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SIP questions

SIP questions. Henning Schulzrinne Columbia University IRT Lab Siemens Munich -- January 2003. SIP bugs. http://www.sipwg.org/sipwg/ 54 bugs listed likely lead to either an addendum or re-issue at Proposed

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SIP questions

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  1. SIP questions Henning Schulzrinne Columbia University IRT Lab Siemens Munich -- January 2003

  2. SIP bugs • http://www.sipwg.org/sipwg/ • 54 bugs listed • likely lead to either an addendum or re-issue at Proposed • Draft Standard requires promotion of DNS SRV, NAPTR, TLS, …  unlikely anytime soon • currently, in bug-gathering phase, since none are critical • a few require discussion • very few seem to have backwards-compatibility issues

  3. SIP bugs - editorial

  4. SIP bugs – editorial

  5. SIP bugs - security

  6. SIP bugs – error behavior

  7. SIP bugs – error behavior

  8. SIP bugs – protocol format issues

  9. SIP bugs – URI & transport

  10. SIP bugs – tags and branches

  11. SIP bugs – Record-Route

  12. SIP bugs – dynamic behavior

  13. SIP bugs – early dialogs and media

  14. SIP bugs – events

  15. Session model for instant messaging • No recent public discussion, but apparently design team at work • moving towards SDP specific to CPIM messages, instead of comedia • Primary motivation: • bypass SIP proxies • reliable channel  avoid concerns about message size • more efficient encryption and signing • integration into multimedia sessions • Open issues: • proxies ever needed or is B2BUA good enough here? • Proposals: • SIP-lite  • BEEP  • CPIM (draft-campbell-simple-im-sessions, draft-campbell-simple-cpimmsg-sessions)

  16. CPIM session model • draft-ietf-impp-cpim-msgfmt-07 • Some annoying formatting differences to SIP, but close • SDP negotiation uses comedia, but doesn't fit well (port reuse, security)  in transition c=IN IP4 alice.example.com m=message 7394 cpim/tcp text/plain a=direction:both a=uri:im:2s93i9@alice.example.com c=IN IP4 bob.example.com m=message 8493 cpim/tcp text/plain text/html a=direction:active a=uri:im:849ro3@bob.example.com

  17. CPIM example From: MR SANDERS <im:piglet@100akerwood.com> To: Depressed Donkey <im:eeyore@100akerwood.com> DateTime: 2000-12-13T13:40:00-08:00 Subject: the weather will be fine today Subject:;lang=fr beau temps prevu pour aujourd'hui NS: MyFeatures <mid:MessageFeatures@id.foo.com> Require: MyFeatures.VitalMessageOption MyFeatures.VitalMessageOption: Confirmation-requested MyFeatures.WackyMessageOption: Use-silly-font header encapsulated MIME content Content-type: text/xml; charset=utf-8 Content-ID: <1234567890@foo.com> <body> Here is the text of my message. </body>

  18. Security • Not much movement – considered first-stage complete • Work on Authenticated Identity Body (AIB) • signed subset of SIP • not clear how proxies can do the signing • Some murmurings on reviving Basic authentication • avoids storage of valuable key on server • What about untrusted end systems? • Proposal for public key retrieval via OPTIONS • Single sign-on: SAML and SIP?

  19. Transport protocols • Clear movement towards TCP (and TLS) • No indications that SCTP is widely implemented in SIP proxies • not much performance benefit in practice (see Camarillo/Schulzrinne paper on performance comparison) • UDP not likely to disappear from spec (gratuitous incompatibility)

  20. Stray responses • Responses without proxy state are routed statelessly to UAC • Problem: 408 (timeout) for unreachable UAS can trigger an avalanche of responses • Proposal: suppress error responses or at least 408 • Evaluation: 4xx needed by caller • avoid special cases • usually, only last proxy needs short (user-oriented) timeout  let it clean up the chain • however, not clear if proxy knows its position in chain

  21. What is CASP? • Genericsignalingservice • establishes state along path of data • one sender, typically one receiver • can be multiple receivers  multicast • can be used for QoS per-flow or per-class reservation • anything that establishes temporary state roughly along a data path • but not restricted to QoS: • node and link discovery (link speed and properties, packet loss) • deposit active network code • control firewalls and NATs • avoid restricting users of protocol (and religious arguments): • sender vs. receiver orientation • more or less closely tied to data path • router-by-router (on-path) • network (AS) path (off-path)

  22. Network friendly congestion-controlled re-use of state across applications mobility friendly handles path changes transport neutral any reliable protocol initially, TCP and SCTP policy neutral no particular AAA policy or protocol interaction with COPS, DIAMETER needs work soft state per-node time-out explicit removal extensible data format negotiation Topology hiding not recommended, but possible Light weight implementation complexity security associations (re-use) may not need kernel implementation CASP properties

  23. CASP network model – on-path selective ("vegetarian") • CASP nodes form CASP chain • not every node processes all client protocols: • non-CASP node: regular router • omnivorous: processes all CASP messages • selective: bypassed by CASP messages with unknown client protocols CASP chain QoS QoS QoS midcom omnivorous

  24. CASP network model – out-of-path Bandwidth broker NAC CASP • Also route network-by-network • can combine router-by-router with out-of-path messaging • Not well defined if several in one AS • basically, an inter-domain service location problem • IETF tool kit for distributed solutions: • SLP (with extension in progress) • DNS SRV/NAPTR (see SIP) • Probably not: multicast, anycast, LDAP AS15465 AS17 AS 1249 data

  25. client layer does the real work: reserve resources open firewall ports … messaging layer: establishes and tears down state negotiates features and capabilities transport layer: reliable transport CASP protocol structure client layer (C) scout protocol messaging layer (M) messaging layer (M) transport layer (T) UDP IP router alert

  26. CASP messages • Regular CASP messages • establish or tear down state • carry client protocol • Scout messages • discover next hop • Hop-by-hop reliability • Generated by any node along the chain

  27. Message forwarding • Route stateless or state-full: • stateless: record route and retrace • state-full: based on next-hop information in CASP node • Destination: • address  look at destination address • address + record  record route • route  based on recorded route • state forward  based on next-hop state • state backward  based on previous-hop state • State: • no-op  leave state as is • ADD  add message (and maybe client) state • DEL  delete message state

  28. Next-node discovery: path-coupled • Discovery behavior options: • straight-through • hop-by-hop NI NR NE NE non NE • routing protocol with probing • service discovery, e.g., SLP • first hop, e.g., router advertisements • DHCP • scout protocol

  29. Next-node discovery: path-coupled hop-by-hop discovery (“incremental”) NI NE NE NR non NE

  30. Combining flow-through and transport sessions NI NE NE NR non NE use existing transport connection if available

  31. Mobility and route changes • avoids session identification by end point addresses • avoid use of traffic selector as session identifier • remove dead branch DEL (B=2) discovers new route on refresh B=1 ADD B=2

  32. SIP resilience • Similar to DNS  provide multiple servers, on different networks • failover via DNS SRV/NAPTR • with state replication • database replication may be too heavy-weight • copy REGISTER to all backup proxies • issue: digest authentication  third-party authentication • copy service logic, configuration to other servers • minimize call state bound to one server • specify server cluster as Route, not one server • SCTP between servers for failover

  33. CASP & IMS • Suitable for • RAN resource reservation • air interface control? • edge resource reservation • MIDCOM-style services

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