Sip Instant Messaging - Cisco FirePOWER ASA 5500 series Configuration Manual

SIP Inspection
To support SIP calls through the security appliance, signaling messages for the media connection
addresses, media ports, and embryonic connections for the media must be inspected, because while the
signaling is sent over a well-known destination port (UDP/TCP 5060), the media streams are
dynamically allocated. Also, SIP embeds IP addresses in the user-data portion of the IP packet. SIP
inspection applies NAT for these embedded IP addresses.
The following limitations and restrictions apply when using PAT with SIP:
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SIP Instant Messaging

Instant Messaging refers to the transfer of messages between users in near real-time. SIP supports the
Chat feature on Windows XP using Windows Messenger RTC Client version 4.7.0105 only. The
MESSAGE/INFO methods and 202 Accept response are used to support IM as defined in the following
RFCs:
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MESSAGE/INFO requests can come in at any time after registration/subscription. For example, two
users can be online at any time, but not chat for hours. Therefore, the SIP inspection engine opens
pinholes that time out according to the configured SIP timeout value. This value must be configured at
least five minutes longer than the subscription duration. The subscription duration is defined in the
Contact Expires value and is typically 30 minutes.
Because MESSAGE/INFO requests are typically sent using a dynamically allocated port other than port
5060, they are required to go through the SIP inspection engine.
Only the Chat feature is currently supported. Whiteboard, File Transfer, and Application Sharing are not
Note
supported. RTC Client 5.0 is not supported.
SIP inspection translates the SIP text-based messages, recalculates the content length for the SDP
portion of the message, and recalculates the packet length and checksum. It dynamically opens media
connections for ports specified in the SDP portion of the SIP message as address/ports on which the
endpoint should listen.
SIP inspection has a database with indices CALL_ID/FROM/TO from the SIP payload. These indices
identify the call, the source, and the destination. This database contains the media addresses and media
ports found in the SDP media information fields and the media type. There can be multiple media
addresses and ports for a session. The security appliance opens RTP/RTCP connections between the two
endpoints using these media addresses/ports.
Cisco Security Appliance Command Line Configuration Guide
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If a remote endpoint tries to register with a SIP proxy on a network protected by the security
appliance, the registration fails under very specific conditions, as follows:
PAT is configured for the remote endpoint.
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– The SIP registrar server is on the outside network.
The port is missing in the contact field in the REGISTER message sent by the endpoint to the
–
proxy server.
If a SIP device transmits a packet in which the SDP portion has an IP address in the owner/creator
field (o=) that is different than the IP address in the connection field (c=), the IP address in the o=
field may not be properly translated. This is due to a limitation in the SIP protocol, which does not
provide a port value in the o= field.
Session Initiation Protocol (SIP)-Specific Event Notification, RFC 3265
Session Initiation Protocol (SIP) Extension for Instant Messaging, RFC 3428
Chapter 25 Configuring Application Layer Protocol Inspection
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