value negotiations until it has no incomplete key text. This way one avoids having both negotiating entities assuming the originator role in a negotiation. Julo
Martins, Comments referenced by the same items Martins used. 1. Julian sent an email saying he would put the text I proposed in (though the text you quoted is not the whole text). 2. I think that the principle we have been using on text negotiation was that each key negotion is a separate item. Your proposal would be counter to that and I don't think it would be an improvement. The target should be allowed to respond to any complete key-value pair it has received. When a key-value pair is straddling the PDU bondary, then it shouldn't respond to that key until the complete key-value pair has been received. There is one potential corner case issue that should to be covered. Targets can initiate keys. If key-value pairs didn't straddle PDU boundaries, then ensuring that there is a clear originator for each offered key is easy. You can originate any key that you haven't received an offer of from your partner. But now keys can straddle PDUs. If the text between the last separater and the end of the PDU is Ma, then you don't know what key your partner has started to offer. If the partner was starting to offer MaxBurstSize and you offer it in the next PDU of the exchange, both sides may think they are originator. I suggest one of the following a) don't allow keys to straddle PDU boundaries. b) don't allow originating a key when the last login PDU ended in a partial key. c) don't allow offering a key where the start of the key matches a partial key at the end of the last login PDU. 3. Yes, we noticed a little while ago that losing a packet at the end of negotiation could hang things up though the concern is mainly for a full feature phase negotiation. Looking at 6.8, any timeout during negotiation causes the login and its TCP connection to be terminated. The whole negotiation process (see the point about origninators in 2) depends upon a one-by-one exchange of PDUs. PDU loss has to terminate it. Therefore, the target commits to the end of login as described for T5 target. It has sent the final login response with a status of zero. Moves to S5. If the login response doesn't get to the initiator, then either the initiator will close the connection due to the timeout. Since the target is in S4, loss of the transport connection will cause it to go to S8 and R1 of the cleanup state machine. It presumably will not take the M2 transition because the intiator isn't going to do cleanup for a connection it thinks wasn't in full feature phase. It will take M1 due to timeout - not elegant but good enough. The concern was Full Feature Phase negotation. Until negotiation ends, it can be reset and no values change. When the target sends the last Text Response PDU, then it thinks negotiation has ended and it applies the new values. If that PDU doesn't reach the initiatior, then it terminates the entire negotiation and continues to use the old values. The two ends are using different values. We decided to not raise this as an issue because it is such a corner case - we are operating over a reliable connection so PDUs shouldn't be lost (unless the whole path goes down in which case it doesn't matter). Also, there are few values exchanged during full feature so it isn't worthwhile to add complexity. Regards, Pat -----Original Message----- From: Martins Krikis [mailto:mkrikis@yahoo.com] Sent: Friday, May 24, 2002 12:39 PM To: Julian Satran Cc: ips@ece.cmu.edu Subject: iSCSI: Negotiation clarifications still needed The previous thread went on too long, but since it has now quieted down, I'll conjecture that the following are the aspects that still need to be addressed. 1. Not everybody seemed to have noticed that it is NOT legal to send the same key again, if it has once been negotiated (including negotiations that end with a reserved value (Reject, Irrelevant or NotUnderstood). I think it would benefit the draft to add the sentence that Pat proposed to paragraph 5 or page 72 in 12-92: "Sending the key again would be a re-negotiation". That I think would make it crystal clear. 2. When the key=value pairs that Originator is sending are broken across multiple PDUs, it is not clear whether the Responder may start responding to the keys as soon as it receives them or whether it should send blank PDUs back (as in the example on page 164 of 12-92) until it gets a PDU, the data part of which ends in a NUL byte (thus signaling that there are no broken key=value pairs at the end of it). I am proposing that the draft should make it explicit that only blank PDUs are to be sent. This allows decoupling of key=value generation from their encapsulation in PDUs (i.e., the generating logic need not worry about whether a key=value pair will fit and go out in this PDU or has to be retained to go out in the next). I can explain in detail why this is important (it has to do with teh possibility of receiving the "just-about outgoing" keys) but I'm keeping this "brief". Furthermore, it is my feeling that instead of checking the last bytes of a PDU for NUL, it would be better if the end-of-data was marked by a flag in the header. This way encapsulation will be simpler---just put as much data in the PDU as fits there and raise the flag if it isn't all, instead of checking whether it ends in a NUL and possibly shortening data to make it not end like that. 3. There is an opinion that on page 73 of 12-92, the phrase that says "or the responder may select an admissible value" is in contradiction to the very next sentence. There is also an opinion that this phrase is entirely unnecessary and detrimental to achieving broad interoperability (I call it "cutting slack to misbehaving or incompatible originators"). I don't have a suggestion since I consider the "feature" that this phrase allows of little importance to a properly built iSCSI node. 4. This is new. When doing Text Request/Response negotiations (i.e., in FFP), it seems that the Initiator commits to the new values when it receives a response from the Target with the F bit set. It is unclear when the Target should commit. Should it switch to using the new values as soon as it sends its response with the F-bit set, or should it do so only when it knows that the Initiator received its response? Commiting right away is simpler and since responses with F-bits set have TTT=0xffffffff and thus may not be reset, sounds plausible. If the values have importance on the next reception, it may also be important to commit timely. However, what if the Initiator doesn't get this response? Target now has committed, Initiator hasn't. Committing later puts the burden on Initiator to send something effectively telling "I've received your final response". Otherwise the Target will time out and not commit. This response can get lost too. Basically, it is beginning to look a bit like (what was it called?) "distributed consensus problem"? I think it goes like this: Two generals that are on oposite sides of the enemy want to synchronize their attack, and start sending messengers through with messages like "attack at dawn->", "<-ok, attack at dawn", "I know you know we attack at dawn->", "<-, I know you know I know we attack at dawn", etc., etc., ... But at no point can they commit yet... Is anybody else worried about this? Anyway, so when should a target commit? Page 83 of 12-92 is the relevant reference. Thanks, Martins Krikis Disclaimer: these opinions are my own and may not be those of my employer. __________________________________________________ Do You Yahoo!? LAUNCH - Your Yahoo! 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