Overload in SIP

Jonathan Rosenberg

Cisco Systems

Problem Definition

SIPClient

ProxyA

Proxy 1

Proxy 2

Proxy 3INVI

TE

INVITE

503

INVITE

ElementOverloaded

SIP allows request to be retriedat another server upon receipt of 503Can include Retry-After header sayingthat this server should be left alonefor some period of time

Problem Definition

SIPClient

ProxyA

Proxy 1

Proxy 2

Proxy 3INVI

TE

INVITE

503

ElementOverloaded

When all elements are overloaded, 503 creates MORE trafficAmplified further by retransmits ofINVITE since 503 is delayed or lost

Oscillation Problem 1

SIPClient

ProxyA

Proxy 1

Proxy 2

INVI

TE

INVITE

503 Retry After 20

ElementOverloaded

Proxy 1 is overloaded, rejects requestwith 503 and Retry After of 20 seconds,moving ALL work to proxy 2

Oscillation Problem 2

SIPClient

ProxyA

Proxy 1

Proxy 2

INVI

TE

ElementOverloaded

Proxy 2 is now overloaded, and rejectsall work, even though proxy 1 is nowfreed up

INVITE

503

Solution Requirements

• Keep throughput at a good level when elements are overloaded

• Failures should be isolated and not cause widespread outages

• Minimize configuration to work• Deal with malicious elements• Inform upstream elements of overload• Throttle upstream traffic in granular fashions• Fairness across upstream elements

Sounds Familiar?

• Many of these are traditional congestion control issues, applied to the SIP application plane

• Input from TSV community is much desired!