Network Working Group T. Creighton
Internet-Draft Comcast Cable
G. Khandpur
Comcast Cable
Expires: February 15, 2007 August 2006
Use of DNS SRV and NAPTR Records for SPEERMINT
draft-ietf-speermint-srv-naptr-use-00.txt
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Copyright Notice
Copyright (C) The Internet Society (2006).
Abstract
The objective of this document is to specify the Best Current
Practice (BCP) adopted by a Voice Over IP (VoIP) service provider in
order to locate another VoIP service provider to peer with in the
context of Session PEERing for Multimedia INTerconnect.
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Table of Contents
1. Introduction...................................................2
2. Terminology....................................................2
3. Session Peering Setup..........................................3
3.1 TARGET Determination.......................................5
3.2 NAPTR Lookup...............................................5
3.3 SRV Lookup.................................................5
3.4 Using SRV Results..........................................6
4. High Availability..............................................6
4.1 SPP1 Fails to Reach SPP2...................................6
4.2 SPP2 Fails to Reach SPP1...................................6
5. Caching/TTL....................................................7
6. Acknowledgements...............................................7
7. Security Considerations........................................7
8. IANA Considerations............................................7
9. References.....................................................7
9.1 Normative References.......................................7
9.2 Informative References.....................................8
Authors Addresses................................................8
Intellectual Property and Copyright Statements....................8
1. Introduction
A service provider needs to identify the ingress Session Initiation
Protocol (SIP) (RFC 3261 [1]) server of a peering network before it
can signal and route SIP based real-time communication sessions.
This function of locating the ingress SIP server of peering network
is typically performed by the egress SIP server of the service
provider originating the SIP session. Also, the ingress server in
the peering network needs to locate the originating service
providers egress server in situations where the peering connection
to it gets terminated after receiving the SIP requests or if the
egress SIP server of originating service provider fails. The SIP
servers at originating and peering side use the DNS procedures, using
both SRV [2] and NAPTR [3] records, in order to locate each other.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [2] and
indicate requirement levels for compliant implementations.
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3. Session Peering Setup
SIP systems are represented by user agents (UA). In the diagram
below, a user agent (UA1), hosted by a service provider SP1,
initiates a call to a User Agent (UA2), hosted by service provider
SP2. The egress SIP server of SP1 is a SIP stateful Session Peering
Proxy (SPP), called SPP1, that interfaces with session peering
service provider SP2. The call initiated by UA1 is received by this
network element SPP1. The resource to which the call needs to be
routed by SPP1 is identified by a SIP or SIPS URI. This could be the
SIP URI of UA2 found in the Request-URI of the SIP INVITE received by
SPP1, or the next hop from SPP1 found in the Route header of SIP
INVITE. In order to determine the resource to route the call to, SP1
MAY make use of ENUM [4] lookup services or an internal lookup to
determine the SIP URI of the resource. This lookup MAY be performed
by SPP1 or another network element of SP1.
............................ .............................
. +------+ . . +------+ .
. | | . . | | .
. | SPP 1|--------------| SPP 2| .
. / | | . . | | \ .
. / +------+ . . +------+ \ .
. +------+ / || . . || \ +------+ .
. | | / || . . || \ | | .
. | UA 1 | || . . || | UA 2 | .
. | | || . . || | | .
. +------+ || . . || +------+ .
. +-------+ . . +-------+ .
. | | . . | | .
. | DNS 1 | . . | DNS 2 | .
. | | . . | | .
. +-------+ . . +-------+ .
. . . .
. SP 1 . . SP 2 .
............................ .............................
Figure 1: Logical Peering Scenario
In order to route the call to this resource in SP2, SPP1 needs to
determine the ingress SIP Session Peering Proxy for SP2, called SPP2,
by resolving the SIP or SIPS URI in DNS. SPP1 makes use of the NAPTR
and DNS SRV mechanism defined in [5] to determine the IP address,
port, and transport protocol for peering with the SP2 ingress SIP
peering proxy server (i.e. SPP2). SPP1 and SPP2 which are involved
in the session peering, support a set of protocols and have list of
preferences for these protocols. UDP, TCP and TLS MUST be supported
by these proxies.
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As a best practice, SPP1 and SPP2 SHOULD be deployed in a highly
scalable and highly available manner, such as a cluster of servers.
These servers are of different prioritization and weight, to ensure
capacity-based load balancing.
The figure below shows a high level SIP call flow setting up a SIP
peering session between SP1 and SP2. All SIP signaling MUST go
through the SPP1 and SPP2 as these are the ingress and egress points
in SP1 and SP2 network.
UA 1 SPP 1 DNS 1 DNS 2 SPP 2 UA 2
| | | | | |
|INVITE| | | | |
|----->| | | | |
| NAPTR Query | | |
| |----->| | | |
| NAPTR Response | | |
| |<-----| | | |
| SRV Query | | |
| |----->| | | |
| SRV+A Response | | |
| |<-----| | | |
| | INVITE | |
| |------------------->| |
| | | | |INVITE|
| | | | |----->|
| | | | |200 OK|
| | | | |<-----|
| | 200 OK | |
| |<-------------------| |
|200 OK| | | | |
|<-----| | | | |
| ACK | | | | |
|----->| | | | |
| | ACK | |
| |------------------->| |
| | | | | ACK |
| | | | |----->|
| 2-Way Media |
|<================================>|
| | | | | |
| | | | | |
Figure 2: Example Call Flow
[DO WE NEED THIS CALL FLOW]
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The target, to which the request is sent, is determined by SPP1 as
follows:
3.1 TARGET Determination
The target resource is identified with a SIP or SIPS URI. This is
the URI in the Route header, if present, or the URI from the request
URI of the SIP INVITE received by SPP1.
The host value of the hostport component of the URI is the TARGET.
This TARGET is the domain to be contacted.
3.2 NAPTR Lookup
Next the SPP1 determines the transport protocol of the TARGET by
performing a NAPTR query for the TARGET. NAPTR processing as
described in [3] will result in the discovery of the most preferred
transport protocol of a server instance of SPP2 and SRV records.
For example, consider a client that wishes to resolve
sip:user@example.com performs a NAPTR query for that TARGET domain
example.com, and the following NAPTR records are returned:
; order pref flags service regexp replacement
IN NAPTR 50 50 "s" "SIPS+D2T" "" _sips._tcp.example.com
IN NAPTR 90 50 "s" "SIP+D2T" "" _sip._tcp.example.com
IN NAPTR 100 50 "s" "SIP+D2U" "" _sip._udp.example.com
DNS MUST return at least three records - one with "SIP+D2T", one
with "SIP+D2U" and one with "SIPS+D2T" service type for the case of
direct peering (section 4.3 in [6]). "SIPS+D2T" is not a MUST for
indirect (transit) peering (section 4.4 in [6]) since domain validation
as specified in section 26.3.2.2 of [1] for TLS at layer 5 will not work
for indirect peering.
3.3 SRV Lookup
Depending on what transport protocols SPP1 supports, SPP1 selects one
from the preference list of NAPTR results and performs the SRV lookup
to obtain a list of available server instances for SPP2. TLS SHOULD
be the preferred transport protocol for peering between SPP1 and
SPP2.
In our example SPP1 uses TCP, the SRV lookup for
_sip._tcp.example.com would return list of available servers :
;; Priority Weight Port Target
IN SRV 0 1 5060 server1.example.com
IN SRV 0 2 5060 server2.example.com
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Alternatively, if no NAPTR records are found, then SPP1 uses the
preferred transport protocol and issues an SRV query for that
specific transport using sips for SIPS URI and SIP URI with TLS and
sip for SIP URI.
In our example, SPP1 prefers to use TCP and target SIP URI of SP2 is
sip:user@example.com, it sends a SRV query for _sip._tcp.example.com.
3.4 Using SRV Results
RFC 2782 describes the procedures on how to use and interpret the
results obtained from the SRV query. The target entry of the SRV RRs
is looked up by querying the DNS for address records. The SRV response
from DNS MAY also include A records with it which will cut down on round
trips and lookup of DNS again for target entry. On determining
the transport protocol, service, port and address record from the SRV
RRs as described above, the SPP1 will try to connect to the
(protocol, address, service). Once the connection is established to
an available instance of SPP2, SPP1 sends the SIP INVITE to SPP2.
SPP1 MUST act in a stateful manner and any retransmission of SIP
requests for a specific SIP transaction, including ACKS for non-2xx
response or CANCEL for that SIP transaction MUST go to the same
server instance of SPP2.
When SPP1 sends the SIP INVITE to SPP2, it SHOULD set the sent-by
parameter of the topmost Via header in the SIP INVITE to a domain
that identifies SPP1. It MUST not specify the port.
4. High Availability
High Availability is ensured by detecting failures in the ability to
connect to SPP1 and SPP2 server instances. In the event of a
failure, when SPP1 tries to send SIP INVITE to SPP2, the following
failures could occur:
4.1 SPP1 Fails to Reach SPP2
A 503 error response is reported by the transaction layer, or failure
can occur at the transport layer due to TCP disconnect in connection,
ICMP error in UDP or time out at transport layer or SIP layer timeout
when its not receiving any SIP response. In such situations, SPP1
tries a new SIP request transaction to the next available server
instance of SPP2 as determined by SRV RRs entry.
4.2 SPP2 Fails to Reach SPP1
Failure may also occur after the request is received by SPP2 from
SPP1 due to closure of the transport connection the request came in
on at SPP2, before the response can be sent back to SPP1. In this
situation, SPP2 uses the domain value present in the 'sent-by'
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parameter in the top most Via header of the received SIP INVITE, and
queries for SRV records at this domain name using the service
identifier "_sips" if the Via transport is "TLS", "_sip" otherwise.
The sorted list of SRV RRs are obtained and used as described in [2]
to send the response back to SPP1. If the topmost element in the
list of server instances of SPP1 fails, the next available one is
tried.
[SHOULD WE ADD CALL FLOW FOR FAILURE SCENARIO DESCRIBED IN 4.1 AND
4.2]
5. Caching/TTL
SRV RRs have a TTL value based on which the SPP1 caches the entry for
that duration and any further requests to the same TARGET domain are
delivered to the cached server instance. The TTL recommended for SRV
is about 1 hr. The TTL for NAPTR is much higher, about 1 day (24hrs)
since the NAPTR records do not vary that often.
6. Acknowledgements
Special thanks go to Jason Livingood and Yiu Lee for their valuable
input to this document.
7. Security Considerations
This document introduces no new security considerations.
8. IANA Considerations
This document creates no new requirements on IANA namespaces
[RFC2434].
9. References
9.1 Normative References
[1] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
Peterson, J., Sparks, R., Handley, M. and E. Schooler, "SIP:
Session Initiation Protocol", RFC 3261, June 2002.
[2] Gulbrandsen, A., Vixie, P. and L. Esibov, "A DNS RR for
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Specifying the Location of Services (DNS SRV)", RFC 2782,
February 2000.
[3] Mealling, M. and R. Daniel, "The Naming Authority Pointer
(NAPTR) DNS Resource Record", RFC 2915, September 2000.
[4] Faltstrom, P. and M. Mealling, "The E.164 to Uniform
Resource Identifiers (URI) Dynamic Delegation Discovery
System (DDDS) Application (ENUM)", RFC 3761, April2004.
[5] Rosenberg, J., Schulzrinne, H., "Session Initiation Protocol
(SIP): Locating SIP Servers", RFC 3263, June 2002.
[6] Meyer, D., "SPEERMINT Terminology", draft-ietf-speermint-
terminology-03, August 2006.
9.2 Informative References
[RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 2434,
October 1998.
Authors Addresses
Tom Creighton
Comcast Cable Communications
1500 Market Street
Philadelphia, PA 19102
USA
Phone: +1-215-320-8617
Email: tom_creighton@cable.comcast.com
Gaurav Khandpur
Comcast Cable Communications
1500 Market Street
Philadelphia, PA 19102
USA
Phone: +1-215-320-5918
Email: gaurav_khandpur@cable.comcast.com
Intellectual Property and Copyright Statements
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