Network Working Group M. Bangalore

Network Working Group M. Bangalore
Request for Comments: 5140 R. Kumar
Category: Standards Track J. Rosenberg
Cisco
H. Salama
Citex Software
D.N. Shah
Moowee Inc.
March 2008

A Telephony Gateway REgistration Protocol (TGREP)

Status of This Memo

This document specifies an Internet standards track protocol for the
Internet community, and requests discussion and suggestions for
improvements. Please refer to the current edition of the "Internet
Official Protocol Standards" (STD 1) for the standardization state
and status of this protocol. Distribution of this memo is unlimited.

Abstract

This document describes the Telephony Gateway Registration Protocol
(TGREP) for registration of telephony prefixes supported by telephony
gateways and soft switches. The registration mechanism can also be
used to export resource information. The prefix and resource
information can then be passed on to a Telephony Routing over IP
(TRIP) Location Server, which in turn can propagate that routing
information within and between Internet Telephony Administrative
Domains (ITADs). TGREP shares a lot of similarities with the TRIP
protocol. It has similar procedures and finite state machine for
session establishment. It also shares the same format for messages
and a subset of attributes with TRIP.

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Table of Contents

1. Introduction ....................................................3
2. Terminology and Definitions .....................................5
3. TGREP: Overview of Operation ....................................6
4. TGREP Attributes ................................................7
4.1. TotalCircuitCapacity Attribute .............................8
4.2. AvailableCircuits Attribute ................................9
4.3. CallSuccess Attribute .....................................10
4.4. Prefix Attributes .........................................12
4.5. TrunkGroup Attribute ......................................13
4.6. Carrier Attribute .........................................15
5. TrunkGroup and Carrier Address Families ........................16
5.1. Address Family Syntax .....................................17
6. Gateway Operation ..............................................18
6.1. Session Establishment .....................................18
6.2. UPDATE Messages ...........................................19
6.3. KEEPALIVE Messages ........................................19
6.4. Error Handling and NOTIFICATION Messages ..................19
6.5. TGREP Finite State Machine ................................19
6.6. Call Routing Databases ....................................19
6.7. Multiple Address Families .................................20
6.8. Route Selection and Aggregation ...........................20
7. LS/Proxy Behavior ..............................................20
7.1. Route Consolidation .......................................22
7.2. Aggregation ...............................................23
7.3. Consolidation versus Aggregation ..........................23
8. Security Considerations ........................................23
9. IANA Considerations ............................................24
9.1. Attribute Codes ...........................................24
9.2. Address Family Codes ......................................24
10. Acknowledgments ...............................................25
11. References ....................................................25
11.1. Normative References .....................................25
11.2. Informative References ...................................26

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1. Introduction

It is assumed that the reader of this document is familiar with TRIP
[2, 12]. In typical Voice over IP (VoIP) networks, Internet
Telephony Administrative Domains (ITADs) will deploy numerous
gateways for the purposes of geographical diversity, capacity, and
redundancy. When a call arrives at the domain, it must be routed to
one of those gateways. Frequently, an ITAD will break its network
into geographic Points of Presence (POPs), with each POP containing
some number of gateways, and a proxy server element that fronts those
gateways. The proxy element is a SIP proxy [11] or H.323 gatekeeper.
The proxy server is responsible for managing the access to the POP,
and also for determining which of the gateways will receive any given
call that arrives at the POP. In conjunction with the proxy server
that routes the call signaling, there are two components, the
"Ingress LS" (aka "TGREP receiver") and the "Egress LS". The TGREP
receiver component maintains TGREP peering relationship with one or
more gateways. The routing information received from the gateways is
further injected into the Egress LS, which in turn disseminates into
the rest of the network on TRIP. For convenience, gateway and GW are
used interchangeably.

This configuration is depicted graphically in Figure 1.

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Signaling TGREP
-------------> <----------------

+---------+
| |
| GW |
> +---------+
//
//
SIP // +---------+
<----> // | |
+-------------------------+ // | GW |
| | // +---------+
| +-------------+ |/
| | | |
| | Routing | | +---------+ TO PSTN
| | Proxy | | | |
---> | | |-----------> | GW | ----->
|+---+-----+ +-----+----+ | +---------+
|| | | | |
|| <+-+ | |--
||Egress LS| |Ingress LS| | --- +---------+
|| | | | | -- | |
|+---------+ +----------+ | -- | GW |
| | -- +---------+
| | -->
+-------------------------+
TRIP +---------+
<----> | |
| GW |
+---------+

Figure 1: Gateway and LS Configuration

The decision about which gateway to use depends on many factors,
including their availability, remaining call capacity, and call
success statistics to a particular Public Switched Telephone Network
(PSTN) destination (see [14]). For the proxy to do this adequately,
it needs to have access to this information in real-time, as it
changes. This means there must be some kind of communications
between the proxy and the gateways to convey this information.

The TRIP protocol [2] is defined for carrying telephony routing
information between providers, for the purposes of getting a call
routed to the right provider for termination to the PSTN. However,
there is no mechanism defined in TRIP that defines how routes get
injected into the TRIP protocol from within the network. Nor does it

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define mechanisms that would allow the provider to select the
specific gateway for terminating a call when it arrives. Those gaps
are filled by TGREP.

TGREP allows PSTN gateways or soft switches to inform a signaling
server, such as a SIP proxy server or H.323 gatekeeper, of routes it
has to the PSTN. These advertisements include fairly dynamic
information, such as the remaining capacity in a particular trunk,
which is essential for selecting the right gateway.

TGREP is identical in syntax and overall operation to TRIP. However,
it differs in the route processing rules followed by the TGREP
receiver, allowing for a route processing function called
"consolidation". Consolidation combines multiple routes for the same
route destination with different attributes to a single route to
prevent loss of routing information. TGREP also defines a set of new
attributes, usable by TGREP or TRIP. Finally, TGREP only utilizes a
subset of overall TRIP capabilities. Specifically, certain
attributes are not utilized (as described below), and the TGREP
entities (the sender and receiver) operate in an asymmetric
relationship, whereas TRIP allows symmetric and asymmetric.

As a general rule, because of a lot of similarities between TRIP and
TGREP, frequent reference will be made to the terminologies and
formats defined in TRIP [2]. It is suggested that the reader be
familiar with the concepts of TRIP like attributes, flags, route
types, address families, etc.

2. Terminology and Definitions

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].

Some other useful definitions are:

Circuit: A circuit is a discrete (specific) path between two or more
points along which signals can be carried. In this context, a
circuit is a physical path, consisting of one or more wires and
possibly intermediate switching points.

Trunk: In a network, a trunk is a communication path connecting two
switching systems used in the establishment of an end-to-end
connection. In selected applications, it may have both its
terminations in the same switching system.

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TrunkGroup: A trunkgroup is a set of trunks, traffic engineered as a
unit, for the establishment of connections within or between
switching systems in which all of the paths are interchangeable
except where subgrouped.

Carrier: A carrier is a company offering telephone and data
communications between points (end-users and/or exchanges).

3. TGREP: Overview of Operation

TGREP is a route registration protocol for telephony destinations on
a gateway. These telephony destinations could be prefixes,
trunkgroups, or carriers. The TGREP sender resides on the GW and
gathers all the information from the GW to relay to the TRIP Location
Server. A TGREP Receiver is defined, which receives this information
and optionally performs operations like consolidation and aggregation
(see Section 7.3), and hands over the reachability information to a
TRIP Location Server. The routing proxy also uses the information to
select the gateway for incoming calls.

The TGREP sender establishes a session to the TGREP receiver using a
procedure similar to session establishment in TRIP. After the
session establishment, the TGREP sender sends the reachability
information in the UPDATE messages. The UPDATE messages have the
same format as in TRIP. However, certain TRIP attributes are not
relevant in TGREP; a TGREP receiver MAY ignore them if they are
present in a TGREP message. The following TRIP attributes do not
apply to TGREP:

1. AdvertisementPath
2. RoutedPath
3. AtomicAggregate
4. LocalPreference
5. MultiExitDisc
6. ITADTopology
7. ConvertedRoute

In addition, TGREP defines the following new attributes in this
document that can be carried in a TGREP UPDATE message.

- TotalCircuitCapacity: This attribute identifies the total number
of PSTN circuits that are available on a route to complete
calls.

- AvailableCircuits: This attribute identifies the number of PSTN
circuits that are currently available on a route to complete
calls.

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- CallSuccess: This attribute represents information about the
number of normally terminated calls out of a total number of
attempted calls.

- Prefix (E164): This attribute is used to represent the list of
E164 prefixes to which the respective route can complete calls.

- Prefix (Decimal Routing Number): This attribute is used to
represent the list of Decimal prefixes to which the respective
route can complete calls.

- Prefix (Hexadecimal Routing Number): This attribute is used to
represent the list of Hexadecimal prefixes to which the
respective route can complete calls.

- TrunkGroup: This attribute enables providers to route calls to
destinations through preferred trunks.

- Carrier: This attribute enables providers to route calls to
destinations through preferred carriers.

In the rest of the document, we specify attributes and address
families used in TGREP. The new attributes and address families
introduced are also applicable for general usage in TRIP except where
noted (AvailableCircuits attribute, for example).

4. TGREP Attributes

Due to its usage within a service provider network, TGREP makes use
of a subset of the attributes defined for TRIP, in addition to
defining several new ones. In particular, the following attributes
from TRIP are applicable to TGREP:

1. WithdrawnRoutes
2. ReachableRoutes
3. NexthopServer
4. Prefix
5. Communities

TGREP also defines several new attributes, described in this section.
These are TotalCircuitCapacity, AvailableCircuits, CallSuccess,
TrunkGroup, and Carrier. As mentioned above, these new attributes
are usable by TRIP unless noted otherwise.

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A TGREP UPDATE supports the following attributes:

1. TotalCircuitCapacity
2. AvailableCircuits
3. CallSuccess
4. Prefix (E.164, Pentadecimal routing number, Decimal routing
number)
5. TrunkGroup
6. Carrier

4.1. TotalCircuitCapacity Attribute

Mandatory: False.
Required Flags: Not well-known.
Potential Flags: None.
TRIP Type Code: 13.

The TotalCircuitCapacity attribute identifies the total number of
PSTN circuits that are available on a route to complete calls. It is
used in conjunction with the AvailableCircuits attribute in gateway
selection by the LS. The total number of calls sent to the specified
route on the gateway cannot exceed this total circuit capacity under
steady state conditions.

The TotalCircuitCapacity attribute is used to reflect the
administratively provisioned capacity as opposed to the availability
at a given point in time as provided by the AvailableCircuits
attribute. Because of its relatively static nature, this attribute
MAY be propagated beyond the LS that receives it.

TotalCircuitCapacity represents the total number of possible calls at
any instant. This is not expected to change frequently. This can
change, for instance, when certain telephony trunks on the gateway
are taken out of service for maintenance purposes.

4.1.1. TotalCircuitCapacity Syntax

The TotalCircuitCapacity attribute is a 4-octet unsigned integer. It
represents the total number of circuits available for terminating
calls through this advertised route. This attribute represents a
potentially achievable upper bound on the number of calls that can be
terminated on this route in total.

4.1.2. Route Origination and TotalCircuitCapacity

Routes MAY be originated containing the TotalCircuitCapacity
attribute.

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4.1.3. Route Selection and TotalCircuitCapacity

The TotalCircuitCapacity attribute MAY be used for route selection.
Since one of its primary applications is load balancing, an LS will
wish to choose a potentially different route (amongst a set of routes
for the same destination), on a call-by-call basis. This can be
modeled as re-running the decision process on the arrival of each
call. The aggregation and dissemination rules for routes with this
attribute ensure that re-running this selection process never results
in propagation of a new route to other peers.

4.1.4. Aggregation and TotalCircuitCapacity

An LS MAY aggregate routes to the same prefix that contains a
TotalCircuitCapacity attribute. It SHOULD add the values of the
individual routes to determine the value for the aggregated route in
the same ITAD.

4.1.5. Route Dissemination and TotalCircuitCapacity

Since this attribute reflects the static capacity of the gateway's
circuit resources, it is not expected to change frequently. Hence,
the LS receiving this attribute MAY disseminate it to other peers,
both internal and external to the ITAD.

4.2. AvailableCircuits Attribute

Mandatory: False.
Required Flags: Not well-known.
Potential Flags: None.
TRIP Type Code: 14.

The AvailableCircuits attribute identifies the number of PSTN
circuits that are currently available on a route to complete calls.
The number of additional calls sent to that gateway for that route
cannot exceed the circuit capacity. If it does, the signaling
protocol will likely generate errors, and calls will be rejected.

The AvailableCircuits attribute is used ONLY between a gateway and
its peer LS responsible for managing that gateway. If it is received
in a route, it is not propagated.

4.2.1. AvailableCircuits Syntax

The AvailableCircuits attribute is a 4-octet unsigned integer. It
represents the number of circuits remaining for terminating calls to
this route.

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4.2.2. Route Origination and AvailableCircuits

Routes MAY be originated containing the AvailableCircuits attribute.
Since this attribute is highly dynamic, changing with every call,
updates MAY be sent as it changes. However, it is RECOMMENDED that
measures be taken to help reduce the messaging load from route
origination. It is further RECOMMENDED that a sufficiently large
window of time be used to provide a useful aggregated statistic.

4.2.3. Route Selection and AvailableCircuits

The AvailableCircuits attribute MAY be used for route selection.
Since one of its primary applications is load balancing, an LS will
wish to choose a potentially different route (amongst a set of routes
for the same prefix) on a call-by-call basis. This can be modeled as
re-running the decision process on the arrival of each call. The
aggregation and dissemination rules for routes with this attribute
ensure that re-running this selection process never results in
propagation of a new route to other peers.

4.2.4. Aggregation and AvailableCircuits

Not applicable.

4.2.5. Route Dissemination and AvailableCircuits

Routes MUST NOT be disseminated with the AvailableCircuits attribute.
The attribute is meant to reflect capacity at the originating gateway
only. Its highly dynamic nature makes it inappropriate to
disseminate in most cases.

4.3. CallSuccess Attribute

Mandatory: False.
Required Flags: Not well-known.
Potential Flags: None.
TRIP Type Code: 15.

The CallSuccess attribute is an attribute used ONLY between a gateway
and its peer LS responsible for managing that gateway. If it is
received in a route, it is not propagated.

The CallSuccess attribute provides information about the number of
normally terminated calls out of a total number of attempted calls.

CallSuccess is to be determined by the gateway based on the
Disconnect cause code at call termination. For example, a call that
reaches the Alerting stage but does not get connected due to the

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unavailability of the called party, or the called party being busy,
is conventionally considered a successful call. On the other hand, a
call that gets disconnected because of a circuit or resource being
unavailable is conventionally considered a failed call. The exact
mapping of disconnect causes to CallSuccess is at the discretion of
the gateway reporting the attribute.

The CallSuccess attribute is used by the LS to keep track of failures
in reaching certain telephony destinations through a gateway(s) and
to use that information in the gateway selection process to enhance
the probability of successful call termination.

This information can be used by the LS to consider alternative
gateways to terminate calls to those destinations with a better
likelihood of success.

4.3.1. CallSuccess Syntax

The CallSuccess attribute is composed of two component fields -- each
represented as a 4-octet unsigned integer. The first component field
represents the total number of calls terminated successfully for the
advertised destination on a given address family over a given window
of time. The second component field represents the total number of
attempted calls for the advertised destination within the same window
of time.

When the value for the total number of attempted calls wraps around,
the counter value for the number of successful calls is reset to keep
it aligned with the other component over a given window of time. The
TGREP receiver using this information should obtain this information
frequently enough to prevent loss of significance.

4.3.2. Route Origination and CallSuccess

Routes MAY be originated containing the CallSuccess attribute. This
attribute is expected to get statistically significant with passage
of time as more calls are attempted. It is RECOMMENDED that
sufficiently large windows be used to provide a useful aggregated
statistic.

4.3.3. Route Selection and CallSuccess

The CallSuccess attribute MAY be used for route selection. This
attribute represents a measure of success of terminating calls to the
advertised destination(s). This information MAY be used to select
from amongst a set of alternative routes to increase the probability
of successful call termination.

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4.3.4. Aggregation and CallSuccess

Not applicable.

4.3.5. Route Dissemination and CallSuccess

Routes MUST NOT be disseminated with the CallSuccess attribute. Its
potential to change dynamically does not make it suitable to
disseminate.

4.4. Prefix Attributes

Mandatory: False.
Required Flags: Not well-known.
Potential Flags: None.
TRIP Type Codes: 16 for E164 Prefix, 17 for Pentadecimal Routing
Number Prefix, and 18 for Decimal Routing Number Prefix.

The Prefix attribute is used to represent the list of prefixes to
which the respective route can complete calls. This attribute is
intended to be used with the Carrier or TrunkGroup address family
(discussed in Section 5).

Though it is possible that all prefix ranges may be reachable through
a given carrier, administrative issues could make certain ranges
preferable to others.

4.4.1. Prefix Attribute Syntax

The Prefix attribute could be E.164, Decimal, or Pentadecimal (refer
to TRIP [2]), each of them having its own type code. The Prefix
attribute is encoded as a sequence of prefix values in the attribute
Value field. The prefixes are listed sequentially with no padding as
shown in Figure 2. Each prefix includes a 2-octet Length field that
represents the length of the Address field in octets. The order of
prefixes in the attribute is not significant.

The presence of the Prefix Attribute with the Length field of the
attribute as 0 signifies that the TGREP GW can terminate ALL prefixes
of that prefix type (E.164, Decimal, or Pentadecimal) for the given
Reachable route(s). This is not equivalent to excluding the Prefix
attribute in the TGREP update.

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< 2 octets > < Length1 octets > < 2 octets > < Length2 octets >

+------------+--------------//--+------------+--------------//--+-
| Length1 | Prefix1 | Length2 | Prefix2 | ...
+------------+--------------//--+------------+--------------//--+-

Figure 2: Prefix Format

4.4.2. Route Origination and Prefix

Routes MAY be originated containing the Prefix attribute.

4.4.3. Route Selection and Prefix

The Prefix attribute MAY be used for route selection.

4.4.4. Aggregation and Prefix

Routes with differing Prefix attributes MUST NOT be aggregated.
Routes with the same value in the Prefix attribute MAY be aggregated
and the same Prefix attribute attached to the aggregated object.

4.4.5. Route Dissemination and Prefix

The LS receiving this attribute should disseminate to other peers,
both internal and external to the ITAD.

4.5. TrunkGroup Attribute

Mandatory: False.
Required Flags: Not well-known.
Potential Flags: None.
TRIP Type Code: 19.

The TrunkGroup attribute is used to represent the list of trunkgroups
on the gateway used to complete calls. It enables providers to route
calls to destinations through preferred trunks. This attribute is
relatively static.

4.5.1. TrunkGroup Syntax

The TrunkGroup attribute is a variable-length attribute that is
composed of a sequence of trunkgroup identifiers. It indicates that
the gateway can complete the call through any trunkgroup identifier
indicated in the sequence.

Each trunkgroup identifier is encoded as a Length-Value field (shown
in Figure 3 below). The Length field is a 1-octet unsigned numeric

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value. The Value field is a variable-length field consisting of two
subfields, a trunkgroup label followed by a trunk context, the two
subfields separated by the delimiter ";" (semicolon). Both the
trunkgroup label and the trunk context subfields are of variable
length. The Length field represents the total size of the Value
field including the delimiter.

The permissible character set for the trunk group label and the
trunkgroup context subfields and the associated ABNF [8] is per rules
outlined in [4].

The presence of the TrunkGroup attribute with the Length field of the
attribute as 0 signifies that the TGREP GW can terminate ALL
trunkgroups for the given Reachable route(s).

< 1 octet > < Length1 octets > < 1 octet > < Length2 octets >

+-----------+--------------//--+-----------+--------------//--+-
| Length1 | TrunkGroup 1 | Length2 | TrunkGroup 2 | ...
+-----------+--------------//--+-----------+--------------//--+-

Figure 3: TrunkGroup Syntax

4.5.2. Route Origination and TrunkGroup

Routes MAY be originated containing the TrunkGroup attribute.

4.5.3. Route Selection and TrunkGroup

The TrunkGroup attribute MAY be used for route selection. Certain
trunkgroups MAY be preferred over others based on provider policy.

4.5.4. Aggregation and TrunkGroup

Routes with differing TrunkGroup attributes MUST NOT be aggregated.
Routes with the same value in the TrunkGroup attribute MAY be
aggregated and the same TrunkGroup attribute attached to the
aggregated object.

4.5.5. Route Dissemination and TrunkGroup

This attribute is not expected to change frequently. Hence, the LS
receiving this attribute MAY disseminate it to other peers, internal
to ITAD. Routes SHOULD not be disseminated external to the ITAD,
with TrunkGroup attribute.

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4.6. Carrier Attribute

Mandatory: False.
Required Flags: Not well-known.
Potential Flags: None.
TRIP Type Code: 20.

The Carrier attribute is used to represent the list of carriers that
the gateway uses to complete calls. It enables providers to route
calls to destinations through preferred carriers. This attribute is
relatively static.

4.6.1. Carrier Syntax

The Carrier attribute is a variable-length attribute that is composed
of a sequence of carrier identifiers. It indicates that the route
can complete the call to any of the carriers represented in the
sequence of carrier identifiers [13].

Each carrier identifier is encoded as a Length-Value field (shown in
Figure 4 below). The Length field is a 1-octet unsigned numeric
value. The Value field is a variable-length field.

The permissible character set for the Value field and the associated
ABNF [8] is per rules outlined in [5]. Specifically, it carries
"global-cic" or "local-cic" [5]. In case of "local-cic", the
"phonedigit-hex" part and the "cic-context" part would be separated
by the delimiter ";". Hence, absence or presence of the delimiter
can be used to determine if the value is a "global-cic" or a "local-
cic". The Length field represents the total size of the Value field
including the delimiter.

The presence of the Carrier attribute with the Length field of the
attribute as 0 signifies that the TGREP GW can terminate ALL Carriers
for the given Reachable route(s).

< 1 octet > < Length1 octets > < 1 octet > < Length2 octets >

+-----------+--------------//--+-----------+--------------//--+-
| Length1 | Carrier 1 | Length2 | Carrier 2 | ...
+-----------+--------------//--+-----------+--------------//--+-

Figure 4: Carrier Syntax

4.6.2. Route Origination and Carrier

Routes MAY be originated containing the Carrier attribute.

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4.6.3. Route Selection and Carrier

The Carrier attribute MAY be used for route selection. Certain
carriers MAY be preferred over others based on provider policy.

4.6.4. Aggregation and Carrier

Routes with differing Carrier attributes MUST NOT be aggregated.
Routes with the same value in the Carrier attribute MAY be aggregated
and the same Carrier attribute attached to the aggregated object.

4.6.5. Route Dissemination and Carrier

This attribute is not expected to change frequently. Hence, the LS
receiving this attribute MAY disseminate it to other peers, both
internal and external to the ITAD.

5. TrunkGroup and Carrier Address Families

As described in TRIP [2], the Address Family field gives the type of
address for the route. Two new address families and their codes are
defined in this section:

Code Address Family
4 TrunkGroup
5 Carrier

When a set of GWs is to be managed at the granularity of carriers or
trunkgroups, then it may be more appropriate for a GW to advertise
routes using the Carrier address family or TrunkGroup address family,
respectively. Also, in a TGREP association between the gateway and
the LS responsible for managing that gateway, there are some
attributes that more naturally fit in as advertised properties of
trunkgroups or carriers rather than that of advertised prefixes, for
example, the AvailableCircuit information on a particular trunkgroup
or a particular carrier. To express this relationship, the existing
TRIP address families are inadequate. We need separate address
families that can associate certain properties like AvailableCircuits
information to trunkgroups or carriers.

The primary benefits of this model are as follows:

- It allows a service provider to route calls based strictly on the
trunkgroups or carriers.

- It facilitates more accurate reporting of attributes of a dynamic
nature like AvailableCircuits by providing the ability to manage
resources at the granularity of a trunkgroup or a carrier.

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- It enables scalability as gateways can get really large with the
ability to provision hundreds or a few thousand circuits, and this
can increase the potential for traffic that reports dynamic
resource information between the gateway and the LS. The model
introduced can potentially alleviate this UPDATE traffic, hence
increasing efficiency and providing a scalable gateway registration
model.

5.1. Address Family Syntax

Consider the generic TRIP route format from TRIP [2] shown below.

0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+---------------+---------------+---------------+---------------+
| Address Family | Application Protocol |
+---------------+---------------+---------------+---------------+
| Length | Address (variable) ...
+---------------+---------------+---------------+---------------+

Figure 5: Generic TRIP Route Format

The Address Family field will be used to represent the type of the
address associated for this family, which is based on the TrunkGroup
or Carrier. The codes for the new address families have been
allocated by IANA.

The code for the TrunkGroup address family is 4, and the code for the
Carrier address family is 5.

The Application Protocol field is the same as the one for the
Decimal, Pentadecimal, and E.164 address families defined in TRIP
[2]. The Length field represents the total size of the Address field
in bytes.

The value in the Address field represents either the TrunkGroup or
Carrier address family, and the syntax is as follows:

For the TrunkGroup address family, the Address field represents a
TrunkGroup value that is defined as specified in Section 4.5.1,
"TrunkGroup Syntax".

For the Carrier address family, the Address field represents a
Carrier value. This is the same as the Value field specified in an
earlier Section 4.6.1, "Carrier Syntax".

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The above mentioned address families are not hierarchical, but flat.
If a gateway supports any of these address families, it should
include that address family as one of the Route types supported in
the OPEN message capability negotiation phase.

The following attributes are currently defined to be used with all
the address families including the TrunkGroup and Carrier address
families.

- AvailableCircuits attribute
- TotalCircuitCapacity attribute
- CallSuccess attribute

It is recommended that the above three attributes be used by the
gateway with the TrunkGroup or Carrier address family, if possible.
This will potentially offer a more efficient resource reporting
framework, and a scalable model for gateway provisioning.

However, when the gateway is not using the TrunkGroup or Carrier
address family, it MAY use the above attributes with the Decimal,
Pentadecimal, and E.164 address families.

The Prefix attribute cannot be used when the address family is E164
numbers, Pentadecimal routing numbers, or Decimal routing numbers.

The Carrier attribute cannot be used if the address family type is
Carrier.

The TrunkGroup attribute cannot be used if the address family type is
TrunkGroup.

6. Gateway Operation

A gateway uses TGREP to advertise its reachability to its domain's
Location Server(s) (LS, which are closely coupled with proxies). The
gateway operates in TRIP Send Only mode since it is only interested
in advertising its reachability, but is not interested in learning
about the reachability of other gateways and other domains. Also,
the gateway will not create its own call routing database. In this
section, we describe the operation of TGREP on a gateway.

6.1. Session Establishment

When opening a peering session with a TGREP receiver, a TGREP gateway
follows exactly the same procedures as any other TRIP entity. The
TGREP gateway sends an OPEN message that includes a Send Receive
Capability in the Optional Parameters. The Send Receive Capability

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is set by the gateway to Send Only. The OPEN message also contains
the address families supported by the gateway. The remainder of the
peer session establishment is identical to TRIP.

6.2. UPDATE Messages

Once the peer session has been established, the gateway sends UPDATE
messages to the TRIP LS with the gateway's entire reachability. The
gateway also sends any attributes associated with the routes.

TGREP processing of the UPDATE message at the gateway is identical to
UPDATE processing in TRIP [2]. A TGREP sender MUST support all
mandatory TRIP attributes.

6.3. KEEPALIVE Messages

KEEPALIVE messages are periodically exchanged over the peering
session between the TGREP gateway and the TRIP LS as specified in
Section 4.4 of TRIP [2].

6.4. Error Handling and NOTIFICATION Messages

The same procedures used with TRIP are used with TGREP for error
handling and generating NOTIFICATION messages. The only difference
is that a TGREP gateway will never generate a NOTIFICATION message in
response to an UPDATE message, irrespective of the contents of the
UPDATE message. Any UPDATE message is silently discarded.

6.5. TGREP Finite State Machine

When the TGREP finite state machine is in the Established state and
an UPDATE message is received, the UPDATE message is silently
discarded and the TGREP gateway remains in the Established state.
Other than that, the TRIP finite state machine and the TGREP finite
state machine are identical.

6.6. Call Routing Databases

A TGREP gateway may maintain simultaneous sessions with more than one
TRIP LS. A TGREP gateway maintains one call routing database per
peer TRIP LS. These databases are equivalent to TRIP's Adj-TRIBs-
Out, and hence we will call them Adj-TRIBs-GW-Out. An Adj-TRIB-GW-
Out contains the gateway's reachability information advertised to its
peer TRIP LS. How an Adj-TRIB-GW-Out database gets populated is
outside the scope of this document (possibly by manual
configuration).

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The TGREP gateway does not have databases equivalent to TRIP's
Adj-TRIBs-In and Loc-TRIB, because the TGREP gateway does not learn
routes from its peer TRIP LSs, and hence it does not run call route
selection.

6.7. Multiple Address Families

As mentioned above, TGREP supports various address families in order
to convey the reachability of telephony destinations. A TGREP
session MUST NOT send UPDATEs of more than one of the following
categories (a) Prefix address families (E164, Pentadecimal, and
Decimal), (b) TrunkGroup address family, or (c) Carrier address
family for a given established session. TGREP should specify its
choice address family through the route-type capability in the OPEN
message. And route-type specification in the OPEN message violating
the above rule should be rejected with a NOTIFICATION message.

6.8. Route Selection and Aggregation

TRIP's route selection and aggregation operations MUST NOT be
implemented by TGREP gateways.

7. LS/Proxy Behavior

As mentioned earlier, TGREP can be considered as a protocol
complimentary to TRIP in providing reachability information, which
can then be further fed into the Location Server. The architecture
of an LS/Proxy system is as follows: There exists a TRIP LS
application that functions as a speaker in the I-TRIP/E-TRIP network
as documented in TRIP [2]. This component is termed as "Egress LS"
for the purposes of this discussion. Then, there is a signaling
server fronting a set of gateways. In conjunction with this
signaling server is also a second component operating in receive
mode, which peers with one or more gateways, each of them using TGREP
to advertise routing information. This component on the receiving
end of one or more TGREP sessions is termed as the "Ingress LS" or
"TGREP receiver" for the purposes of this discussion. Also, the
entity (typically, a gateway) advertising the routes on the TGREP
session is termed as the "TGREP sender". The TGREP receiver
receiving the TRIP messages takes the resulting routing information
from each gateway, and "exports" it to another process we define
below, that performs consolidation and aggregation, in that order.
These operations would take as input the collective set of routes
from all the gateways. Subsequently, the resulting TRIB is passed as
input into the LS-Egress process as shown below, that can then
disseminate these via TRIP. The interface between the TGREP receiver
(aka Ingress LS) peering with the GW(s) and the TRIP LS (Egress LS)
is entirely a local matter.

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The nature of the consolidation and aggregation operations and the
accompanying motivation are described in the subsections below. The
order in which the operations are listed represents an implicit
logical sequence in which they are applied. The architecture for an
LS/Proxy entity is shown in Figure 6 below.

+-------------------------------------------------------+
| +-------------------------------+ |
| | +-+ +-+ | | TGREP
| | |A| |C| | | +-----+
| | |g| |o| | | | |
| +-------------+ | |g| |n| +-------------+ | | --| GW |
| | | | |r| |s| | | | | +-----+
| | TRIP | | |e| |o| | | | +---
| | LS <----------|g<--|l<--- TGREP |-++-| +-----+
| | | | |a| |i| | Session | | | | |
| | (I-TRIP/ | | |t| |d| | Management |-++-+----| GW |
| | E-TRIP) | | |i| |a| | | | | +-----+
| | (Egress LS) | | |o| |t| | |-+ +---
| +-----------/-+ | |n| |i| +-------------+ | | +-----+
| / | | | |o| | | --| |
| / | | | |n| (Ingress LS) | | | GW |
| / | +-+ +-+ | | +-----+
| / | TGREP Receiver | |
| / +-------------------------------+ |
| / |
| / |
+-------/-----------------------------------------------+
/ LS/Proxy
/
/
/
/
/
+/----------------+
| |
| |
| |
| LS |
| |
| |
| |
| |
| |
+-----------------+

Figure 6: LS Architecture for TRIP-GW

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7.1. Route Consolidation

The TGREP receiver (Ingress LS) may receive routing information from
one or more gateways. It is possible that multiple routes are
available for the same destination. These different alternative
routes may be received from the same gateway or from multiple
gateways. It is RECOMMENDED that the set of gateway routes for each
destination be consolidated, before presenting a candidate route, to
the Egress LS entity. The motivation for this operation should be to
define a route that can maximally represent the collective routing
capabilities of the set of gateways, managed by this TGREP receiver.
Let us take an example scenario in order to bring out the motivation
for this operation. Let us say, the TGREP receiver maintains peering
sessions with gateways A and B.

- Gateway A advertises a route for destination "SIP 408" on the
E.164 address family with the Carrier attribute value C1.

- Gateway B advertises a route for destination "SIP 408" on the
E.164 address family with Carrier attribute value C2.

The TGREP receiver that receives these routes can consolidate these
constituent routes into a single route for destination "SIP 408" with
its Carrier attribute being a union of the Carrier attribute values
of the individual routes, namely, "C1 C2". This operation is
referred to as consolidation. In the above example, it is possible
that a route to the destination "SIP 408" through one or more
carriers may have been lost if the individual routes were not
consolidated.

Another example is to consolidate the Prefix attribute from multiple
Carrier or TrunkGroup updates received from different gateways for
the same destination. Let us say, there are Carrier address family
(AF) updates from two gateways for Carrier destination X, and the
prefix attribute values are {408, 650} from one update and {919, 973}
from the other. The prefix values from these two updates can be
consolidated into a single Carrier AF route advertisement with prefix
value {408, 650, 919, 973}.

In general, there is a potential for loss of gateway routing
information when TGREP routes from a set of gateways are not
consolidated when a candidate route is presented to the TRIP LS. The
specifics of applying the consolidation operation to different
attributes and routes from different address families is left to the
individual TGREP receiver implementations.

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7.2. Aggregation

The set of gateway routes, which are in a consolidated form or
otherwise, may be aggregated before importing it to the LS instance
that is responsible for I-TRIP/E-TRIP processing (Egress LS). This
operation follows the standard aggregation procedures described in
TRIP [2], while adhering to the aggregation rules for each route
attribute.

7.3. Consolidation versus Aggregation

To highlight the difference between the two operations discussed
above, "consolidation" combines multiple routes for the same route
destination, whereas "aggregation" combines routes for different
route destinations that qualify as candidates to be summarized
resulting in route information reduction.

To take an example, if there are multiple gateways offering routes to
an E.164 destination "408" but with possibly different attributes
(e.g., Carrier), the LS/Proxy can combine these to form one route for
"408" but representing the attribute information collectively. This
process is consolidation.

If, for example, the LS/Proxy receives routes for 4080, 4081, 4082,
... 4089 from amongst a set of gateways, it could aggregate these
different candidate routes to have a summarized route destination
"408" with each of the attributes computed using the aggregation
procedures defined in TRIP.

8. Security Considerations

The security considerations for TGREP are identical to that
identified in TRIP [2] and are just restated here for the purposes of
clarity.

The security mechanism for the peering session between TGREP GW and a
TRIP LS, in an IP network, is IPsec [3]. IPsec uses two protocols to
provide traffic security: Authentication Header (AH) [6] and
Encapsulating Security Payload (ESP) [7].

The AH header affords data origin authentication, connectionless
integrity, and optional anti-replay protection of messages passed
between the peer LSs. The ESP header provides origin authentication,
connectionless integrity, anti-replay protection, and confidentiality
of messages.

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Implementations of the protocol defined in this document employing
the ESP header SHALL comply with Section 3.1.1 of [10], which defines
a minimum set of algorithms for integrity checking and encryption.
Similarly, implementations employing the AH header SHALL comply with
Section 3.2 of [10], which defines a minimum set of algorithms for
integrity checking.

Implementations SHOULD use the Internet Key Exchange Protocol (IKEv2)
[9] to permit more robust keying options. Implementations employing
IKEv2 SHOULD support 3DES-CBC for confidentiality and HMAC-SHA1 for
integrity.

A Security Association (SA) [3] is a simplex "connection" that
affords security services to the traffic carried by it. Security
services are afforded to an SA by the use of AH or ESP, but not both.
Two types of SAs are defined: transport mode and tunnel mode. A
transport mode SA is a security association between two hosts, and is
appropriate for protecting the TRIP session between two peer LSs.

9. IANA Considerations

Both TRIP [2] and TGREP share the same IANA registry for
Capabilities, Attributes, Address Families, and Application
Protocols. IANA has added the following attribute codes and address
family codes to the TRIP [2] registries.

9.1. Attribute Codes

The Attribute Type Codes assigned for the new attributes defined in
this document are listed below:

Code Attribute Reference
---- --------- ---------
13 TotalCircuitCapacity [RFC5140]
14 AvailableCircuits [RFC5140]
15 CallSuccess [RFC5140]
16 E.164 Prefix [RFC5140]
17 Pentadecimal Routing Number Prefix [RFC5140]
18 Decimal Routing Number Prefix [RFC5140]
19 TrunkGroup [RFC5140]
20 Carrier [RFC5140]

9.2. Address Family Codes

The following subsections show the codes that have been assigned for
the two new address families introduced in this document.

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9.2.1. TrunkGroup Address Family

Code Address Family Reference
---- -------------- ---------
4 TrunkGroup [RFC5140]

9.2.2. Carrier Address Family

Code Address Family Reference
---- -------------- ---------
5 Carrier [RFC5140]

10. Acknowledgments

We wish to thank Vijay Gurbani, Li Li, Kevin McDermott, David Oran,
Bob Penfield, Jon Peterson, Anirudh Sahoo, and James Yu for their
insightful comments and suggestions.

11. References

11.1. Normative References

[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.

[2] Rosenberg, J., Salama, H., and M. Squire, "Telephony Routing
over IP (TRIP)", RFC 3219, January 2002.

[3] Kent, S. and K. Seo, "Security Architecture for the Internet
Protocol", RFC 4301, December 2005.

[4] Gurbani, V. and C. Jennings, "Representing Trunk Groups in
tel/sip Uniform Resource Identifiers (URIs)", RFC 4904, June
2007.

[5] Yu, J., "Number Portability Parameters for the "tel" URI", RFC
4694, October 2006.

[6] Kent, S., "IP Authentication Header", RFC 4302, December 2005.

[7] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC 4303,
December 2005.

[8] Crocker, D., Ed., and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", STD 68, RFC 5234, January 2008.

[9] Kaufman, C., Ed., "Internet Key Exchange (IKEv2) Protocol", RFC
4306, December 2005.

Bangalore, et al. Standards Track [Page 25]

RFC 5140 TGREP March 2008

[10] Manral, V., "Cryptographic Algorithm Implementation Requirements
for Encapsulating Security Payload (ESP) and Authentication
Header (AH)", RFC 4835, April 2007.

11.2. Informative References

[11] 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.

[12] Rosenberg, J. and H. Schulzrinne, "A Framework for Telephony
Routing over IP", RFC 2871, June 2000.

[13] ITU-T List of ITU Carrier Codes (published periodically in the
ITU-T Operational Bulletin).

[14] Rosenberg, J., "Requirements for Gateway Registration", Work in
Progress, November 2001.

Bangalore, et al. Standards Track [Page 26]

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Authors' Addresses

Manjunath S. Bangalore
Cisco
Mail Stop SJC-14/2/1
3625 Cisco Way
San Jose, CA 95134
Phone: +1-408-525-7555
EMail: [email protected]

Rajneesh Kumar
Cisco
Mail Stop SJC-14/4/2
3625 Cisco Way
San Jose, CA 95134
Phone: +1-408-527-6148
EMail: [email protected]

Jonathan Rosenberg
Cisco
Edison, NJ 08837
EMail: [email protected]

Hussein F. Salama
Citex Software
Giza, Egypt
EMail: [email protected]

Dhaval Niranjan Shah
Moowee Inc.
4920 El Camino Real
Los Altos, CA 94022
Phone: +1-408-307-7455
EMail: [email protected]

Bangalore, et al. Standards Track [Page 27]

RFC 5140 TGREP March 2008

Full Copyright Statement

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Bangalore, et al. Standards Track [Page 28]