IPTEL Working Group Manjunath Bangalore, Cisco Systems
Internet Draft Rajneesh Kumar, Cisco Systems
draft-ietf-iptel-trip-gw-00.txt Jonathan Rosenberg, dynamicsoft
June 2002 Hussein Salama, Cisco Systems
Expiration Date: December 2002 Dhaval N. Shah, Cisco Systems
Usage of TRIP in Gateways for Exporting Phone Routes
Status of this Memo
This document is an Internet-Draft and is in full conformance with
all provisions of Section 10 of RFC2026.
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Abstract
This document describes a new application of the Telephony Routing
over IP (TRIP) protocol. TRIP was engineered as a tool for inter-
domain exchange of telephone routing information. However, it can
also be used as a means for gateways and soft switches to export
their routing information to a Location Server (LS), which may be
co-resident with a proxy or gatekeeper. This LS can then manage those
gateway resources. We discuss the motivations for this application,
and then show how a subset of TRIP is needed for this application.
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1. 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:
Trunk: In a network, 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.
TrunkGroup: 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: An organization that provides connections for telephony
services between end-users and/or exchanges.
2. Introduction
In typical 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 their network into geographic POPs,
with each POP containing some number of gateways, and a proxy server
element that fronts those gateways. 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.
This configuration is depicted graphically in Figure 1.
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+---------+
| |
| GW |
-> +---------+
//
//
// +---------+
// | |
// | GW |
// +---------+
//
+----------+ // TO PSTN
| | / +---------+
| Routing | ---------------> | | ----->
-------->| Proxy | | GW |
| | -- +---------+
| | --
+----------+ --
--- +---------+
-- | |
-- | GW |
-- +---------+
-->
+---------+
| |
| 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 particular POTS destination. 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.
In this document, we specify a usage of TRIP to communicate this
information from the gateways to the location servers associated with
the proxies that make call routing decisions. Section 3 looks at TRIP
[4,5]. We then describe the details of a TRIP solution in Section 4.
It is assumed that the reader is familiar with TRIP.
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3. TRIP
TRIP was engineered as a tool for interdomain route exchange. At
first glance, it may not seem appropriate for application in a
gateway. However, TRIP provides exactly the features needed to solve
the problem at hand. TRIP allows one entity (in this case, a gateway)
to convey to another (in this case, a proxy) a set of telephony
routes which terminate through it. These routes are represented by
telephony addressing information along with attributes that can
express resource availability and preferences. In TRIP, the routing
tables are exchanged once. Only when they change are updates sent.
This is exactly the capability needed for a gateway to send its
routing information to a proxy, and hence allows TRIP to be leveraged
for this purpose. Since routing information is sent when it changes,
using TRIP does not require communications on the critical path of
call setup signaling.
TRIP also supports a keepalive between peers. This keepalive is a
short message, sent fairly frequently. It does not contain routing
information. The period of the keepalive is negotiated once at
startup time. If one of the entities crashes, the other flushes all
routes received from it.
TRIP can contain attributes describing a route. New attributes can
easily be added. The available capacity of a route is needed by the
proxies to properly load balance and route to a set of gateways. This
capacity can be expressed as an attribute.
TRIP can be run over IPSec or TLS between two peers, providing
authentication, integrity and privacy.
Another advantage of using TRIP here is that it makes the
redistribution of local gateway reachability information into inter-
domain TRIP relatively simple (refer to Figure 6), because it's the
same protocol.
While it is true that TRIP is complex, almost all of this complexity
is related to the processing of routes *received* from other peers.
An element, such as a gateway, which only *sends* routes to a peer
(the proxy), does not need to implement any of those functions. In
particular, any processing related to aggregation, TRIB updates,
route propagation and advertisement, handling of transitivity and
unknown routes, or the decision process need not be implemented. The
resulting set of functions are very small. They are composed of only:
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- The initial OPEN phase, exchange of keepalive timers, and the
process of bringing up the state machine.
- Sending of one or more UPDATE messages containing the routes and
parameters of the gateways.
- Sending of a periodic keepalive.
4. Defining TRIP-GW
We call the subset of TRIP needed for this application "TRIP-GW",
"TRIP-Lite", or TRIP for gateways. We note that this is a valid
subset defined by the specification, so that a TRIP-GW speaker is a
conformant TRIP speaker. In this section, we include the various
attributes, some of them new and also introduce two new address
families. The gateway and proxy behaviors are discussed in more
details in sections 4.9 and 4.10 respectively.
4.1. AvailableCircuits Attribute
Mandatory: False.
Required Flags: optional, non-transitive
Potential Flags: None.
TRIP Type Code: To be assigned by IANA.
The AvailableCircuits attribute is used ONLY between a gateway and
its peer LS responsible for managing that gateway. It is for this
reason that this attribute is non-transitive. If it is received in a
route, it MUST NOT be propagated unless the LS is sure that it is
relatively static.
The AvailableCircuits 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.
AvailableCircuits is measured in integral number of calls. It is very
dynamic.
4.1.1. AvailableCircuits Syntax
The AvailableCircuits attribute is a 4-octet unsigned numeric value.
It represents the number of circuits remaining for terminating calls
to this route.
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4.1.2. 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 a
gateway originating routes with this attribute use thresholds, and
that routes are re-originated only when the attribute moves above or
below a threshold. It is also RECOMMENDED that the thresholds in each
direction (going above a threshold and going below a threshold) be
different, thus achieving a form of hysterisis. Both of these
measures help reduce the messaging load from route origination.
4.1.3. 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.
4.1.4. Aggregation and AvailableCircuits
Not applicable
4.1.5. 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.
4.2. TotalCircuitCapacity Attribute
Mandatory: False.
Required Flags: optional, transitive
Potential Flags: None.
TRIP Type Code: To be assigned by IANA.
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
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attribute. Because of its relatively static nature, this attribute
MAY be propogated beyond the LS that receives it, hence making this
attribute transitive.
The TotalCircuitCapacity 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.
TotalCircuitCapacity is measured in integral number of calls. 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.
4.2.1. TotalCircuitCapacity Syntax
The TotalCircuitCapacity attribute is a 4-octet unsigned numeric
value. 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 which can be terminated on this route in total.
4.2.2. Route Origination and TotalCircuitCapacity
Routes MAY be originated containing the TotalCircuitCapacity
attribute.
4.2.3. 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.
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4.2.4. Aggregation and TotalCircuitCapacity
An LS MAY aggregate routes to the same prefix which contain a
TotalCircuitCapacity attribute. It SHOULD add the values of the
individual routes to determine the value for the aggregated route in
the same ITAD.
4.2.5. 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.
4.3. CallSuccess Attribute
Mandatory: False.
Required Flags: optional, non-transitive
Potential Flags: None.
TRIP Type Code: To be assigned by IANA.
The CallSuccess attribute is a non-transitive attribute used ONLY
between a gateway and its peer LS responsible for managing that
gateway. If it is received in a route, it MUST NOT be 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 because of
called party being unavailable is conventionally considered a
successful call. Similar is the case when the called party is busy.
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
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.
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4.3.1. CallSuccess Syntax
The CallSuccess attribute is comprised of two component fields - each
represented as an unsigned 4-octet numeric value. The first
component field represents the total number of calls terminated
normally for the advertised destination on a given address family.
The second component field represents the total number of attempted
calls for the advertised destination.
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. Therefore, it is RECOMMENDED
that the gateway make a choice based on local thresholds to determine
when to report this attribute in an UPDATE.
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.
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 in most cases.
4.4. Prefix Attributes
Mandatory: Conditional Mandatory (if ReachableRoutes is present).
Required Flags: Well-known.
Potential Flags: None.
TRIP Type Codes: To be assigned by IANA.
The Prefix attribute is used to represent the list of prefixes that
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the respective route can complete calls to. This attribute is
intended to be used with the Carrier or Trunkgroups address family
(discussed in a later section).
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 RFC [4]), each of them having it's 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.
1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 . . . 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4
+-------------------------------+-----------+-------------------------------+-----------
| Length | Prefix1...| Length | 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 attribute 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.
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4.4.5. Route Dissemination and Prefix
The LS receiving this attribute should disseminate it to other peers,
both internal and external to the ITAD.
4.5. TrunkGroups Attribute
Mandatory: False.
Required Flags: optional, transitive
Potential Flags: None.
TRIP Type Code: To be assigned by IANA.
The TrunkGroups attribute is used to represent trunkgroups on the
gateway that the gateway can complete the calls to. It allows
providers to route calls to destinations through preferred trunks.
This attribute is relatively static.
4.5.1. TrunkGroups Syntax
The TrunkGroups attribute is a variable length attribute that is
composed of a sequence of trunkgroup length-value fields. It
indicates that the gateway can complete the call through any
trunkgroup in the sequence.
Each trunkgroup is a length-value field (shown in Figure 3 below).
The length field is a 1-octet unsigned numeric value. The value field
is a variable length alphanumeric field and is also called the
trunkgroup label field. The length field represents the size of the
trunkgroup label in number of octets. The maximum length is 128
octets.
0 1
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 ... 7 8 9 0 1 2 3 4 5 ...
+---------------+--------------------+---------------+---------------------
| Length | TrunkGroup Label...| Length | TrunkGroup Label...
+---------------+--------------------+---------------+---------------------
Figure 3: TrunkGroups Syntax
4.5.2. Route Origination and TrunkGroups
Routes MAY be originated containing the TrunkGroups attribute.
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4.5.3. Route Selection and TrunkGroups
The TrunkGroups attribute MAY be used for route selection. Certain
trunkgroups MAY be preferred over others based on provider policy.
4.5.4. Aggregation and TrunkGroups
Routes with differing TrunkGroups attribute MUST NOT be aggregated.
Routes with the same value in the TrunkGroups attribute MAY be
aggregated and the same TrunkGroups attribute attached to the
aggregated object.
4.5.5. Route Dissemination and TrunkGroups
This attribute is not expected to change frequently. Hence, the LS
receiving this attribute MAY disseminate it to other peers, internal
to ITAD. Routes MUST not be disseminated external to the ITAD, with
TrunkGroups attribute.
4.6. Carrier Attribute
Mandatory: False.
Required Flags: optional, transitive
Potential Flags: None.
TRIP Type Code: To be assigned by IANA.
The Carrier attribute is used to represent the list of Carrier
Identification Codes(CIC's) that the gateway can complete the calls
to. It enables providers to route calls to destinations through
preferred carriers. A CIC represents a unique code assigned to the
carrier or telephony service provider offering the service The CIC
definition is as defined in the ITU specification referenced
below[8,9].
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 identification codes. It indicates that the
route can complete the call to any of the carriers represented by the
carrier identification codes in the sequence.
Each carrier identification code in the sequence is a fixed length
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2-octet unsigned numeric value as 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 2 3
+-------------------------------+-------------------------------+-----
| CarrierIdCode1 | CarrierIdCode2 | ...
+-------------------------------+-------------------------------+-----
Figure 4: Carrier Syntax
4.6.2. Route Origination and Carrier
Routes MAY be originated containing the Carrier attribute.
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 attribute 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.
4.7. TrunkGroup and Carrier Address Families
When a set of GWs are to managed at the granularity of carriers or
trunkgroups then it may be more appropriate for a GW can advertise
routes using the Carrier address family or trunkgroup address family
respectively. Also, in a TRIP-GW 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
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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.
- 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.
4.7.1. Address Family Syntax
Consider the generic TRIP route format from TRIP[4] 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 will be allocated
by IANA.
The Application Protocol field is same as the one for the Decimal,
PentaDecimal and E.164 address families defined in TRIP[4]. The
Length field represents the total size of the Address field in bytes.
The value in the Address field represents either the TrunkGroup or
the Carrier address families and the syntax is as follows:
For the TrunkGroup Address Family, the Address field is a variable
length alphanumeric field (trunkgroup label), where length is
determined by the length field of the route. The maximum value of the
length field for this Address Family is 128 bytes.
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For the Carrier Address Family, the length field represents the
length of the Address field in bytes. The Address field is a fixed
length field that consists of the CarrierIdCode (CIC). Again, the
CIC is as defined in ITU specification M.400[8]. CIC is a fixed
length 2-octet unsigned numeric value as shown in Figure 4 above.
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 families, 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 TrunkGroup or Carrier
address family, it MAY use the above attributes with the Decimal,
PentaDecimal and E.164 address families.
The Prefix attributes MUST NOT be used with the Prefix address
families.
The Carrier attribute MUST NOT be used with the Carrier and
TrunkGroup address families.
The TrunkGroups attribute MUST NOT be used with the TrunkGroup
and Carrier address families.
4.8. Other attribute considerations
4.8.1. Cost/Pricing attribute
In exploring attributes suitable for the GW-LS communications,
Pricing is another attribute that was considered. Though at first
glance, it seems like a useful piece of information to be advertised
by the gateway to express the price offered by carriers to different
destinations, in reality, the computation of pricing can get quite
complex. For example, the price offered by a provider can vary by
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time of day, it can be based on an agreement between two service
providers interconnecting with each other, it could be based on one
price for the first 'n' minutes and a different price after that,
Least Cost routing choices and Grades of service offered by a carrier
can affect pricing. There could be other factors as well. Expressing
this complex interplay between different factors that determine
pricing is non-trivial to represent. It will be a subject of further
investigation to determine whether advertising pricing associated
with carriers in its simple form without any dependencies adds value
to be included as an attribute in the TRIP-GW communications.
4.9. Gateway Operation
A gateway uses TRIP 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. Therefore, the
gateway does not need a complete implementation of TRIP. A
lightweight version of the protocol is sufficient. In this section we
describe the operation of TRIP on a gateway.
4.9.1. Session Establishment
When opening a peering session with a TRIP LS, a TRIP-GW gateway
follows exactly the same procedures as any other TRIP speaker. The
TRIP-GW gateway sends an OPEN message which includes a Send Receive
Capability in the Optional Parameters. The Send Receive Capability is
set by the gateway to Send Only. The OPEN message also contains the
address families supported by the gateway. When the TRIP LS receives
the gateway's OPEN message, it sets its local policy such that no
UPDATE messages are sent to the TRIP-GW gateway. The remainder of the
peer session establishment is identical to TRIP.
4.9.2. 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.
If the gateway's reachability changes at any point in time, the
gateway MUST generate UPDATE message(s) with the change. The
frequency of successive UPDATE messages MUST follow the same rules
specified for TRIP[4]. The TRIP-GW gateway MUST support all
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mandatory TRIP attributes.
If the gateway receives an UPDATE message from the TRIP LS, it MUST
silently discard it as specified for TRIP[4]. No further action
should be taken.
4.9.3. KEEPALIVE Messages
KEEPALIVE messages are periodically exchanged over the peering
session between the TRIP-GW gateway and the TRIP LS as specified in
Section 4.4 of TRIP RFC[4].
4.9.4. Error Handling and NOTIFICATION Messages
The same procedures used with TRIP, are used with TRIP-GW for error
handling and generating NOTIFICATION messages. The only difference is
that a TRIP-GW 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.
4.9.5. TRIP-GW Finite State Machine
When the TRIP-GW finite state machine is in the Established state and
an UPDATE message is received, the UPDATE message is silently
discarded and the TRIP-GW gateway remains in the Established state.
Other than that the TRIP finite state machine and the TRIP-GW finite
state machine are identical.
4.9.6. Call Routing Databases
A TRIP-GW gateway may maintain simultaneous sessions with more than
one TRIP LSs. A TRIP-GW 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-TRIB-GWs-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 draft (possibly by manual
configuration).
The TRIP-GW gateway does not have databases equivalent to TRIP's
Adj-TRIBs-In and Loc-TRIB, because the TRIP-GW gateway does not learn
routes from its peer TRIP LSs, and hence it does not run call route
selection.
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4.9.7. Route Selection and Aggregation
TRIP's route selection and aggregation operations MUST NOT be
implemented by TRIP-GW gateways.
4.10. LS/Proxy Behavior
TRIP completely specifies the behavior of the LS as a TRIP speaker.
However, the primary question is: is an LS connected to a gateway an
internal or external peer of the gateway?
The most obvious choice, internal peer, is not the best choice. If
an LS has ten peer GWs, all of them advertising reachability to
1408*, it will flood all ten routes to all other LSs in the same
ITAD. This won't scale, because each LS in the ITAD will have to
create a separate Adj-TRIB-In for each GW in that ITAD. In addition,
it doesn't allow a SIP Proxy server or a H.323 GK to load balance
among the GWs of its zone/subdomain.
A similar problem exists when an LS is an external peer to the
gateways, and has direct peering relationships with one or more
internal peers. However, an ingress LS to an ITAD does not perform
aggregation. Only the egress LS aggregates routes.
Therefore, it is RECOMMENDED that the LS actually run two instances
of TRIP; one with an external peering relationship to the gateways,
and the other with an internal peering relationship with one or more
LS's within the ITAD. The interface between these instances is
largely a local matter; routes are exported from one and imported to
the other. In the process of exporting routes from the GW instance,
it may be useful to consolidate the routes before importing them to
the other LS instance. Some details and motivation for this operation
are provided below. In addition, the routes may also be aggregated.
This architecture is shown in Figure 6.
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. .
. .
. +----------------.---------------------------+
. | .+-+ +-+ |
. | .|A| |C| | +-----+
. | .|g| |o| | | |
. |+-------------+ .|g| |n| +-------------+ | -- | GW |
. || | .|r| |s| | | | -- +-----+
. || TRIP | .|e| |o| | TRIP | | ---
. || Instance <-.|g<--|l<--- Instance |--+- +-----+
. || | .|a| |i| | | | | |
. || (I-TRIP/ | .|t| |d| | (TRIP-Lite |--+---------| GW |
. || E-TRIP) | .|i| |a| | Receiver) | | +-----+
. || | .|o| |t| | |--+---
. |+-/-----------+ .|n| |i| +-------------+ | --- +-----+
. | / .| | |o| | -- | |
. |/ .| | |n| | | GW |
. / .+-+ +-+ | +-----+
. /| LS . |
. / +----------------.---------------------------+
. / .
. / .
. / .
. / .
. / .
.+/------------+ .
.| | .
.| | .
.| | .
.| LS | .
.| | .
.| | .
.| | .
.| | .
.+\-----------+ .
. \ .
. \ .
. \ .
. \ +-----------------.---------------------------+
. \ | .+-+ +-+ |
. \| .|A| |C| | +-----+
. \ .|g| |o| | | |
. \-------------+ .|g| |n| +-------------+ | -- | GW |
. || | .|r| |s| | | | -- +-----+
. || TRIP | .|e| |o| | TRIP | | ---
. || Instance <-.|g<--|l<--- Instance |--+- +-----+
. || | .|a| |i| | | | | |
. || (I-TRIP/ | .|t| |d| | (TRIP-Lite |--+---------| GW |
. || E-TRIP) | .|i| |a| | Receiver) | | +-----+
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. || | .|o| |t| | |--+---
. |+-------------+ .|n| |i| +-------------+ | --- +-----+
. | .| | |o| | -- | |
. | .| | |n| | | GW |
. | .+-+ +-+ | +-----+
. | LS . |
. +----------------.---------------------------+
. .
. ITAD .
............................
Figure 6: LS Architecture for TRIP-GW
4.10.1. Route consolidation
A signaling server/Location Server(LS) fronting a set of gateways,
receives routing information on several TRIP-Lite sessions.
Subsequently, this routing information is presented as candidate
routes (possibly as local routes) to the TRIP Decision Process, for
every destination for which a route is available. The LS may
potentially receive two or more TRIP-Lite routes for the same
destination. These alternative routes may be received from the same
gateway, or from multiple gateways. It is recommended that the LS
consolidate the set of TRIP-Lite routes for every destination, before
presenting a candidate route, to the TRIP Decision Process. The
purpose of this operation should be to represent the collective
routing capabilities of the set of gateways, managed by this LS, and
subsequently propagate this information into the core of the TRIP
network. An example scenario is shown below. Consider an LS that
maintains TRIP-Lite peering sessions with gateways A and B.
o Gateway A advertises a route for destination "SIP 408" on the E.164
address family with the Carrier attribute being C1
o Gateway B advertises a route for destination "SIP 408" on the E.164
address family with Carrier attribute C2
The LS that receives these TRIP-Lite routes can consolidate these
routes into a single route for destination "SIP 408" with its Carrier
attribute being a union of the the Carrier attribute values of the
individual routes, namely, "C1 C2" This operation is refered 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. In general,
there is a potential for loss of gateway routing information, when
TRIP-Lite routes from a set of gateways are not consolidated, when a
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candidate route is presented to the TRIP Decision process. The
specifics of applying the consolidation operation to different
attributes and routes from different address families, is left to the
individual TRIP-Lite receiver implementations. In addition, the
route selection procedures as documented in the TRIP RFC [4], do not
apply to processing of TRIP-Lite routes received from the gateways.
4.10.2. Aggregation
The set of gateway routes, that 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. This operation
follows the standard aggregation procedures described in the TRIP
RFC[4], while adhering to the aggregation rules for each route
attribute.
5. IANA Considerations
- The Attribute Type Codes for the new attributes:
AvailableCircuits, TotalCircuitCapacity, CallSuccess, Prefix,
TrunkGroups and Carrier described in Sections 4.1, 4.2, 4.3, 4.4,
4.5 and 4.6 above, respectively, are to be assigned by IANA.
- The Address Family Codes for the new address families: TrunkGroup
and Carrier described in Section 4.7, are to be assigned by IANA.
6. Changes since -03
- Removed Carrier-Trunkgroup attribute and address family and
references to it.
- Added Terminology and Definitions section.
- Updated CallSuccess attribute.
- Added Prefix attribute.
- Added Carrier attribute.
- Added TrunkGroups attribute.
- Added TrunkGroup Address Family.
- Added Carrier Address Family.
- Added some more references.
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7. Changes since -02
- Removed the requirements section.
- Discussed the motivation for introducing Carrier information into
TRIP.
- Defined a new attribute for the E.164 address family.
- Defined a new address family for CarrierCode-TrunkGroup
combination .
- Defined new attributes to advertise dynamic gateway
characteristics like resource availability, and call success
rate.
- Added as section to validate the TRIP-GW solution against the
requirements in [7].
8. Changes since -01
- Added requirements.
- Added more formal analysis of REGISTER and added analysis of SLP.
- Removed circuit capacity attribute.
9. Changes since -00
- Added text to stress the value of this proposal for managing a
gateway cluster.
- Added attributes for circuit capacity and DSP capacity.
- Added section on LS operation, discussing aggregation issue.
Authors' Addresses
Manjunath Bangalore
Cisco Systems
Mail Stop SJC-21/2/2
771 Alder Drive
Milpitas, CA 95035
email: manjax@cisco.com
Rajneesh Kumar
Cisco Systems
Mail Stop SJC-21/2/2
771 Alder Drive
Milpitas, CA 95035
email: rajneesh@cisco.com
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Jonathan Rosenberg
dynamicsoft
72 Eagle Rock Avenue
First Floor
East Hanover, NJ 07936
email: jdrosen@dynamicsoft.com
Hussein F. Salama
Cisco Systems
Mail Stop SJC-24/3/2
170 W. Tasman Drive
San Jose, CA 95134
email: hsalama@cisco.com
Dhaval N. Shah
Cisco Systems
Mail Stop SJC-21/2/2
771 Alder Drive
Milpitas, CA 95035
email: dhaval@cisco.com
Acknowledgments
We wish to thank David Oran, Anirudh Sahoo, Kevin McDermott, Randy
Baird, Cullen Jennings, Jon Peterson and Li Li for their
insightful comments and suggestions.
References
[1] Bradner, S., "Keywords for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg, "SIP:
session initiation protocol," Request for Comments 2543, Internet
Engineering Task Force, Mar. 1999.
[3] E. Guttman, C. Perkins, J. Veizades, and M. Day, "Service
location protocol, version 2," Request for Comments 2608, Internet
Engineering Task Force, June 1999.
[4] J. Rosenberg, H. Salama, and M. Squire, "Telephony routing over
IP (TRIP)," Request for Comments 3219, Internet Engineering Task
Bangalore, Kumar, Rosenberg, Salama, Shah [Page 23]
Internet Draft draft-ietf-iptel-trip-gw-00.txt May 2002
Force, January 2002.
[5] J. Rosenberg and H. Schulzrinne, "A framework for telephony
routing over IP," Request for Comments 2871, Internet Engineering
Task Force, June 2000.
[6] International Telecommunication Union, "Packet based multimedia
communication systems," Recommendation H.323, Telecommunication
Standardization Sector of ITU, Geneva, Switzerland, Feb. 1998.
[7] J. Rosenberg, "Requirements for Gateway Registration," Internet
Draft, Internet Engineering Task Force, Nov. 2001. Work in progress.
[8] ITU-T M.1400 Specification titled "Designations for
interconnections among network operators," published October 10,
2001.
[9] ITU-T List of ITU Carrier Codes (published periodically in the
ITU-T Operational Bulletin).
[10] J. Peterson, "An Architecture for Gateway Registration Based on
Trunk Groups," Internet Draft, Internet Engineering Task Force, Feb.
2002. Work in progress.
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Bangalore, Kumar, Rosenberg, Salama, Shah [Page 24]
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TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
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Bangalore, Kumar, Rosenberg, Salama, Shah [Page 25]
