Network Working Group T. Creighton
Internet-Draft Comcast Cable
G. Khandpur
Comcast Cable
Expires: February 15, 2007 October 2006
Use of DNS SRV and NAPTR Records for SPEERMINT
draft-ietf-speermint-srv-naptr-use-01.txt
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Copyright (C) The Internet Society (2006).
Abstract
The objective of this document is to specify the Best Current
Practice (BCP) adopted by a service provider providing multimedia
communication services such as Voice over Internet Protocol(VoIP) in
order to locate another 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 ................................ 6
3.2 NAPTR Lookup ....................................... 6
3.3 SRV Lookup......................................... 6
3.4 Using SRV Results................................... 7
4. High Availability....................................... 7
4.1 SBE1 Fails to Reach SBE2............................. 8
4.2 SBE2 Fails to Reach SBE1............................. 8
5. Caching/TTL............................................ 8
6. Acknowledgements ....................................... 9
7. Security Considerations................................. 9
8. IANA Considerations..................................... 9
9. References............................................. 9
9.1 Normative References ................................ 9
9.2 Informative References............................... 9
Authors Addresses........................................10
Intellectual Property and Copyright Statements................10
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). The diagram below
shows the case of direct peering where a user agent (UA1), hosted by
a service provider SP1, initiates a SIP session to a User Agent
(UA2), hosted by service provider SP2. The egress SIP server of SP1
is a SIP signaling path border element (SBE) as defined in section 3
of[6], called SBE1, that interfaces with session peering service
provider SP2. The SIP session initiated by UA1 is received by this
network element SBE1. The resource to which the SIP request needs to
be routed by SBE1 is identified by a SIP or SIPS URI. This could be
the SIP URI of UA2 found in the Request-URI of the SIP request
received by SBE1, or the next hop from SBE1 found in the topmost
Route header of SIP request. In order to determine the resource to
route the request 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 SBE1 or another network element of SP1.
............................ .............................
. +------+ . . +------+ .
. | | . . | | .
. | SBE 1|--------------| SBE 2| .
. / | | . . | | \ .
. / +------+ . . +------+ \ .
. +------+ / || . . || \ +------+ .
. | | / || . . || \ | | .
. | UA 1 | || . . || | UA 2 | .
. | | || . . || | | .
. +------+ || . . || +------+ .
. +-------+ . . +-------+ .
. | | . . | | .
. | DNS 1 | . . | DNS 2 | .
. | | . . | | .
. +-------+ . . +-------+ .
. . . .
. SP 1 . . SP 2 .
............................ .............................
Figure 1: Logical Peering Scenario (direct peering)
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In order to route the SIP request to this resource in SP2, SBE1 needs
to determine the ingress SIP signaling path border element for SP2,
called SBE2, by resolving the SIP or SIPS URI in DNS. SBE1 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 proxy server (i.e. SBE2). SBE1 and SBE2 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.
As a best current practice, SBE1 and SBE2 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 the case of indirect/transit peering where
SBE2 is the ingress SIP server of a transit service provider. The
mechanism to locate SBE2 is the same as described for direct peering
scenario.
............................ .............. . .
. +------+ . . +------+ . . .
. | | . . | | . . .
. | SBE 1|---------| SBE 2| . . .
. / | | . . | | . . .
. / +------+ . . +------+ . . .
. +------+ / || . . || . . +------+ .
. | | / || . . || . . | | .
. | UA 1 | || . . || . . | UA 2 | .
. | | || . . || . . | | .
. +------+ || . . || . . +------+ .
. +-------+ . . +-------+ . . .
. | | . . | | . . .
. | DNS 1 | . . | DNS 2 | . . .
. | | . . | | . . .
. +-------+ . . +-------+ . . .
. . . . . .
. SP 1 . . Transit SP . . SP 2 .
............................ .............. ...............
Figure 1: Logical Peering Scenario (indirect peering)
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The figure below shows a high level SIP call flow setting up a direct
SIP peering session between SP1 and SP2. In this call flow a VoIP
session is established between a caller, Bob (sip:bob@sp.com), in SP1
and callee, Alice (sip:alice@sp2.com), in SP2 using SIP INVITE
request. All SIP signaling MUST go through the SBE1 and SBE2 as these
are the ingress and egress points in SP1 and SP2 network.
Bob(UA 1) SBE 1 DNS 1 DNS 2 SBE 2 Alice(UA 2)
| | | | | |
|INVITE| | | | |
|----->| | | | |
| NAPTR Query | | |
| |----->| | | |
| NAPTR Response | | |
| |<-----| | | |
| SRV Query | | |
| |----->| | | |
| SRV Response | | |
| |<-----| | | |
| A Query | | |
| |----->| | | |
| A Response | | |
| |<-----| | | |
| | INVITE | |
| |------------------->| |
| | | | |INVITE|
| | | | |----->|
| | | | |200 OK|
| | | | |<-----|
| | 200 OK | |
| |<-------------------| |
|200 OK| | | | |
|<-----| | | | |
| ACK | | | | |
|----->| | | | |
| | ACK | |
| |------------------->| |
| | | | | ACK |
| | | | |----->|
| 2-Way Media |
|<================================>|
| | | | | |
| | | | | |
Figure 2: Example Call Flow
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The target, to which the request is sent, is determined by SBE1 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 request received by SBE1.
The host value of the hostport component of the URI is the TARGET.
This TARGET is the domain to be contacted. The NAPTR/SRV/A lookup as
described in the following section should be skipped if
transport/port/IP address is already specified for the target URI.
3.2 NAPTR Lookup
Next the SBE1 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 SBE2 and SRV records.
Considering our example call flow setup above, SBE1 wishes to resolve
sip:alice@sp2.com and performs a NAPTR query for that TARGET domain
sp2.com, and the following NAPTR records are returned:
; order pref flags service regexp replacement
IN NAPTR 50 50 "s" "SIPS+D2T" "" _sips._tcp.sp2.com
IN NAPTR 90 50 "s" "SIP+D2T" "" _sip._tcp.sp2.com
IN NAPTR 100 50 "s" "SIP+D2U" "" _sip._udp.sp2.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
and indirect peering (section 4.3 in [6]). 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, SIPS over
TLS cannot be used.
3.3 SRV Lookup
Depending on what transport protocols SBE1 supports, SBE1 selects one
from the preference list of NAPTR results and performs the SRV lookup
to obtain a list of available server instances for SBE2. TLS SHOULD
be the preferred transport protocol for peering between SBE1 and
SBE2.
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In our example SBE1 uses TCP, the SRV lookup for _sip._tcp.sp2.com
would return list of available servers :
;; Priority Weight Port Target
IN SRV 0 1 5060 server1.sp2.com
IN SRV 0 2 5060 server2.sp2.com
Alternatively, if no NAPTR records are found, then SBE1 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 as the SRV domain prefix.
In our example, SBE1 prefers to use TCP and target SIP URI of SP2 is
sip:alice@sp2.com, it sends a SRV query for _sip._tcp.sp2.com.
The SRV responses MAY also include A records with it.
3.4 Using SRV Results
If A records are not returned with the SRV responses, procedure from
RFC 2782 describes 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. If the SRV response from DNS
includes A records with it, it 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 SBE1 will try to connect to the (protocol,
address, service). Once the connection is established to an
available instance of SBE2, SBE1 sends the SIP request to SBE2. SBE1
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 SBE2.
When SBE1 sends the SIP request to SBE2, it SHOULD set the sent-by
parameter of the topmost Via header in the SIP request to a domain
that identifies SBE1. It MUST not specify the port.
4. High Availability
High Availability is ensured by detecting failures in the ability to
connect to SBE1 and SBE2 server instances. In the event of a
failure, when SBE1 tries to send SIP INVITE to SBE2, the following
failures could occur:
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4.1 SBE1 Fails to Reach SBE2
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, SBE1
tries a new SIP request transaction to the next available server
instance of SBE2 as determined by SRV RRs entry. The SIP T1 timer on
SBE1 SHOULD be configurable with a upper limit value of 500ms. A
shorter value of T1, say 100ms, reflects a faster failover support.
4.2 SBE2 Fails to Reach SBE1
Failure may also occur after the request is received by SBE2 from
SBE1 due to closure of the transport connection the request came in
on at SBE2, before the response can be sent back to SBE1. In this
situation, SBE2 uses the domain value present in the 'sent-by'
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 SBE1. If the topmost element in the
list of server instances of SBE1 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
5.1 Caching
SBE SHOULD use caching of DNS results to eliminate unnecessary DNS
queries.
5.2 TTL
SRV RRs have a TTL value based on which the SBE1 caches the entry
for that duration, if it supports caching, 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 as compared to SRV.
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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
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.
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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
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