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Congestion SafetyChanges and Issues
draft-ietf-sip-congestsafe-01
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Background
• SIP can use UDP, and a proxy can switch from TCP/SCTP to UDP without the knowledge or consent of the originating UA
• UAs consequently do not have knowledge of end-to-end MTU or link conditions
• SIP messages (requests or responses) can be large enough and frequent enough to create congestion issues on some links
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Previous draft
• Defined Require header for congestion safety insertable by UAs, ostensibly for sending large requests safely
• Defined policy for transmitting requests in a relatively safe manner that includes proxies
• Did not address transmitting responses, which has been added in current draft
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Why are Safe Responses hard?
• Response routing must follow inverse of path taken by requests (the VIA path), no options.
• Responses may be larger than requests, creating problems (ex. fragmentation) that didn’t affect the request.
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Response Handling in Draft
• Hypothesis: if the request arrived ok, a response of equal or smaller size is no less likely to be ok.
• Rule: A “safe” UAS responding to a request may not send a response that is larger than the request unless the request was marked congestion safe using Require mechanism
• A “safe” UAS responds with 514 “Response Could Not Be Sent Safely” if rule not met
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Issues with new approach
• Responses not “guaranteed” congestion-safe unless requests use safety mechanism
• Better than what we have now?
• Is it good enough?