One document matched: draft-ietf-iptel-trip-01.txt
Differences from draft-ietf-iptel-trip-00.txt
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1. Abstract
This document presents the Telephony Routing over IP (TRIP). TRIP
is a policy driven inter-administrative domain protocol for
advertising the reachability of telephony destinations between
location servers, and for advertising attributes of the routes to
those destinations. TRIP's operation is independent of any
signaling protocol, hence TRIP can serve as the telephony routing
protocol for any signaling protocol.
The Border Gateway Protocol (BGP-4) is used to distribute routing
information between administrative domains. TRIP is used to
distribute telephony routing information between telephony
administrative domains. The similarity between the two protocols is
obvious, and hence TRIP is modeled after BGP-4.
2. Terminology
A framework for a Telephony Routing over IP (TRIP) is described in
[1]. We assume the reader is familiar with the framework and
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terminology of [1]. We define and use the following terms in
addition to those defined in [1].
Telephony Routing Information Base (TRIBTRIB): The database of
reachable telephony destinations built and maintained at an LS as a
result of its participation in TRIP.
IP Telephony Administrative Domain (ITAD): The set of resources
(gateways, location servers, etc.) under control of a single
administrative authority. End users are customers of an ITAD.
Less/More Specific Route. A route X is said to be less specific
than a route Y if every destination in Y is also a destination in
X, and X and Y are not equal. In this case, Y is also said to be
more specific than X.
Peers: Two LSs that share a logical association (a transport
connection). If the LSs are in the same ITAD, they are internal
peers. Otherwise, they are external peers. The logical
association between two peer LSs is called a peering session.
Telephony Routing Information Protocol (TRIP): The protocol
defined in this specification. The function of TRIP is to
advertise the reachability of telephony destinations, attributes
associated with the destinations, as well as the attributes of the
path towards those destinations.
TRIP destination: TRIP can be used to manage routing tables for
multiple protocols (SIP, H323, etc.). In TRIP, a destination is
the combination of (a) a set of addresses (given by and address
family and address prefix), and (b) an application protocol (SIP,
H323, etc).
3. Introduction
The gateway location and call routing problem has been introduced in
[1]. It is considered one of the more difficult problems in IP
telephony. The selection of an egress gateway for a telephony call,
traversing an IP network towards an ultimate destination in the
PSTN, is driven in large part by the policies of the various parties
along the path, and by the relationships established between these
parties. As such, a global directory of egress gateways in which
users look up destination phone numbers is not a feasible solution.
Rather, information about the availability of egress gateways is
exchanged between providers, and subject to policy, made available
locally and then propagated to other providers in other ITADs, thus
creating call routes towards these egress gateways. This would allow
each provider to create its own database of reachable phone numbers
and the associated call routes - such a database could be very
different for each provider depending on policy.
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TRIP is an inter-domain (i.e., inter-ITAD) gateway location and call
routing protocol. The primary function of a TRIP speaker, called a
location server (LS), is to exchange information with other LSs.
This information includes the reachability of telephony
destinations, the call routes towards these destinations, and
information about gateways towards those telephony destinations
residing in the PSTN. The TRIP requirements are set forth in [1].
LSs exchange sufficient call routing information to construct a
graph of ITAD connectivity so that call routing loops may be
prevented. In addition, TRIP can be used to exchange attributes
necessary to enforce policies and to select call routes based on
path or gateway characteristics. This specification defines TRIP's
transport and synchronization mechanisms, its finite state machine,
and the TRIP data. This specification defines the basic attributes
of TRIP. The TRIP attribute set is extendible, so additional
attributes may be defined in future drafts.
TRIP is modeled after the Border Gateway Protocol 4 (BGP-4) [2] and
enhanced with some link state features as in the Open Shortest Path
First (OSPF) protocol [3], IS-IS [XXX], and the Server Cache
Synchronization Protocol (SCSP) [4]. TRIP uses BGP's inter-domain
transport mechanism, BGP's peer communication, BGP's finite state
machine, and similar formats and attributes as BGP. Unlike BGP
however, TRIP permits generic intra-domain LS topologies, which
simplifies configuration and increases scalability in contrast to
BGP's full mesh requirement of internal BGP speakers. TRIP uses an
intra-domain flooding mechanism similar to that used in OSPF [3],
IS-IS [XXX], and SCSP [4].
TRIP permits aggregation of routes as they are advertised through
the network. TRIP does not define a specific route selection
algorithm.
TRIP runs over a reliable transport protocol. This eliminates the
need to implement explicit fragmentation, retransmission,
acknowledgment, and sequencing. The error notification mechanism
used in TRIP assumes that the transport protocol supports a graceful
close, i.e., that all outstanding data will be delivered before the
connection is closed.
TRIP's operation is independent of any particular telephony
signaling protocol. Therefore, TRIP can be used as the routing
protocol for any of these protocols, e.g., H.323 [5] and SIP [6].
The LS peering topology is independent of the physical topology of
the network. In addition, the boundaries of ITAD are independent of
the boundaries of the layer 3 routing autonomous systems. Neither
internal nor external TRIP peers need be physically adjacent.
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4. Summary of Operation
This section summarizes the operation of TRIP. Details are provided
in later sections.
4.1 Peering Session Establishment and Maintenance
Two peer LSs form a transport protocol connection between one
another. They exchange messages to open and confirm the connection
parameters, and to negotiate the capabilities of each LS as well as
the type of information to be advertised over this connection.
KeepAlive messages are sent periodically to ensure adjacent peers
are operational. Notification messages are sent in response to
errors or special conditions. If a connection encounters an error
condition, a Notification message is sent and the connection is
closed.
4.2 Database Exchanges
Once the peer connection has been established, the initial data flow
is the LS's entire routing table. Incremental updates are sent as
the TRIP routing tables change. TRIP does not require periodic
refresh of the routes. Therefore, an LS must retain the current
version of all routing entries.
If a particular ITAD has multiple LSs and is providing transit
service for other ITADs, then care must be taken to ensure a
consistent view of routing within the ITAD. When synchronized the
TRIP routing tables of all internal peers are identical.
4.3 Internal Versus External Synchronization
As with BGP, TRIP distinguishes between internal and external peers.
Within an ITAD, internal TRIP uses link-state mechanisms to flood
database updates over an arbitrary topology. Externally, TRIP uses
point-to-point peering relationships to exchange database
information.
To achieve internal synchronization, internal peer connections are
configured between LSs of the same ITAD such that the resulting
intra-domain LS topology is connected and sufficiently redundant.
This is different from BGP's approach that requires all internal
peers to be connected in a full mesh topology, which may result in
scaling problems. When an update is received from an internal peer,
the routes in the update are checked to determine if they are newer
than the version already in the database. Newer routes are then
flooded to all other peers in the same domain.
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4.4 Advertising TRIP Routes
In TRIP, a route is defined as the combination of (a) a set of
destination addresses (given by an address family indicator and an
address prefix), and (b) an application protocol (e.g. SIP, H323,
etc.). Generally, there are additional attributes associated with
each route (for example, the next-hop server).
TRIP routes are advertised between a pair of LSs in UPDATE messages.
The destination addresses are included in the ReachableRoutes
attribute of the UPDATE, while other attributes describe things like
the path or egress gateway.
If an LS chooses to advertise the TRIP route, it may add to or
modify the attributes of the route before advertising it to a peer.
TRIP provides mechanisms by which an LS can inform its peer that a
previously advertised call route is no longer available for use.
There are three methods by which a given LS can indicate that a
route has been withdrawn from service:
a) Include the route in the WithdrawnRoutes Attribute in an UPDATE
message, thus marking the associated destinations as being no
longer available for use.
b) Advertise a replacement route with the same set of destinations
in the ReachableRoutes Attribute.
c) For external peers where flooding is not in use, the LS-to-LS
peer connection can be closed, which implicitly removes from
service all call routes which the pair of speakers had advertised
to each other. Note that terminating an internal peering session
does not necessarily remove the information advertised by the
peer LS as the same information may have been received from
multiple internal peers.
4.5 Telephony Routing Information Bases
The Telephony Routing Information Base (TRIB) within an LS consists
of three distinct parts:
a) Adj-TRIBs-In: The Adj-TRIBs-In store routing information that
has been learned from inbound UPDATE messages. Their contents
represent TRIP routes that are available as an input to the
Decision Process. These are the `unprocessed' routes received.
The routes from each external peer LS and each internal LS are
maintained in this database independently, so that updates from
one peer do not effect the routes received from another LS. Note
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that there is an Adj-TRIBs-In for every LS within the domain,
even those with which the LS is not directly peering.
b) Loc-TRIB: The Loc-TRIB contains the local TRIP routing
information that the LS has selected by applying its local
policies to the call routing information contained in its Adj-
TRIBs-In.
c) Adj-TRIBs-Out: The Adj-TRIBs-Out store the information that the
local LS has selected for advertisement to its external peers.
The call routing information stored in the Adj-TRIBs-Out will be
carried in the local LS's UPDATE messages and advertised to its
peers.
Figure 1 illustrates the relationship between the three parts of the
call routing information base.
Loc-TRIB
/\
|
Decision Process
/\ |
| \/
Adj-TRIBs-In Adj-TRIBs-Out
Figure 1 TRIB Relationships
Although the conceptual model distinguishes between Adj-TRIBs-In,
Loc-TRIB, and Adj-TRIBs-Out, this neither implies nor requires that
an implementation must maintain three separate copies of the routing
information. The choice of implementation (for example, 3 copies of
the information vs. 1 copy with pointers) is not constrained by the
protocol.
5. Message Formats
This section describes message formats used by TRIP. Messages are
sent over a reliable transport protocol connection. A message MUST
be processed only after it is entirely received. The maximum message
size is 4096 octets. All implementations MUST support this maximum
message size. The smallest message that MAY be sent consists of a
TRIP header without a data portion, or 3 octets.
5.1 Message Header Format
Each message has a fixed-size header. There may or may not be a data
portion following the header depending on the message type. The
layout of the header fields is shown in Figure 2.
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0 1 2
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 2 TRIP Header
Length:
This 2-octet unsigned integer indicates the total length of the
message, including the header, in octets. Thus, it allows one to
locate in the transport-level stream the beginning of the next
message. The value of the Length field must always be at least 3
and no greater than 4096, and may be further constrained depending
on the message type. No padding of extra data after the message is
allowed, so the Length field must have the smallest value possible
given the rest of the message.
Type:
This 1-octet unsigned integer indicates the type code of the
message. The following type codes are defined
1 - OPEN
2 - UPDATE
3 - NOTIFICATION
4 - KEEPALIVE
5.2 OPEN Message Format
After a transport protocol connection is established, the first
message sent by each side is an OPEN message. If the OPEN message is
acceptable, a KEEPALIVE message confirming the OPEN is sent back.
Once the OPEN is confirmed, UPDATE, KEEPALIVE, and NOTIFICATION
messages may be exchanged.
The minimum length of the OPEN message is 14 octets (including
message header). OPEN messages not meeting this minimum requirement
are handled as defined in Section 7.2.
In addition to the fixed-size TRIP header, the OPEN message contains
the following fields:
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Version | Reserved | My ITAD |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TRIP Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Hold Time | Optional Parameters Len |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Optional Parameters (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 3 TRIP OPEN Header
Version:
This 1-octet unsigned integer indicates the protocol version of the
message. The current TRIP version number is 1.
My ITAD:
This 2-octet unsigned integer indicates the ITAD number of the
sender. The ITAD number must be unique for this domain within this
confederation of cooperating LSs.
Hold Time:
This 2-octet unsigned integer indicates the number of seconds that
the sender proposes for the value of the Hold Timer. Upon receipt
of an OPEN message, an LS MUST calculate the value of the Hold
Timer by using the smaller of its configured Hold Time and the Hold
Time received in the OPEN message. The Hold Time MUST be either
zero or at least three seconds. An implementation MAY reject
connections on the basis of the Hold Time. The calculated value
indicates the maximum number of seconds that may elapse between the
receipt of successive KEEPALIVE and/or UPDATE messages by the
sender.
TRIP Identifier:
This 4-octet unsigned integer indicates the TRIP Identifier of the
sender. The TRIP Identifier MUST uniquely identify this LS within
its ITAD. A given LS MAY set the value of its TRIP Identifier to
an IPv4 address assigned to that LS. The value of the TRIP
Identifier is determined on startup and MAY be different for
different external peer connections, but MUST be the same for all
internal peer connections. When comparing two TRIP identifiers,
the TRIP Identifier is interpreted as a numerical 4-octet unsigned
integer.
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Editor's Note [BGP]. Is the sentence about the TRIP ID
restrictions ok(ie can it be different to different
external peers)? Is it useful?
Optional Parameters Length:
This 2-octet unsigned integer indicates the total length of the
Optional Parameters field in octets. If the value of this field is
zero, no Optional Parameters are present.
Optional Parameters:
This field may contain a list of optional parameters, where each
parameter is encoded as a <Parameter Type, Parameter Length,
Parameter Value> triplet.
0 1 2
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter Type | Parameter Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Parameter Value (variable)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 4 Optional Parameter Encoding
Parameter Type is a 2-octet field that unambiguously identifies
individual parameters.
Parameter Length is a 2-octet field that contains the length of the
Parameter Value field in octets.
Parameter Value is a variable length field that is interpreted
according to the value of the Parameter Type field.
5.2.1 Open Message Optional Parameters
This document defines the following Optional Parameters for the OPEN
message.
5.2.1.1 Capability Information
Capability Information uses Optional Parameter type 1. This is an
optional parameter used by an LS to convey to its peer the list of
capabilities supported by the LS. This permits an LS to learn of
the capabilities of its peer LSs. Capability negotiation is defined
in Section 9.
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The parameter contains one or more triples <Capability Code,
Capability Length, Capability Value>, where each triple is encoded
as shown below:
0 1 2
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Capability Code | Capability Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Capability Value (variable)...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 6 Capability Optional Parameter
Capability Code:
Capability Code is a 2-octet field that unambiguously identifies
individual capabilities.
Capability Length:
Capability Length is a 2-octet field that contains the length of
the Capability Value field in octets.
Capability Value:
Capability Value is a variable length field that is interpreted
according to the value of the Capability Code field.
Any particular capability, as identified by its Capability Code, may
appear more than once within the Optional Parameter.
This document reserves Capability Codes 32768-65536 for vendor-
specific applications (these are the codes with the first bit of the
code value equal to 1). This document reserves value 0. Capability
Codes (other than those reserved for vendor specific use) are
controlled by IANA. See Section XXX for IANA considerations.
The following Capability Codes are defined by this specification.
a) Route Types Supported. The Route Types Supported Capability Code
lists the route types supported in this peering session by the
transmitting LS. An LS MUST NOT use route types that are not
supported by the peer LS in any particular peering session. If
the route types supported by a peer are not satisfactory, an LS
MAY terminate the peering session. The format for a Route Type
is:
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0 1 2
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 Family1 | Application Protocol |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Family2 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 7 Route Types Supported Capability
The Address Family and Application Protocol are as defined in
Section 6.1.1. Address Family1 gives the address family being
routed (within the ReachableRoutes attribute). The second
occurrence gives the underlying network address type (the type of
address for the NextHopServer). The application protocol lists
the application for which the routes apply. As an example, a
route type for TRIP could be <E164, SIP, IPv4>, indicating a set
of E164 destinations for the SIP protocol being routed over an
IPv4 network.
The Route Types Supported Capability MAY contain multiple route
types in the capability. The number of route types within the
capability is the maximum number that can fit given the capability
length. The Capability Code is 1 and the length is variable.
Editor's Note: Any other useful capabilities?
5.3 UPDATE Message Format
UPDATE messages are used to transfer routing information between
LSs. The information in the UPDATE packet can be used to construct
a graph describing the relationships of the various ITADs. By
applying rules to be discussed, routing information loops and some
other anomalies may be prevented.
An UPDATE message is used to both advertise and withdraw routes from
service. An UPDATE message may simultaneously advertise and
withdraw TRIP routes.
In addition to the TRIP header, the TRIP UPDATE contains a list of
routing attributes as shown in Figure 8. There is no padding
between routing attributes.
+------------------------------------------------+--...
| First Route Attribute | Second Route Attribute | ...
+------------------------------------------------+--...
Figure 8 TRIP UPDATE Format
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The minimum length of an UPDATE message 11 octets (the TRIP header
plus at least the WithdrawnRoutes and ReachableRoutes attributes).
5.3.1 Routing Attributes
A variable length sequence of routing attributes is present in every
UPDATE message. Each attribute is a triple <attribute type,
attribute length, attribute value> of variable length.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr. Flags |Attr. Type Code| Attr. Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute Value (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 9 Routing Attribute Format
Attribute Type is a two-octet field that consists of the Attribute
Flags octet followed by the Attribute Type Code octet.
The Attribute Type Code defines the type of attribute. The basic
TRIP-defined Attribute Type Codes are discussed later in this
section. Attributes MUST appear in the UPDATE message in numerical
order of the attribute Type Code. Attribute Flags are used to
control attribute processing when the attribute type is unknown.
Attribute Flags are further defined in Section 5.3.2.
The third and the fourth octets of the call route attribute contain
the length of the attribute value field in octets.
The remaining octets of the attribute represent the Attribute Value
and are interpreted according to the Attribute Flags and the
Attribute Type Code. The basic supported attribute types, their
values, and their uses are defined in this specification. These are
the attributes necessary for proper loop free operation of TRIP,
both inter-domain and intra-domain. Additional attributes may be
defined in a future documents.
5.3.2 Attribute Flags
It is clear that the set of attributes for TRIP will evolve over
time. Hence it is essential that mechanisms be provided to handle
attributes with unrecognized types. The handling of unrecognized
attributes is controlled via the flags field of the attribute.
Recognized attributes should be processed according to their
specific definition.
The following are the attribute flags defined by this specification:
Bit Flag
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0) Optional Flag
1) Transitive Flag
2) Dependent Flag
3) Partial Flag
4) Link-state Encapsulated Flag
The high-order bit (bit 0) of the Attribute Flags octet is the
Optional Bit. It defines whether the attribute is optional (if set
to 1) or well-known (if set to 0). Implementations are not required
support optional attributes, but MUST support well-known attributes.
The second high-order bit (bit 1) of the Attribute Flags octet is
the Transitive bit. It defines whether an optional attribute is
transitive (if set to 1) or non-transitive (if set to 0). For well-
known attributes, the Transitive bit MUST be zero on transmit and
MUST be ignored on receipt.
The third high-order bit (bit 2) of the Attribute Flags octet is the
Dependent bit. It defines whether a transitive attribute is
dependent (if set to 1) or independent (if set to 0). For well-known
attributes and for non-transitive attributes, the Dependent bit is
irrelevant, and MUST be set to zero on transmit and MUST be ignored
on receipt.
The fourth high-order bit (bit 3) of the Attribute Flags octet is
the Partial bit. It defines whether the information contained in the
optional transitive attribute is partial (if set to 1) or complete
(if set to 0). For well-known attributes and for non-transitive
attributes the Partial bit MUST be set to 0 on transmit and MUST be
ignored on receipt.
The fifth high-order bit (bit 4) of the Attribute Flags octet is the
Link-state Encapsulation bit. This bit is only applicable to
certain attributes (ReachableRoutes and WithdrawnRoutes) and
determines the encapsulation of the routes within those attributes.
If this bit is set,link-state encapsulation is used within the
attribute. Otherwise, standard encapsulation is used within the
attribute. The Link-state Encapsulation technique is described in
Section 5.3.2.4. This flag is only valid on the ReachableRoutes and
WithdrawnRoutes attributes. It MUST be cleared on transmit and MUST
be ignored on receipt for all other attributes.
The other bits of the Attribute Flags octet are unused. They MUST be
zeroed on transmit and ignored on receipt.
5.3.2.1 Attribute Flags and Route Selection
If an LS receives an UPDATE with a well-known attribute that has an
unrecognized type, then the LS MUST ignore the ReachableRoutes
within that message. If an LS receives an optional attribute with
an unrecognized type, then it MUST process the attribute according
to the Attribute Flags.
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If a mandatory attribute is received for which the flags are not
properly set, then the Update message should be discarded. If a
recognized non-mandatory attribute is received for which the flags
are not properly set, that attribute should be ignored and not
propagated. Any recognized attribute can be used as input to the
route selection process, although the utility of some attributes in
route selection is minimal.
5.3.2.2 Attribute Flags and Route Dissemination
TRIP provides for two variations of transitivity due to the fact
that intermediate LSs need not modify the NextHopServer when
propagating routes. Attributes may be non-transitive, dependent
transitive, or independent transitive. An attribute cannot be both
dependent transitive and independent transitive.
Unrecognized *independent* transitive attributes may be propagated
by any intermediate LS. Unrecognized *dependent* transitive
attributes MAY only be propagated if the LS is NOT changing the
next-hop server. The transitivity variations permit some
unrecognized attributes to be carried end-to-end (independent
transitive), some to be carried between adjacent next-hop servers
(dependent transitive), and other to be restricted to peer LSs (non-
transitive).
An LS that passes an unrecognized transitive attribute to a peer
MUST set the Partial flag on that attribute. Any LS along a path
MAY insert a transitive attribute into a route. If any LS except
the originating LS inserts a new independent transitive attribute
into a route, then it MUST set the Partial flag on that attribute.
If any LS except an LS that modifies the NextHopServer inserts a new
dependent transitive attribute into a route, then it MUST set the
Partial flag on that attribute. The Partial flag indicates that not
every LS along the relevant path has processed and understood the
attribute. For independent transitive attributes, the "relevant
path" is the path given in the AdvertisementPath attribute. For
dependent transitive attributes, the relevant path consists only of
those domains thru which this object has passed since the
NextHopServer was last modified. The Partial flag in an independent
transitive attribute MUST NOT be unset by any other LS along the
path. The Partial flag in a dependent transitive attribute MUST be
reset whenever the NextHopServer is changed, but MUST NOT be unset
by any LS that is not changing the NextHopServer.
The rules governing the addition of new non-transitive attributes
are defined independently for each non-transitive attribute.
Any attribute MAY be updated by an LS in the path.
5.3.2.3 Attribute Flags and Route Aggregation
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Each attribute defines how it is to be handled during route
aggregation.
The rules governing the handling of unknown attributes are guided by
the Attribute Flags. Unrecognized transitive attributes are dropped
during aggregation. There should be no unrecognized non-transitive
attributes during aggregation because non-transitive attributes must
be processed by the local LS in order to be propagated.
5.3.2.4 Attribute Flags and Encapsulation
Normally attributes have the simple format as described in Section
5.3.1. If the Link-state Encapsulation Flag is set, then the two
additional fields are added to the attribute header as shown in
Figure 10.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr. Flags |Attr. Type Code| Attr. Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originator TRIP Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Sequence Number |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute Value (variable) |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 10 Link State Encapsulation
The Originator TRIP ID and Sequence Number are used to control the
flooding of routing updates within a collection of servers. These
fields are used to detect duplicate and old routes so that they are
not further propagated within the servers. The use of these fields
is defined in Section 11.1.
5.3.3 Mandatory Attributes
Certain attributes are mandatory; they must be in every UPDATE
message. Mandatory attributes are identified in their definition.
By definition, mandatory attributes are also well-known. UPDATE
messages that do not include all mandatory attributes are discarded.
5.3.4 TRIP UPDATE Attributes
This section summarizes the attributes that may be carried in an
UPDATE message. Attributes MUST appear in the UPDATE message in
increasing order of the Attribute Type Code. Additional details are
provided in Section 6.
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5.3.4.1 WithdrawnRoutes
This attribute lists a set of routes that are being withdrawn from
service. The transmitting LS has determined that these routes
should no longer be advertised, and is propagating this information
to its peers.
5.3.4.2 ReachableRoutes
This attribute lists set of routes that are being added to service.
These routes have the potential to be inserted into the Adj-TRIBs-In
of the receiving LS.
5.3.4.3 NextHopServer
This attribute gives the network address of the entity to which
messages should be sent along this routed path. The NextHopServer
is specific to the set of destinations and application protocol
defined in the ReachableRoutes attribute. Note that this is NOT the
address to which media (voice, video, etc.) should be transmitted,
only the application protocol given in ReachableRoutes.
5.3.4.4 AdvertisementPath
The AdvertisementPath is analogous to the AS_PATH in BGP4 [2]. The
attribute records the sequence of domains through which this
advertisement has passed. The attribute is used to detect when the
routing advertisement is looping. This attribute does NOT reflect
the path through which messages following this route would traverse.
Since the next-hop need not be modified by each LS, the actual path
to the destination might not have to traverse every domain in the
AdvertisementPath.
5.3.4.5 RoutedPath
The RoutedPath attribute is analogous to the AdvertisementPath
attribute, except that it records the actual path (given by the list
of domains) *to* the destinations. Unlike AdvertisementPath, which
is modified each time the route is propagated, RoutedPath is only
modified when the NextHopServer attribute changes. Thus, it records
the subset of the AdvertisementPath over which messages following
this particular route would traverse.
5.3.4.6 AtomicAggregate
The AtomicAggregate attribute indicates that a route may actually
include domains not listed in the RoutedPath. If an LS, when
presented with a set of overlapping routes from a peer LS, selects a
less specific route without selecting the more specific route, then
the LS MUST include the AtomicAggregate attribute with the route.
An LS receiving a route with an AtomicAggregate attribute MUST NOT
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make the set of destinations more specific when advertising it to
other LSs.
5.3.4.7 LocalPreference
The LocalPreference attribute is an intra-domain attribute used to
inform other LSs of the local LSs preference for a given route. The
preference of a route is calculated at the ingress to a domain and
passed as an attribute with that route throughout the domain. Other
LSs within the same ITAD use this attribute in their route selection
process. This attribute has no significance between domains.
Editor's Note. Want/need Community attribute?
5.3.4.8 MultiExitDisc
There may be more than one LS peering relationship between
neighboring domains. The MultiExitDisc attribute is used by an LS
to express a preference for one link between the domains over
another link between the domains. The use of the MultiExitDisc
attribute is controlled by local policy.
5.3.4.9 ITAD Topology
The ITAD topology attribute is an intra-domain attribute that is
used by LSs to indicate their intra-domain topology to other LSs in
the domain.
5.3.4.10 Authentication
TRIP allows the originator of a particular attribute to include a
signature so that the receiver may validate the originator and
contents of the attribute. The Authentication attribute includes a
list of the signatures for all signed attributes in the UPDATE.
5.4 KEEPALIVE Message Format
TRIP does not use any transport-based keep-alive mechanism to
determine if peers are reachable. Instead, KEEPALIVE messages are
exchanged between peers often enough as not to cause the Hold Timer
to expire. A reasonable maximum time between KEEPALIVE messages
would be one third of the Hold Time interval. KEEPALIVE messages
MUST NOT be sent more than once per XX seconds. An implementation
SHOULD adjust the rate at which it sends KEEPALIVE messages as a
function of the negotiated Hold Time interval.
Editor's Note: Need to examine timer values in TRIP
context - are BGP defaults satisfactory?
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If the negotiated Hold Time interval is zero, then periodic
KEEPALIVE messages MUST NOT be sent.
KEEPALIVE message consists of only message header and has a length
of 3 octets.
5.5 NOTIFICATION Message Format
A NOTIFICATION message is sent when an error condition is detected.
The TRIP transport connection is closed immediately after sending a
NOTIFICATION message
In addition to the fixed-size TRIP header, the NOTIFICATION message
contains the following fields:
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Error Code | Error Subcode | Data... (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 11 TRIP NOTIFICATION Format
Error Code:
This 1-octet unsigned integer indicates the type of NOTIFICATION.
The following Error Codes have been defined:
Error Code Symbolic Name Reference
1 Message Header Error Section 7.1
2 OPEN Message Error Section 7.2
3 UPDATE Message Error Section 7.3
4 Hold Timer Expired Section 7.5
5 Finite State Machine Error Section 7.6
6 Cease Section 7.7
Error Subcode:
This 1-octet unsigned integer provides more specific information
about the nature of the reported error. Each Error Code may have
one or more Error Subcodes associated with it. If no appropriate
Error Subcode is defined, then a zero (Unspecific) value is used
for the Error Subcode field.
Message Header Error Subcodes:
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1 - Bad Message Length.
2 - Bad Message Type.
OPEN Message Error Subcodes:
1 - Unsupported Version Number.
2 - Bad Peer ITAD.
3 - Bad TRIP Identifier.
4 - Unsupported Optional Parameter.
5 - Unacceptable Hold Time.
6 -
- Unsupported Capability.
UPDATE Message Error Subcodes:
1 - Malformed Attribute List.
2 - Unrecognized Well-known Attribute.
3 - Missing Well-known Mandatory Attribute.
4 - Attribute Flags Error.
5 - Attribute Length Error.
6 - Invalid Attribute.
Data:
This variable-length field is used to diagnose the reason for the
NOTIFICATION. The contents of the Data field depend upon the Error
Code and Error Subcode.
Note that the length of the data can be determined from
the message length field by the formula:
Data Length = Message Length - 5
The minimum length of the NOTIFICATION message is 5 octets
(including message header).
6. TRIP Attributes
This section provides details on the syntax and semantics of each
TRIP UPDATE attribute.
6.1 WithdrawnRoutes
Mandatory: TRUE.
Required Flags: Well-known.
Potential Flags: Link-State Encapsulation (when flooding).
TRIP Type Code: 1
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The WithdrawnRoutes attribute MUST be included in every UPDATE
message. It specifies a set of routes that are to be removed from
service by the receiving LS(s). The set of routes MAY be empty,
indicated by a length field of zero.
6.1.1 Syntax of WithdrawnRoutes
The WithdrawnRoutes Attribute encodes a sequence of routes in its
value field. The format for individual routes is given in Section
6.1.1.1. The WithdrawnRoutes Attribute lists the individual routes
sequentially with no padding as shown in Figure 12. Each route
includes a length field so that the individual routes within the
attribute can be delineated.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| WithdrawnRoute1... | WithdrawnRoute2... | ...
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 12 WithdrawnRoutes Format
6.1.1.1 Generic TRIP Route Format
The generic format for a TRIP route is given in Figure 13.
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 13 Generic TRIP Route Format
Address Family:
The address family field gives the type of address for the route.
Address families are defined in RFC 1700 [XXX].
Application Protocol:
The application protocol gives the protocol for which this routing
table is maintained. The currently defined application protocols
are:
1) SIP
2) H323
Additional application protocols may be defined in the future.
Length:
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The length of the address field, in bytes.
Address:
This is an address (prefix) of the family type given by Address
Family. The octet length of the address is variable and is
determined by the length field of the route.
6.1.1.2 Encoding of E164 Numbers
A set of telephone numbers is specified by an E164 number prefix.
E164 prefixes are represented by a string of digits, each digit
encoded by its ASCII character representation. This routing object
covers all phone numbers starting with this prefix.
The syntax for the phone number prefix is:
phone-number-bound = *phone-digit
phone-digit = DIGIT
DIGIT = '0'|'1'|'2'|'3'|'4'|'5'|'6'|'7'|'8'|'9'
This format is similar to the format for a global telephone number
as defined in SIP [6] without visual separators and without the
international `+' prefix. This format facilitates efficient
comparison when using TRIP to route SIP or H323, both of which use
character based representations of phone numbers. The prefix length
is determined from the length field of the route.
6.2 ReachableRoutes
Mandatory: TRUE.
Required Flags: Well-known.
Potential Flags: Link-State Encapsulation (when flooding).
Trip Type Code: 2
The ReachableRoutes attribute MUST be included in every UPDATE
message. It specifies a set of routes that are to be added to
service by the receiving LS(s). The set of routes MAY be empty,
this is indicated by setting the length field to zero.
6.2.1 Syntax of ReachableRoutes
The ReachableRoutes Attribute has the same syntax as the
WithdrawnRoutes Attribute. See Section 6.1.1.
6.2.2 Route Origination and ReachableRoutes
Routes are injected into TRIP by a method outside the scope of this
specification. Possible methods include a front-end protocol, an
intra-domain routing protocol, or static configuration.
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6.2.3 Route Selection and ReachableRoutes
The routes in ReachableRoutes are necessary for route selection.
6.2.4 Aggregation and ReachableRoutes
To aggregate multiple routes, the set of ReachableRoutes to be
aggregated MUST combine to form a less specific set.
There is no mechanism within TRIP to communicate that a particular
address prefix is not used and thus that these addresses could be
skipped during aggregation. LSs MAY use methods outside of TRIP to
learn of invalid prefixes that may be ignored during aggregation.
6.2.5 Route Dissemination and ReachableRoutes
The ReachableRoutes attribute is recomputed at each LS except where
flooding is being used (e.g., within a domain).
6.2.6 E164 Number Specifics
A gateway that can reach all valid numbers in a specific prefix
SHOULD advertise that prefix as the ReachableRoutes, even if there
are more specific prefixes that do not actually exist on the PSTN.
Generally, it takes 10 E164 prefixes of length n to aggregate into a
prefix of length n-1. However, if an LS is aware that a prefix is
an invalid PSTN prefix, then the LS MAY aggregate by skipping this
prefix. For example, if the prefix +19191 is known not to exist,
then an LS can aggregate to +1919 without +19191. A prefix
representing an invalid set of PSTN destinations is sometimes
referred to as a "black-hole". The method by which an LS is aware
of black-holes is not within the scope of TRIP, but if an LS has
such knowledge, it can use the knowledge when aggregating.
6.3 NextHopServer
Mandatory: True.
Required Flags: Well-known.
Potential Flags: None.
TRIP Type Code: TBD.
Given a route with application protocol A and destinations D, the
NextHopServer indicates the next-hop that messages of protocol A
destined for D should be sent. This may or may not represent the
ultimate destination of those messages.
6.3.1 NextHopServer Syntax
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For generality, the address of the next-hop server may be of various
types (IPv4, IPv6, etc). The NextHopServer attribute includes an
address type identifier, address length, and a next-hop address.
RFC1700 [XXX] defines the address types with the Address Family
Identifier.
The syntax for the NextHopServer is given in Figure 14.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Next Hop ITAD | Address Family |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Length | Address_ (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 14 NextHopServer Syntax
The Next-Hop ITAD indicates the domain of the next-hop. The Address
Family field gives the type of address in use, the Length field
gives the number of octets in the Address field, and the Address
field contains the network address of the next-hop server.
Editor's Note. Would be nice to be able to indicate DNS
name of next-hop server. Need to get DNS an the Address
Family identifier.
6.3.2 Route Origination and NextHopServer
When an LS originates a routing object into TRIP, it MUST include a
NextHopServer within its domain. The NextHopServer could be an
address of the egress gateway or of a signaling proxy.
6.3.3 Route Selection and NextHopServer
LS policy may prefer certain next-hops or next-hop domains over
others.
6.3.4 Aggregation and NextHopServer
When aggregating multiple routing objects into a single routing
object, an LS MUST insert a new signaling server from within its
domain as the new NextHopServer unless all of the routes being
aggregated have the same next-hop.
6.3.5 Route Dissemination and NextHopServer
When propagating routing objects to peers, an LS may choose to
insert an address of a signaling proxy within its domain as the new
next-hop, or it may leave the next-hop unchanged. Inserting a new
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address as the next-hop will cause the signaling messages to be sent
to that address, and will provide finer control over the signaling
path. Leaving the next-hop unchanged will yield a more efficient
signaling path (fewer hops). It is a local policy decision of the
LS to decide whether to propagate or change the NextHopServer.
6.4 AdvertisementPath
Mandatory: TRUE.
Required Flags: Well-known.
Potential Flags: Partial.
TRIP Type Code: TBD.
This attribute identifies the ITADs through which routing
information carried in an advertisement has passed. The
AdvertisementPath attribute is analogous to the AS_PATH attribute in
BGP. The attributes differ in that BGP's AS_PATH also reflects the
path to the destination. In TRIP, not every domain need modify the
next-hop, so the AdvertisementPath may include many more hops than
the actual path to the destination. The RoutedPath attribute
(Section 6.5) reflects the actual path to the destination.
6.4.1 AdvertisementPath Syntax
AdvertisementPath is a variable length attribute that is composed of
a sequence of ITAD path segments. Each ITAD path segment is
represented by a type-length-value triple.
The path segment type is a 1-octet long field with the following
values defined:
Value Segment Type
1. AP_SET: unordered set of ITADs a route in the advertisement
message has traversed
2. AP_SEQUENCE: ordered set of ITADs a route in the advertisement
message has traversed
The path segment length is a 1-octet long field containing the
number of ITADs in the path segment value field.
The path segment value field contains one or more ITAD numbers, each
encoded as a 2-octets long field. ITAD numbers uniquely identify an
Internet Telephony Administrative Domain, and must be obtained from
IANA. See Section XXX for procedures to obtain an ITAD number from
IANA.
6.4.2 Route Origination and AdvertisementPath
When an LS originates a route then:
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a) The originating LS shall include its own ITAD number in the
AdvertisementPath attribute of all advertisements sent to LSs
located in neighboring ITADs. In this case, the ITAD number of
the originating LS's ITAD will be the only entry in the
AdvertisementPath attribute.
b) The originating LS shall include an empty AdvertisementPath
attribute in all advertisements sent to LSs located in its own
ITAD. An empty AdvertisementPath attribute is one whose length
field contains the value zero.
6.4.3 Route Selection and AdvertisementPath
The AdvertisementPath may be used for route selection. Possible
criteria to be used are the number of hops on the path and the
presence or absence of particular ITADs on the path.
As discussed in Section 11, the AdvertisementPath is used to prevent
routing information from looping. If an LS receives a route with
its own ITAD already in the AdvertisementPath, the route MUST be
discarded.
6.4.4 Aggregation and AdvertisementPath
The rules for aggregating AdvertisementPath attributes are given in
the following sections, where the term `path' used in Section
6.4.4.1 and 6.4.4.2 is understood to mean AdvertisementPath.
6.4.4.1 Aggregating Routes with Identical Paths
If all routes to be aggregated have identical path attributes, then
the aggregated route has the same path attribute as the individual
routes.
6.4.4.2 Aggregating Routes with Different Paths
For the purpose of aggregating path attributes we model each ITAD
within the path as a pair <type, value>, where "type" identifies a
type of the path segment (AP_SEQUENCE or AP_SET), and "value" is the
ITAD number. Two ITADs are said to be the same if their
corresponding <type, value> are the same.
If the routes to be aggregated have different path attributes, then
the aggregated path attribute shall satisfy all of the following
conditions:
All pairs of the type AP_SEQUENCE in the aggregated path MUST
appear in all of the paths of routes to be aggregated.
All pairs of the type AP_SET in the aggregated path MUST appear in
at least one of the paths of the initial set (they may appear as
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either AP_SET or AP_SEQUENCE types).
For any pair X of the type AP_SEQUENCE that precedes pair Y in the
aggregated path, X precedes Y in each path of the initial set that
contains Y, regardless of the type of Y.
No pair with the same value shall appear more than once in the
aggregated path, regardless of the pair's type.
An implementation may choose any algorithm that conforms to these
rules. At a minimum a conformant implementation MUST be able to
perform the following algorithm that meets all of the above
conditions:
Determine the longest leading sequence
of tuples (as defined above) common to all the paths of the
routes to be aggregated. Make this sequence the leading sequence
of the aggregated path.
Set the type of the rest of the tuples
from the paths of the routes to be aggregated to AP_SET, and
append them to the aggregated path.
If the aggregated path has more than
one tuple with the same value (regardless of tuple's type),
eliminate all, but one such tuple by deleting tuples of the type
AP_SET from the aggregated path.
An implementation that chooses to provide a path aggregation
algorithm that retains significant amounts of path information may
wish to use the procedure of Section 6.4.4.3.
6.4.4.3 Example Path Aggregation Algorithm
An example algorithm to aggregate two paths works as follows:
a) Identify the ITADs (as defined in Section 6.4.1) within each path
attribute that are in the same relative order within both path
attributes. Two ITADs, X and Y, are said to be in the same order
if either X precedes Y in both paths, or if Y precedes X in both
paths.
b) The aggregated path consists of ITADs identified in (a) in
exactly the same order as they appear in the paths to be
aggregated. If two consecutive ITADs identified in (a) do not
immediately follow each other in both of the paths to be
aggregated, then the intervening ITADs (ITADs that are between
the two consecutive ITADs that are the same) in both attributes
are combined into an AP_SET path segment that consists of the
intervening ITADs from both paths; this segment is then placed in
between the two consecutive ITADs identified in (a) of the
aggregated attribute. If two consecutive ITADs identified in (a)
immediately follow each other in one attribute, but do not follow
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in another, then the intervening ITADs of the latter are combined
into an AP_SET path segment; this segment is then placed in
between the two consecutive ITADs identified in (a) of the
aggregated path.
If as a result of the above procedure a given ITAD number appears
more than once within the aggregated path, all, but the last
instance (rightmost occurrence) of that ITAD number should be
removed from the aggregated path.
6.4.5 Route Dissemination and AdvertisementPath
When an LS propagates a route which it has learned from another LS,
it shall modify the route's AdvertisementPath attribute based on the
location of the LS to which the route will be sent.
a) When a LS advertises a route to another LS located in its own
ITAD, the advertising LS MUST NOT modify the AdvertisementPath
attribute associated with the route.
b) When a LS advertises a route to an LS located in a neighboring
ITAD, then the advertising LS MUST update the AdvertisementPath
attribute as follows:
1) If the first path segment of the AdvertisementPath is of type
AP_SEQUENCE, the local system shall prepend its own ITAD number
as the last element of the sequence (put it in the leftmost
position).
2) If the first path segment of the AdvertisementPath is of type
AP_SET, the local system shall prepend a new path segment of
type AP_SEQUENCE to the AdvertisementPath, including its own
ITAD number in that segment.
6.5 RoutedPath
Mandatory: True.
Required Flags: Well-known.
Potential Flags: Partial.
TRIP Type Code: TBD.
This attribute identifies the ITADs through which messages sent
using this route would pass. The ITADs in this path are a subset of
those in the AdvertisementPath.
6.5.1 RoutedPath Syntax
The syntax of the RoutedPath attribute is the same as that of the
AdvertisementPath attribute. See Section 6.4.1.
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6.5.2 Route Origination and RoutedPath
When an LS originates a route it MUST include the RoutedPath
attribute.
a) The originating LS shall include its own ITAD number in the
RoutedPath attribute of all advertisements sent to LSs located in
neighboring ITADs. In this case, the ITAD number of the
originating LS's ITAD will be the only entry in the RoutedPath
attribute.
b) The originating LS shall include an empty RoutedPath attribute in
all advertisements sent to LSs located in its own ITAD. An empty
RoutedPath attribute is one whose length field contains the value
zero.
6.5.3 Route Selection and RoutedPath
The RoutedPath MAY be used for route selection, and in most cases is
preferred over the AdvertisementPath for this role. Some possible
criteria to be used are the number of hops on the path and the
presence or absence of particular ITADs on the path.
6.5.4 Aggregation and RoutedPath
The rules for aggregating RoutedPath attributes are given in Section
6.4.4.1 and 6.4.4.2, where the term `path' used in Section 6.4.4.1
and 6.4.4.2 is understood to mean RoutedPath.
6.5.5 Route Dissemination and RoutedPath
When an LS propagates a route that it learned from another LS, it
modifies the route's RoutedPath attribute based on the location of
the LS to which the route is sent.
a) When a LS advertises a route to another LS located in its own
ITAD, the advertising LS MUST NOT modify the RoutedPath attribute
associated with the route.
b) If the LS has not changed the NextHopServer attribute, then the
LS MUST NOT change the RoutedPath attribute.
c) Otherwise, the LS changed the NextHopServer and is advertising
the route to an LS in another ITAD. The advertising LS MUST
update the RoutedPath attribute as follows:
1) If the first path segment of the RoutedPath is of type
AP_SEQUENCE, the local system shall prepend its own ITAD number
as the last element of the sequence (put it in the leftmost
position).
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2) If the first path segment of the RoutedPath is of type AP_SET,
the local system shall prepend a new path segment of type
AP_SEQUENCE to the RoutedPath, including its own ITAD number in
that segment.
6.6 AtomicAggregate
Mandatory: False.
Required Flags: Well-known.
Potential Flags: None.
TRIP Type Code: TBD.
The AtomicAggregate attribute indicates that a route may traverse
domains not listed in the RoutedPath. If an LS, when presented with
a set of overlapping routes from a peer LS, selects the less
specific route without selecting the more specific route, then the
LS includes the AtomicAggregate attribute with the routing object.
6.6.1 AtomicAggregate Syntax
This attribute has length zero (0); the value field is empty.
6.6.2 Route Origination and AtomicAggregate
Routes are never originated with the AtomicAggregate attribute.
6.6.3 Route Selection and AtomicAggregate
The AtomicAggregate attribute may be used in route selection -
- it
indicates that the RoutedPath may be incomplete.
6.6.4 Aggregation and AtomicAggregate
If any of the routes to aggregate has the AtomicAggregate attribute,
then so should the resultant aggregate.
6.6.5 Route Dissemination and AtomicAggregate
If an LS, when presented with a set of overlapping routes from a
peer LS, selects the less specific route (see Section 2) without
selecting the more specific route, then the LS MUST include the
AtomicAggregate attribute with the routing object (if it is not
already present).
An LS receiving a routing object with an AtomicAggregate attribute
MUST NOT make the set of destinations more specific when advertising
it to other LSs, and MUST NOT remove the attribute when propagating
this object to a peer LS.
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6.7 LocalPreference
Mandatory: False.
Required Flags: Well-known.
Potential Flags: None.
TRIP Type Code: TBD.
The LocalPreference attribute is only used intra-domain, it
indicates the local LS's preference for the routing object to other
LSs within the same domain. This attribute MUST NOT be included
when communicating to an LS in another domain, and MUST be included
over intra-domain links.
6.7.1 LocalPreference Syntax
The LocalPreference attribute is a 4-octet unsigned numeric value.
A higher value indicates a higher preference.
6.7.2 Route Origination and LocalPreference
Routes MUST NOT be originated with the LocalPreference attribute to
inter-domain peers. Routes to intra-domain peers MUST be originated
with the LocalPreference attribute.
6.7.3 Route Selection and LocalPreference
The LocalPreference attribute allows one LS in a domain to calculate
a preference for a route, and to communicate this preference to
other LSs within the domain.
6.7.4 Aggregation and LocalPreference
The LocalPreference attribute is not affected by aggregation.
6.7.5 Route Dissemination and LocalPreference
An LS MUST include the LocalPreference attribute when communicating
with peer LSs within its own domain. An LS MUST NOT include the
LocalPreference attribute when communicating with LSs in other
domains. LocalPreference attributes received from inter-domain
peers MUST be ignored.
6.8 MultiExitDisc
Mandatory: False.
Required Flags: Well-known.
Potential Flags: None.
TRIP Type Code: TBD.
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When two ITADs are connected by more than one set of peers, the
MultiExitDisc attribute may be used to specify preferences for
routes received over one of those links versus routes received over
other links. The MultiExitDisc parameter is used only for route
selection.
6.8.1 MultiExitDisc Syntax
The MultiExitDisc attribute carries a 4-octet unsigned numeric
value. A lower value represents a more preferred routing object.
6.8.2 Route Origination and MultiExitDisc
Routes originated to intra-domain peers MUST NOT be originated with
the MultiExitDisc attribute. When originating a route to an inter-
domain peer, the MultiExitDisc attribute may be included.
6.8.3 Route Selection and MultiExitDisc
The MultiExitDisc attribute is used to express a preference when
there are multiple links between two domains. If all other factors
are equal, then a route with a lower MultiExitDisc attribute is
preferred over a route with a higher MultiExitDisc attribute.
6.8.4 Aggregation and MultiExitDisc
Routes with differing MultiExitDisc parameters MUST NOT be
aggregated. Routes with the same value in the MultiExitDisc
attribute MAY be aggregated and the same MultiExitDisc attribute
attached to the aggregated object.
6.8.5 Route Dissemination and MultiExitDisc
If received from a peer LS in another domain, an LS MAY propagate
the MultiExitDisc to other LSs within its domain. The MultiExitDisc
attribute MUST NOT be propagated to LSs in other domains.
An LS may add the MultiExitDisc attribute when propagating routing
objects to an LS in another domain. The inclusion of the
MultiExitDisc attribute is a matter of policy, as is the value of
the attribute.
6.9 ITAD Topology
Mandatory: False.
Required Flags: Well-known, Link-State encapsulated.
Potential Flags: None.
TRIP Type Code: TBD.
Within an ITAD, each LS must know the status of other LSs so that LS
failure can be detected. To do this, each LS advertises its
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internal topology to other LSs within the domain. When an LS
detects that another LS is no longer active, the information sourced
by that LS can be deleted (the Adj-TRIB-In for that peer may be
cleared). The ITAD Topology attribute is used to communicate this
information to other LSs within the domain.
Editor's Note. Two methods for this function are
possible. One method advertises the topology, requires
LSs to update their topology only when their internal
peer set changes, and requires LSs to calculate to which
LSs are active within their domain via a connectivity
algorithm on the topology. The second option would
require an LS to periodically issue a `keep-alive' type
advertisement that gets flooded within the domain. LSs
would determine which LSs are active by the set of
received keep-alives. We are suggesting the former
method as it allows faster detection of failure.
6.9.1 ITAD Topology Syntax
The ITAD Topology attribute indicates the LSs with which the LS is
currently peering. The attribute consists of a list of the TRIP
Identifiers with which the LS is currently peering, the format is
given in Figure 15. This attribute MUST use the link-state
encapsulation as defined in Section 5.3.2.4.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TRIP Identifier 1 |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| TRIP Identifier 2 ... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 15 ITAD Topology Syntax
6.9.2 Route Origination and ITAD Topology
The ITAD Topology attribute is independent of any routes in the
UPDATE. Whenever the set of internal peers of a LS changes, it MUST
originate an UPDATE with the ITAD Topology Attribute included
listing the current set of internal peers. The LS MUST include
this attribute in the first UPDATE it sends to a peer after the
peering session is established.
6.9.3 Route Selection and ITAD Topology
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This attribute is independent of any routing information in the
UPDATE. When an LS receives an UPDATE with an ITAD Topology
attribute, it MUST compute the set of LSs currently active in the
domain by performing a connectivity test on the ITAD topology as
given by the set of originated ITAD Topology attributes. The LS
MUST locally purge the Adj-TRIB-In for any LS that is no longer
active in the domain. The LS MUST NOT propagate this purging
information to other LSs as they will make a similar decision.
6.9.4 Aggregation and ITAD Topology
This information is not aggregated.
6.9.5 Route Dissemination and ITAD Topology
An LS MUST ignore the attribute if received from a peer in another
domain. An LS MUST NOT send this attribute to an inter-domain
peer.
6.10 Authentication
Mandatory: False.
Required Flags: Well-known.
Potential Flags: None.
TRIP Type Code: TBD.
In some situations, LSs may wish to verify the originator of an
attribute and that the contents of that attribute have not been
altered by other intermediate LSs. The Authentication attribute
carries signatures so that a receiving LS may validate particular
attributes.
6.10.1 Authentication Syntax
The Authentication attribute contains a list of Attribute
Signatures. Each attribute signature has the following format.
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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attribute Signature Length | Originating ITAD |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Originating TRIP Identifier |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Attr Code | Auth Mech | Authentication Data (variable)
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Figure 16 Attribute Signature Syntax
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The Attribute Signature Length is the length of the entire attribute
signature, including the length field. The Originating ITAD and
TRIP identifier indicate the LS that inserted the attribute. The
Attribute code indicates the attribute this signature covers, and
the Authentication Mechanism indicates the algorithm used to compute
the Authentication Data. The valid Authentication Mechanisms are:
Editor's Note. List authentication mechanisms.
The Authentication Mechanism is performed over the following fields
to compute the Authentication Data. The fields are considered in
this order.
1. Value of the ReachableRoutes attribute.
2. Value of the attribute given by the Attribute Code.
6.10.2 Route Origination and Authentication
An LS MAY include the signature attribute with routes that it
originates, covering any subset of the attributes of the route.
6.10.3 Route Selection and Authentication
An LS MAY be required (via configuration or some other means) to
verify the authenticity of certain attributes. An LS MAY use
attribute authentication when calculating the preference of a route.
Possible uses of the Authentication attribute include:
1. Ignoring routes that do not contain authentication for a
particular attribute.
2. Ignoring routes that for which attribute verification cannot be
performed due to unsupported authentication mechanisms or
invalid authentication data.
Other uses are also possible.
6.10.4 Aggregation and Authentication
Aggregation and Authentication are mutually exclusive. Since
attribute signatures cover the routes in the ReachableRoutes field,
aggregating routes together eliminates the validity of signatures.
Authentication attributes MUST NOT be propagated on aggregated
routes. The relative importance of authentication and aggregation
is an administrative decision.
6.10.5 Route Dissemination and Authentication
The Authentication attribute MUST be examined before propagating to
other LSs. For any attributes that have been changed by the local
LS, the LS should strip the Attribute Signature (if they exist) from
the Authentication attribute. The LS MAY insert its own signatures
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into the Authentication attribute if it desires to do so. The LS
MAY propagate Attribute Signatures for attributes that it does not
alter. The decision to add or propagate attribute signatures is a
local policy decision.
6.11 Considerations for Defining new TRIP Attributes
Editor's Note: Text to be added.
7. TRIP Error Detection and Handling
This section describes errors to be detected and the actions to be
taken while processing TRIP messages.
When any of the conditions described here are detected, a
NOTIFICATION message with the indicated Error Code, Error Subcode,
and Data fields MUST be sent, and the TRIP connection MUST be
closed. If no Error Subcode is specified, then a zero Subcode MUST
be used.
The phrase "the TRIP connection is closed" means that the transport
protocol connection has been closed and that all resources for that
TRIP connection have been de-allocated. If the connection was
inter-domain, then routing table entries associated with the remote
peer MUST be marked as invalid. Routing table entries MUST NOT be
marked as invalid if an internal peering session is terminated. The
fact that the routes have been marked as invalid is passed to other
TRIP peers before the routes are deleted from the system.
Unless specified explicitly, the Data field of the NOTIFICATION
message that is sent to indicate an error MUST be empty.
7.1 Message Header Error Detection and Handling
All errors detected while processing the Message Header are
indicated by sending the NOTIFICATION message with Error Code
`Message Header Error'. The Error Subcode elaborates on the specifies
nature of the error. The error checks in this section MUST be
performed by each LS on receipt of every message.
If the Length field of the message header is less than 3 or greater
than 4096, or if the Length field of an OPEN message is less than
the minimum length of the OPEN message, or if the Length field of an
UPDATE message is less than the minimum length of the UPDATE
message, or if the Length field of a KEEPALIVE message is not equal
to 3, or if the Length field of a NOTIFICATION message is less than
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the minimum length of the NOTIFICATION message, then the Error
Subcode MUST be set to "Bad Message Length." The Data field
contains the erroneous Length field.
If the Type field of the message header is not recognized, then the
Error Subcode MUST be set to "Bad Message Type." The Data field
contains the erroneous Type field.
7.2 OPEN Message Error Detection and Handling
All errors detected while processing the OPEN message are indicated
by sending the NOTIFICATION message with Error Code "OPEN Message
Error." The Error Subcode elaborates on the specific nature of the
error. The error checks in this section MUST be performed by each LS
on receipt of every OPEN message.
If the version number contained in the Version field of the received
OPEN message is not supported, then the Error Subcode MUST be set to
"Unsupported Version Number." The Data field is a 1-octet unsigned
integer, which indicates the largest locally supported version
number less than the version the remote TRIP peer bid (as indicated
in the received OPEN message).
If the ITAD field of the OPEN message is unacceptable, then the
Error Subcode MUST be set to "Bad Peer ITAD." The determination of
acceptable ITAD numbers is outside the scope of this protocol.
If the Hold Time field of the OPEN message is unacceptable, then the
Error Subcode MUST be set to "Unacceptable Hold Time." An
implementation MUST reject Hold Time values of one or two seconds.
An implementation MAY reject any proposed Hold Time. An
implementation that accepts a Hold Time MUST use the negotiated
value for the Hold Time.
If the TRIP Identifier field of the OPEN message is not valid, then
the Error Subcode MUST be set to "Bad TRIP Identifier." A TRIP
identifier is 4-octets and can take any value. An LS considers the
TRIP Identifier invalid if it has an already open connection with
another peer LS that has the same ITAD and TRIP Identifier.
Any two LSs within the same ITAD MUST NOT have equal TRIP Identifier
values. This restriction does not apply to LSs in differrent ITADs
since the purpose is to uniquely identify an LS using its TRIP
Identifier and its ITAD number.
If one of the Optional Parameters in the OPEN message is not
recognized, then the Error Subcode MUST be set to "Unsupported
Optional Parameters."
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If the Optional Parameters of the OPEN message include Capability
Information with an unsupported capability (unsupported in either
capability type or value), then the Error Subcode MUST be set to
"Unsupported Capability," and the entirety of the unsupported
capabilities are listed in the Data field of the NOTIFICATION
message.
7.3 UPDATE Message Error Detection and Handling
All errors detected while processing the UPDATE message are
indicated by sending the NOTIFICATION message with Error Code
`UPDATE Message Error.' The Error Subcode elaborates on the specifies
nature of the error. The error checks in this section MUST be
performed by each LS on receipt of every UPDATE message. These
error checks MUST occur before flooding procedures are invoked with
internal peers.
If any recognized attribute has Attribute Flags that conflict with
the Attribute Type Code, then the Error Subcode MUST be set to
"Attribute Flags Error." The Data field contains the erroneous
attribute (type, length and value).
If any recognized attribute has Attribute Length that conflicts with
the expected length (based on the attribute type code), then the
Error Subcode MUST be set to "Attribute Length Error." The Data
field contains the erroneous attribute (type, length and value).
If any of the mandatory well-known attributes are not present, then
the Error Subcode MUST be set to "Missing Well-known Mandatory
Attribute." The Data field contains the Attribute Type Code of the
missing well-known mandatory attributes.
If any of the well-known attributes are not recognized, then the
Error Subcode MUST be set to "Unrecognized Well-known Attribute."
The Data field contains the unrecognized attribute (type, length and
value).
If any attribute has a syntactically incorrect value, or an
undefined value, then the Error Subcode is set to "Invalid
Attribute." The Data field contains the incorrect attribute (type,
length and value). Such a NOTIFICATION message is sent, for example,
when a NextHopServer attribute is received with an invalid address.
The information carried by the AdvertisementPath attribute is
checked for ITAD loops. ITAD loop detection is done by scanning the
full AdvertisementPath, and checking that the ITAD number of the
local ITAD does not appear in the AdvertisementPath. If the local
ITAD number appears in the AdvertisementPath, then the route MAY be
stored in the Adj-TRIB-In, but unless the LS is configured to accept
call routes with its own ITAD in the advertisement path, the call
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route MUST not be passed to the TRIP Decision Process. The operation
of an LS that is configured to accept call routes with its own ITAD
number in the advertisement path are outside the scope of this
document.
If the UPDATE message was received from an internal peer and either
the WithdrawnRoutes, ReachableRoutes, or ITAD Topology attribute
does not have the Link-State Encapsulation flag set, then the Error
Subcode is set to "Invalid Attribute" and the data field contains
the attribute. Likewise, the attribute is invalid if received from
an external peer and the Link-State Flag is set.
If any attribute appears more than once in the UPDATE message, then
the Error Subcode is set to "Malformed Attribute List."
7.4 NOTIFICATION Message Error Detection and Handling
If a peer sends a NOTIFICATION message, and there is an error in
that message, there is unfortunately no means of reporting this
error via a subsequent NOTIFICATION message. Any such error, such as
an unrecognized Error Code or Error Subcode, should be noticed,
logged locally, and brought to the attention of the administration
of the peer. The means to do this, however, are outside the scope of
this document.
7.5 Hold Timer Expired Error Handling
If a system does not receive successive messages within the period
specified by the negotiated Hold Time, then a NOTIFICATION message
with "Hold Timer Expired" Error Code MUST be sent and the TRIP
connection MUST be closed.
7.6 Finite State Machine Error Handling
An error detected by the TRIP Finite State Machine (e.g., receipt of
an unexpected event) MUST result in sending a NOTIFICATION message
with Error Code "Finite State Machine Error" and the TRIP connection
MUST be closed.
7.7 Cease
In the absence of any fatal errors (that are indicated in this
section), a TRIP peer MAY choose at any given time to close its TRIP
connection by sending the NOTIFICATION message with Error Code
"Cease." However, the Cease NOTIFICATION message MUST NOT be used
when a fatal error indicated by this section exists.
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7.8 Connection Collision Detection
If a pair of LSs try simultaneously to establish a transport
connection to each other, then two parallel connections between this
pair of speakers might well be formed. We refer to this situation as
connection collision. Clearly, one of these connections must be
closed.
Based on the value of the TRIP Identifier a convention is
established for detecting which TRIP connection is to be preserved
when a collision occurs. The convention is to compare the TRIP
Identifiers of the peers involved in the collision and to retain
only the connection initiated by the LS with the higher-valued TRIP
Identifier.
Upon receipt of an OPEN message, the local LS MUST examine all of
its connections that are in the OpenConfirm state. An LS MAY also
examine connections in an OpenSent state if it knows the TRIP
Identifier of the peer by means outside of the protocol. If among
these connections there is a connection to a remote LS whose TRIP
Identifier equals the one in the OPEN message, then the local LS
MUST perform the following collision resolution procedure:
1. The TRIP Identifier and ITAD of the local LS is compared to
the TRIP Identifier and ITAD of the remote LS (as specified
in the OPEN message). TRIP Identifiers are treated as 4-
octet unsigned integers for comparison.
2. If the value of the local TRIP Identifier is less than the
remote one, or if the two TRIP Identifiers are equal and the
value of ITAD of the local LS is less than value of the ITAD
of the remote LS, then the local LS MUST close the TRIP
connection that already exists (the one that is already in
the OpenConfirm state), and accepts the TRIP connection
initiated by the remote LS.
3. Otherwise, the local LS closes newly created TRIP connection
(the one associated with the newly received OPEN message),
and continues to use the existing one (the one that is
already in the OpenConfirm state).
If a connection collision occurs with an existing TRIP connection
that is in the Established state, then the LS MUST unconditionally
close of the newly created connection. Note that a connection
collision cannot be detected with connections that are in Idle,
Connect, or Active states.
To close the TRIP connection (that results from the collision
resolution procedure), an LS MUST send a NOTIFICATION message with
the Error Code "Cease" and the TRIP connection MUST be closed.
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8. TRIP Version Negotiation
Peer LSs may negotiate the version of the protocol by making
multiple attempts to open a TRIP connection, starting with the
highest version number each supports. If an open attempt fails with
an Error Code "OPEN Message Error" and an Error Subcode "Unsupported
Version Number," then the LS has available the version number it
tried, the version number its peer tried, the version number passed
by its peer in the NOTIFICATION message, and the version numbers
that it supports. If the two peers support one or more common
versions, then this will allow them to rapidly determine the highest
common version. In order to support TRIP version negotiation, future
versions of TRIP must retain the format of the OPEN and NOTIFICATION
messages.
9. TRIP Capability Negotiation
An LS MAY include the Capabilities Option in its OPEN message to a
peer to indicate the capabilities supported by the LS. An LS
receiving an OPEN message MUST NOT use any capabilities that were
not included in the OPEN message of the peer when communicating with
that peer.
10. TRIP Finite State Machine
This section specifies TRIP operation in terms of a Finite State
Machine (FSM). Following is a brief summary and overview of TRIP
operations by state as determined by this FSM. A condensed version
of the TRIP FSM is found in Appendix 1. There is a TRIP FSM per
peer and these FSMs operate independently.
Idle state:
Initially TRIP is in the Idle state for each peer. In this state,
TRIP refuses all incoming connections. No resources are allocated
to the peer. In response to the Start event (initiated by either
the system or the operator), the local system initializes all TRIP
resources, starts the ConnectRetry timer, initiates a transport
connection to the peer, starts listening for a connection that may
be initiated by the remote TRIP peer, and changes its state to
Connect. The exact value of the ConnectRetry timer is a local
matter, but should be sufficiently large to allow TCP
initialization.
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If an LS detects an error, it closes the transport connection and
changes its state to Idle. Transitioning from the Idle state
requires generation of the Start event. If such an event is
generated automatically, then persistent TRIP errors may result in
persistent flapping of the LS. To avoid such a condition, Start
events MUST NOT be generated immediately for a peer that was
previously transitioned to Idle due to an error. For a peer that
was previously transitioned to Idle due to an error, the time
between consecutive Start events, if such events are generated
automatically, MUST exponentially increase. The value of the
initial timer SHOULD be 60 seconds, and the time SHOULD be at
least doubled for each consecutive retry up to some maximum value.
Any other event received in the Idle state is ignored.
Connect state:
In this state, an LS is waiting for a transport protocol
connection to be completed to the peer, and is listening for
inbound transport connections from the peer.
If the transport protocol connection succeeds, the local LS clears
the ConnectRetry timer, completes initialization, sends an OPEN
message to its peer, sets its Hold Timer to a large value, and
changes its state to OpenSent. A Hold Timer value of 4 minutes is
suggested.
If the transport protocol connect fails (e.g., retransmission
timeout), the local system restarts the ConnectRetry timer,
continues to listen for a connection that may be initiated by the
remote LS, and changes its state to Active state.
In response to the ConnectRetry timer expired event, the local LS
cancels any outstanding transport connection to the peer, restarts
the ConnectRetry timer, initiates a transport connection to the
remote LS, continues to listen for a connection that may be
initiated by the remote LS, and stays in the Connect state.
If the local LS detects that a remote peer is trying to establish
a connection to it and the IP address of the peer is not an
expected one, then the local LS rejects the attempted connection
and continues to listen for a connection from its expected peers
without changing state.
If an inbound transport protocol connection succeeds, the local LS
clears the ConnectRetry timer, completes initialization, sends an
OPEN message to its peer, sets its Hold Timer to a large value,
and changes its state to OpenSent. A Hold Timer value of 4
minutes is suggested.
The Start event is ignored in the Connect state.
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In response to any other event (initiated by either the system or
the operator), the local system releases all TRIP resources
associated with this connection and changes its state to Idle.
Active state:
In this state, an LS is listening for an inbound connection from
the peer, but is not in the process of initiating a connection to
the peer.
If an inbound transport protocol connection succeeds, the local LS
clears the ConnectRetry timer, completes initialization, sends an
OPEN message to its peer, sets its Hold Timer to a large value,
and changes its state to OpenSent. A Hold Timer value of 4
minutes is suggested.
In response to the ConnectRetry timer expired event, the local
system restarts the ConnectRetry timer, initiates a transport
connection to the TRIP peer, continues to listen for a connection
that may be initiated by the remote TRIP peer, and changes its
state to Connect.
If the local LS detects that a remote peer is trying to establish
a connection to it and the IP address of the peer is not an
expected one, then the local LS rejects the attempted connection
and continues to listen for a connection from its expected peers
without changing state.
Start event is ignored in the Active state.
In response to any other event (initiated by either the system or
the operator), the local system releases all TRIP resources
associated with this connection and changes its state to Idle.
OpenSent state:
In this state, an LS has sent an OPEN message to its peer and is
waiting for an OPEN message from its peer. When an OPEN message is
received, all fields are checked for correctness. If the TRIP
message header checking or OPEN message checking detects an error
(see Section 7.2) or a connection collision (see Section
7.8), the local system sends a NOTIFICATION message and changes
its state to Idle.
If there are no errors in the OPEN message, TRIP sends a KEEPALIVE
message and sets a KeepAlive timer. The Hold Timer, which was
originally set to a large value (see above), is replaced with the
negotiated Hold Time value (see Section 5.2). If the negotiated
Hold Time value is zero, then the Hold Time timer and KeepAlive
timers are not started. If the value of the ITAD field is the same
as the local ITAD number, then the connection is an "internal"
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connection; otherwise, it is "external" (this will effect UPDATE
processing). Finally, the state is changed to OpenConfirm.
If the local LS detects that a remote peer is trying to establish
a connection to it and the IP address of the peer is not an
expected one, then the local LS rejects the attempted connection
and continues to listen for a connection from its expected peers
without changing state.
If a disconnect notification is received from the underlying
transport protocol, the local LS closes the transport connection,
restarts the ConnectRetry timer, continues to listen for a
connection that may be initiated by the remote TRIP peer, and goes
into the Active state.
If the Hold Timer expires, the local LS sends NOTIFICATION message
with Error Code "Hold Timer Expired" and changes its state to
Idle.
In response to the Stop event (initiated by either system or
operator) the local LS sends NOTIFICATION message with Error Code
"Cease" and changes its state to Idle.
The Start event is ignored in the OpenSent state.
In response to any other event the local LS sends NOTIFICATION
message with Error Code "Finite State Machine Error" and changes
its state to Idle.
Whenever TRIP changes its state from OpenSent to Idle, it closes
the transport connection and releases all resources associated
with that connection.
OpenConfirm state:
In this state, an LS has sent an OPEN to its peer, received an
OPEN from its peer, and sent a KEEPALIVE in response to the OPEN.
The LS is now waiting for a KEEPALIVE or NOTIFICATION message in
response to its OPEN.
If the local LS receives a KEEPALIVE message, it changes its state
to Established.
If the Hold Timer expires before a KEEPALIVE message is received,
the local LS sends NOTIFICATION message with Error Code "Hold
Timer Expired" and changes its state to Idle.
If the local LS receives a NOTIFICATION message, it changes its
state to Idle.
If the KeepAlive timer expires, the local LS sends a KEEPALIVE
message and restarts its KeepAlive timer.
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If a disconnect notification is received from the underlying
transport protocol, the local LS closes the transport connection,
restarts the ConnectRetry timer, continues to listen for a
connection that may be initiated by the remote TRIP peer, and goes
into the Active state.
In response to the Stop event (initiated by either the system or
the operator) the local LS sends NOTIFICATION message with Error
Code "Cease" and changes its state to Idle.
Start event is ignored in the OpenConfirm state.
In response to any other event the local LS sends NOTIFICATION
message with Error Code "Finite State Machine Error" and changes
its state to Idle.
Whenever TRIP changes its state from OpenConfirm to Idle, it
closes the transport connection and releases all resources
associated with that connection.
Established state:
In the Established state, an LS can exchange UPDATE, NOTIFICATION,
and KEEPALIVE messages with its peer.
If the negotiated Hold Timer is zero, then no procedures are
necessary for keeping a peering session alive. If the negotiated
Hold Time value is non-zero, the procedures of this paragraph
apply. If the Hold Timer expires, the local LS sends a
NOTIFICATION message with Error Code "Hold Timer Expired" and
changes its state to Idle. If the KeepAlive Timer expires, then
the local LS sends a KeepAlive message and restarts the KeepAlive
Timer. If the local LS receives an UPDATE or KEEPALIVE message,
then it restarts its Hold Timer. Each time the LS sends an UPDATE
or KEEPALIVE message, it restarts its KeepAlive Timer.
If the local LS receives a NOTIFICATION message, it changes its
state to Idle.
If the local LS receives an UPDATE message and the UPDATE message
error handling procedure (see Section7.3) detects an error, the
local LS sends a NOTIFICATION message and changes its state to
Idle.
If a disconnect notification is received from the underlying
transport protocol, the local LS changes its state to Idle.
In response to the Stop event (initiated by either the system or
the operator), the local LS sends a NOTIFICATION message with
Error Code "Cease" and changes its state to Idle.
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The Start event is ignored in the Established state.
In response to any other event, the local LS sends NOTIFICATION
message with Error Code `
`Finite State Machine Error'
' and changes
its state to Idle.
Whenever TRIP changes its state from Established to Idle, it
closes the transport) connection, releases all resources
associated with that connection. Additionally, if the peer is an
external peer, the LS deletes all routes derived from that
connection.
11. UPDATE Message Handling
An UPDATE message may be received only in the Established state.
When an UPDATE message is received, each field is checked for
validity as specified in Section 7.3. The rest of this section
presumes that the UPDATE message has passed the error-checking
procedures of Section 7.3.
If the UPDATE message was received from an internal peer, the
flooding procedures of Section 11.1 MUST be applied. The flooding
process synchronizes the databases of all LSs within the domain.
Certain routes within the UPDATE may be marked as old or duplicates
by the flooding process and are ignored during the rest of the
UPDATE processing.
If the UPDATE message contains withdrawn call routes, then the
corresponding previously advertised call routes shall be removed
from the Adj-TRIB-In. This LS MUST run its Decision Process since
the previously advertised call route is no longer available for use.
If the UPDATE message contains a call route, then the route MUST be
placed in the appropriate Adj-TRIB-In, and the following additional
actions MUST be taken:
i) If its destinations are identical to those of a call route
currently stored in the Adj-TRIB-In, then the new call route
MUST replace the older route in the Adj-TRIB-In, thus
implicitly withdrawing the older call route from service.
The LS MUST run its Decision Process since the older call
route is no longer available for use.
ii) If the new call route is more specific than an earlier route
contained in the Adj-TRIB-In and has identical attributes,
then no further actions are necessary.
iii) If the new call route is more specific than an earlier call
route contained in the Adj-TRIB-In but does not have
identical attributes, then the LS MUST run its Decision
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Process since the more specific call route has implicitly
made a portion of the less specific call route unavailable
for use.
iv) If the new call route has destinations that are not present
in any of the routes currently stored in the Adj-TRIB-In,
then the LS MUST run its Decision Process.
v) If the new call route is less specific than an earlier call
route contained in the Adj-TRIB-In, the LS MUST run its
Decision Process on the set of destinations that are
described only by the less specific call route.
11.1 Flooding Process
When an LS receives an UPDATE message from an internal peer, the LS
floods the new information from that message to all of its other
internal peers. Flooding is used to efficiently synchronize all of
the LSs within a domain without putting any constraints on the
domain's internal topology. The flooding mechanism is based on the
techniques used in OSPF [3] and SCSP [4].
11.1.1 Database Information
The LS MUST maintain the sequence number and originating TRIP
identifier for each link-state encapsulated attribute in an internal
Adj-TRIB-In. These values are included with the route in the
ReachableRoutes, WithdrawnRoutes, and ITAD Topology attributes. The
originating TRIP identifier gives the internal LS that originated
this route into the ITAD, the sequence number gives the version of
this route at the originating LS.
11.1.2 Determining Newness
For each route in the ReachableRoutes or WithdrawnRoutes field, the
LS decides if the route is new or old. This is determined by
comparing the Sequence Number of the route in the UPDATE with the
Sequence Number of the route saved in the Adj-TRIB-In. The route is
new if either the route does not exist in the Adj-TRIB-In for the
originating LS, or if the route does exist in the Adj-TRIB-In but
the Sequence Number in the UPDATE is greater than the Sequence
Number saved in the Adj-TRIBs-In. Note that the newness test is
independently applied to each link-state encapsulated attribute in
the UPDATE (WithdrawnRoutes or ReachableRoutes).
11.1.3 Flooding
Each route in the ReachableRoutes or WithdrawnRoutes field that is
determined to be old is ignored in further processing. If the route
is determined to be new then the following actions occur.
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Internet Draft Telephony Routing over IP January 2000
If the route is being withdrawn, then the LS MUST flood the
withdrawn route to all other internal peers, and MUST mark the route
as withdrawn. An LS MUST maintain routes marked as withdrawn in its
databases for MaxPurgeTime seconds.
If the route is being updated, then the LS MUST update the route in
the Adj-TRIB-In and MUST flood it to all other internal peers.
If these procedures result in changes to the Adj-TRIB-In, then the
route is also made available for local route processing as described
early in Section 11.
To implement flooding, the following is recommended. All routes
received in a single UPDATE message that are determined to be new
may be forwarded to all other internal peers in a single UPDATE
message. Other variations on flooding are possible, but the local
LS MUST ensure that each new route (and any associated attributes)
received from an internal peer get forwarded to every other internal
peer.
11.1.4 Sequence Number Considerations
The Sequence Number is used to determine when one version of a route
is newer than another version of a route. A larger Sequence Number
indicates a newer version. The Sequence Number is assigned by the
LS originating the route into the local ITAD. The Sequence Number
is an unsigned 4-octet integer in the range of 1 thru 2^31-1
(MinSequenceNum thru MaxSequenceNum). The value 0 is reserved.
When an LS first originates a route into its ITAD, it MUST originate
it with a Sequence Number of MinSequenceNum. Each time the route is
updated within the ITAD by the originator, the Sequence Number MUST
be increased.
If it is ever the case that the sequence number is MaxSequenceNum-1
and it needs to be increased, then the TRIP module of the LS MUST be
disabled for a period of TripDisableTime so that all routes
originated by this LS with high sequence numbers can be removed.
11.1.5 Purging a Route Within the ITAD
To withdraw a route that it originated within the ITAD, an LS
includes the route in the WithdrawnRoutes field of an UPDATE
message. The Sequence Number MUST be greater than the last valid
version of the route. The LS MAY choose to use a sequence number of
MaxSequenceNum when withdrawing routes within its ITAD, but this is
not required.
After withdrawing a route, an LS MUST mark the route as `withdrawn'
in its database, and maintain the withdrawn route in its database
for MaxPurgeTime seconds. If the LS needs to re-originate a route
that had been purged but is still in its database, it can either re-
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Internet Draft Telephony Routing over IP January 2000
originate the route immediately using a Sequence Number that is
greater than that used in the withdraw, or the LS may wait until
MaxPurgeTime seconds have expired since the route was withdrawn.
11.1.6 Receiving Self-Originated Routes
It is common for an LS to receive UPDATES for routes that it
originated within the ITAD via the flooding procedure. If the LS
receives an UPDATE for a route that it originated that is newer (has
a higher sequence number) than the LSs current version, then special
actions must be taken. This should be a relatively rare occurrence
and indicates that a route still exists within the ITAD since the
LSs last restart/reboot.
If an LS receives a self-originated route update that is newer than
the current version of the route at the LS, then the following
actions MUST be taken. If the LS still wishes to advertise the
information in the route, then the LS MUST the increase the Sequence
Number of the route to a value greater than that received in the
UPDATE and re-originate the route. If the LS does not wish to
continue to advertise the route, then it MUST purge the route as
described in Section 11.1.5.
11.1.7 Removing Withdrawn Routes
An LS SHOULD ensure that routes marked as withdrawn are removed from
the database in a timely fashion after the MaxPurgeTime has expired.
This could be done, for example, by periodically sweeping the
database, and deleting those entries that were withdrawn more than
MaxPurgeTime seconds ago.
11.2 Decision Process
The Decision Process selects call routes for subsequent
advertisement by applying the policies in the local Policy
Information Base (PIB) to the call routes stored in its Adj-TRIBs-
In. The output of the Decision Process is the set of call routes
that will be advertised to all peers; the selected call routes will
be stored in the local LS's Adj-TRIBs-Out.
The selection process is formalized by defining a function that
takes the attributes of a given call route as an argument and
returns a non-negative integer denoting the degree of preference for
the call route. The function that calculates the degree of
preference for a given call route shall not use as its inputs any of
the following: the existence of other call routes, the non-
existence of other call routes, or the attributes of other call
routes. Call route selection then consists of individual application
of the degree of preference function to each feasible call route,
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followed by the choice of the one with the highest degree of
preference.
The Decision Process operates on call routes contained in each Adj-
TRIBs-In, and is responsible for:
- selection of call routes to be advertised to internal peers
- selection of call routes to be advertised to external peers
- call route aggregation and call route information reduction
The Decision Process takes place in three distinct phases, each
triggered by a different event:
a) Phase 1 is responsible for calculating the degree of
preference for each call route received from an external peer,
and for advertising to all the internal peers the call routes
from external peers that have the highest degree of preference
for each distinct destination.
b) Phase 2 is invoked on completion of phase 1. It is
responsible for choosing the best call route out of all those
available for each distinct destination, and for installing
each chosen call route into the Loc-TRIB.
c) Phase 3 is invoked after the Loc-TRIB has been modified. It
is responsible for disseminating call routes in the Loc-TRIB
to each external peer, according to the policies contained in
the PIB. Call route aggregation and information reduction can
optionally be performed within this phase.
11.2.1 Phase 1: Calculation of Degree of Preference
The Phase 1 decision function shall be invoked whenever the local LS
receives from a peer an UPDATE message that advertises a new call
route, a replacement call route, or a withdrawn call route.
The Phase 1 decision function is a separate process that completes
when it has no further work to do.
The Phase 1 decision function shall lock an Adj-TRIB-In prior to
operating on any call route contained within it, and shall unlock it
after operating on all new or replacement call routes contained
within it.
The local LS MUST determine a degree of preference for each newly
received or replacement call route. If the call route is learned
from an internal peer, the value of the LocalPreference attribute
MUST be taken as the degree of preference. If the call route is
learned from an external peer, then the degree of preference MUST be
computed based on pre-configured policy information and used as the
LocalPreference value in any intra-domain TRIP advertisement. The
exact nature of this policy information and the computation involved
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is a local matter. The local LS MUST then run the internal update
process of 11.3.1 to select and advertise the most preferable call
routes.
The output of the degree of preference determination process is the
local preference of a call route. The local LS computes the local
preference of call routes learned from external peers or originated
internally at that LS. The local preference of a call route learned
from an internal peer is included in the LocalPreference attribute
associated with that call route.
11.2.2 Phase 2: Call Route Selection
The Phase 2 decision function shall be invoked on completion of
Phase 1. The Phase 2 function is a separate process that completes
when it has no further work to do. The Phase 2 process MUST consider
all call routes that are present in the Adj-TRIBs-In, including
those received from both internal and external peers.
The Phase 2 decision function MUST be blocked from running while the
Phase 3 decision function is in process. The Phase 2 function MUST
lock all Adj-TRIBs-In prior to commencing its function, and MUST
unlock them on completion.
If the LS determines that the NextHopServer listed in a call route
is unreachable, then the call route MAY be excluded from the Phase 2
decision function. The means by which such a determination is made
is not mandated here.
For each set of destinations for which a call route exists in the
Adj-TRIBs-In, the local LS MUST identify the call route that has:
a) the highest degree of preference of any call route to the
same set of destinations, or
b) is selected as a result of the Phase 2 tie breaking rules
specified in 11.2.2.1.
The local LS MUST then install that call route in the Loc-TRIB,
replacing any call route to the same destination that is currently
being held in the Loc-TRIB.
Withdrawn call routes MUST be removed from the Loc-TRIB and the Adj-
TRIBs-In.
11.2.2.1 Breaking Ties (Phase 2)
In its Adj-TRIBs-In an LS may have several call routes to the same
destination that have the same degree of preference. The local LS
can select only one of these call routes for inclusion in the
associated Loc-TRIB. The local LS considers all call routes with the
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same degrees of preference, both those received from internal peers,
and those received from external peers. The following algorithm
shall be used to break ties.
(a) If the local LS is configured to use the MultiExitDisc
attribute to break ties, and the candidate routes differ in
the value of the MultiExitDisc attribute, then select the
route that has the larger value of MultiExitDisc.
(b) If at least one of the routes was advertised by an LS in a
neighboring ITAD, then select the route that was advertised
by the LS that has the smallest TRIP ID.
(c) Otherwise, select the route that was advertised by the
internal LS that has the lowest TRIP ID.
11.2.3 Phase 3: Route Dissemination
The Phase 3 decision function MUST be invoked on completion of Phase
2, or when any of the following events occur:
a) when locally generated call routes learned by means outside of
TRIP have changed, or
b) when a new LS-to-LS peer connection has been established.
The Phase 3 function is a separate process that completes when it
has no further work to do. The Phase 3 Call Routing Decision
function MUST be blocked from running while the Phase 2 decision
function is in process.
All call routes in the Loc-TRIB shall be processed into a
corresponding entry in the associated Adj-TRIBs-Out. Call route
aggregation and information reduction techniques (see 11.3.4) MAY
optionally be applied.
When the updating of the Adj-TRIBs-Out is complete, the local LS
MUST run the external update process of 11.3.2.
11.2.4 Overlapping Call Routes
When overlapping call routes are present in the same Adj-TRIB-In,
the more specific call route shall take precedence, in order from
more specific to least specific.
The set of destinations described by the overlap represents a
portion of the less specific call route that is feasible, but is not
currently in use. If a more specific call route is later withdrawn,
the set of destinations described by the overlap will still be
reachable using the less specific call route.
If an LS receives overlapping routes, the Decision Process MUST take
into account the semantics of the overlapping routes. In particular,
if an LS accepts the less specific route while rejecting the more
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specific route from the same peer, then the destinations represented
by the overlap may not forward along the domains listed in the
AdvertisementPath attribute of that route. Therefore, an LS has the
following choices:
a) Install both the less and the more specific routes
b) Install the more specific route only
c) Install the non-overlapping part of the less specific route
only (that implies de-aggregation)
d) Aggregate the two routes and install the aggregated route
e) Install the less specific route only
f) Install neither route
If an LS chooses e), then it SHOULD add AtomicAggregate attribute to
the route. A route that carries AtomicAggregate attribute MUST NOT
be de-aggregated. That is, the route cannot be made more specific.
Forwarding along such a route does not guarantee that route
traverses only domains listed in the AdvertisementPath of the route.
If an LS chooses a), then it MUST NOT advertise the more general
route without the more specific route.
11.3 Update-Send Process
The Update-Send process is responsible for advertising UPDATE
messages to all peers. For example, it distributes the call routes
chosen by the Decision Process to other LSs that may be located in
either the same ITAD or a neighboring ITAD. Rules for information
exchange between peer LSs located in different ITADs are given in
11.3.2; rules for information exchange between peer LSs located in
the same ITAD are given in 11.3.1.
Before forwarding routes to peers, an LS MUST determine which
attributes should be forwarded along with that route. If an
optional non-transitive attribute is unrecognized, it is quietly
ignored. If an optional dependent-transitive attribute is
unrecognized, and the NextHopServer attribute has been changed by
the LS, the unrecognized attribute is quietly ignored. If an
optional dependent-transitive attribute is unrecognized, and the
NextHopServer attribute has not been modified by the LS, the Partial
bit in the attribute flags octet is set to 1, and the attribute is
retained for propagation to other TRIP speakers. Similarly, if an
optional independent-transitive attribute is unrecognized, the
Partial bit in the attribute flags octet is set to 1, and the
attribute is retained for propagation to other TRIP speakers.
If an optional attribute is recognized, and has a valid value, then,
depending on the type of the optional attribute, it is updated, if
necessary, for possible propagation to other TRIP speakers.
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11.3.1 Internal Updates
The Internal update process is concerned with the distribution of
call routing information to internal peers.
When an LS receives an UPDATE message from another BGP speaker
located in its own autonomous system, it is flooded as described in
Section 11.1.
When an LS receives a new route from an LS in a neighboring ITAD, it
determines the preference of that route. If the new route has the
highest degree of preference for all routes to some destination
received from external peers by that LS, or if the route was
selected via a tie-breaking procedure as specified in 11.3.1.1), the
LS MUST advertise that route to all other LSs in its ITAD by means
of an UPDATE message.
When an LS receives an UPDATE message with a non-empty
WithdrawnRoutes attribute from an external peer, the LS MUST remove
from its Adj-RIB-In all routes whose destinations were carried in
this field. If the route had been previously advertised, the LS
MUST take the following additional steps:
i) if a new route is selected for advertisement for those
destinations, then the local LS MUST advertise the
replacement route
ii) if a replacement route is not available for advertisement,
then the LS MUST include the destinations of the route in
the WithdrawnRoutes attribute of an UPDATE message, and
MUST send this message to each internal peer.
All routes that are advertised MUST be placed in the appropriate
Adj-RIBs-Out, and all routes that are withdrawn MUST be removed from
the Adj-RIBs-Out.
11.3.1.1 Breaking Ties (Internal Updates)
If an LS has connections to several external peers, there will be
multiple Adj-TRIBs-In associated with these peers. These databases
might contain several equally preferable call routes to the same
destination, all of which were advertised by external peers. The
local LS shall select one of these routes according to the following
rules:
(a) If the LS is configured to use the MultiExitDisc attribute to
break ties, and the candidate routes differ in the value of the
MultiExitDisc attribute, then select the route that has the
lowest value of MultiExitDisc, else
(b) Select the route that was advertised by the external LS that
has the lowest TRIP Identifier.
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11.3.2 External Updates
The external update process is concerned with the distribution of
routing information to external peers. As part of Phase 3 call
route selection process, the LS has updated its Adj-TRIBs-Out. All
newly installed call routes and all newly unfeasible call routes for
which there is no replacement call route MUST be advertised to
external peers by means of UPDATE messages.
Any routes in the Loc-TRIB marked as withdrawn MUST be removed.
Changes to the reachable destinations within its own ITAD shall also
be advertised in an UPDATE message.
11.3.3 Controlling Routing Traffic Overhead
The TRIP protocol constrains the amount of call routing traffic
(that is, UPDATE messages) in order to limit both the link bandwidth
needed to advertise UPDATE messages and the processing power needed
by the Decision Process to digest the information contained in the
UPDATE messages.
11.3.3.1 Frequency of Call Route Advertisement
The parameter MinCallRouteAdvInterval determines the minimum amount
of time that must elapse between advertisements of call routes to a
particular destination from a single LS. This rate limiting
procedure applies on a per-destination basis, although the value of
MinCallRouteAdvInterval is set on a per LS peer basis.
Two UPDATE messages sent from a single LS that advertise feasible
call routes to some common set of destinations received from
external peers must be separated by at least
MinCallRouteAdvInterval. Clearly, this can only be achieved
precisely by keeping a separate timer for each common set of
destinations. This would be unwarranted overhead. Any technique
which ensures that the interval between two UPDATE messages sent
from a single LS that advertise feasible call routes to some common
set of destinations received from external peers will be at least
MinCallRouteAdvInterval, and will also ensure a constant upper bound
on the interval is acceptable.
Since fast convergence is needed within an autonomous system, this
procedure does not apply for call routes received from other
internal peers. To avoid long-lived black holes, the procedure does
not apply to the explicit withdrawal of routes (that is, routes
whose destinations explicitly withdrawn by UPDATE messages.
This procedure does not limit the rate of call route selection, but
only the rate of call route advertisement. If new call routes are
selected multiple times while awaiting the expiration of
MinCallRouteAdvInterval, the last call route selected shall be
advertised at the end of MinCallRouteAdvInterval.
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11.3.3.2 Frequency of Route Origination
The parameter MinITADOriginationInterval determines the minimum
amount of time that must elapse between successive advertisements of
UPDATE messages that report changes within the advertising LS's own
ITAD.
11.3.3.3 Jitter
To minimize the likelihood that the distribution of TRIP messages by
a given LS will contain peaks, jitter should be applied to the
timers associated with MinITADOriginationInterval, KeepAlive, and
MinCallRouteAdvInterval. A given LS shall apply the same jitter to
each of these quantities regardless of the destinations to which the
updates are being sent; that is, jitter will not be applied on a
"per peer" basis.
The amount of jitter to be introduced shall be determined by
multiplying the base value of the appropriate timer by a random
factor that is uniformly distributed in the range from 0.75 to 1.0.
11.3.4 Efficient Organization of Routing Information
Having selected the call routing information that it will advertise,
a TRIP speaker may use methods to organize this information in an
efficient manner. These methods are discussed in the following
sections.
11.3.4.1 Information Reduction
Information reduction may imply a reduction in granularity of policy
control - after information is collapsed, the same policies will
apply to all destinations and paths in the equivalence class.
The Decision Process may optionally reduce the amount of information
that it will place in the Adj-TRIBs-Out by any of the following
methods:
a) ReachableRoutes:
A set of destinations can be usually represented in compact form.
For example, a set of E.164 phone numbers can be represented in more
compact form using E.164 prefixes.
b) AdvertisementPath:
AdvertisementPath information can be represented as ordered
AP_SEQUENCEs or unordered AP_SETs. AP_SETs are used in the call
route aggregation algorithm described in 11.2.4.2. They reduce the
size of the AP_PATH information by listing each ITAD number only
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once, regardless of how many times it may have appeared in multiple
advertisement paths that were aggregated.
An AP_SET implies that the destinations advertised in the UPDATE
message can be reached through paths that traverse at least some of
the constituent ITADs. AP_SETs provide sufficient information to
avoid call route looping; however their use may prune potentially
feasible paths, since such paths are no longer listed individually
as in the form of AP_SEQUENCEs. In practice this is not likely to be
a problem, since once an call arrives at the edge of a group of
ITADs, the LS at that point is likely to have more detailed path
information and can distinguish individual paths to destinations.
11.3.4.2 Aggregating Call Routing Information
Aggregation is the process of combining the characteristics of
several different call routes in such a way that a single call route
can be advertised. Aggregation can occur as part of the decision
process to reduce the amount of call routing information that is
placed in the Adj-TRIBs-Out.
Aggregation reduces the amount of information an LS must store and
exchange with other LSs. Call routes can be aggregated by applying
the following procedure separately to attributes of like type.
Call routes that have the following attributes shall not be
aggregated unless the corresponding attributes of each call route
are identical: MultiExitDisc, NextHopServer.
Attributes that have different type codes cannot be aggregated.
Attributes of the same type code may be aggregated. The rules for
aggregating each attribute MUST be provided together with attribute
definition. For example, aggregation rules for TRIP's basic
attributes, e.g., ReachableRoutes and AdvertisementPath, are given
in 6.
11.4 Call Route Selection Criteria
Generally speaking, additional rules for comparing call routes among
several alternatives are outside the scope of this document. There
are two exceptions:
- If the local ITAD appears in the AdvertisementPath of the new
call route being considered, then that new call route cannot
be viewed as better than any other call route. If such a call
route were ever used, a call routing loop could result (see
Section 7.3).
- In order to achieve successful distributed operation, only call
routes with a likelihood of stability can be chosen. Thus, an
ITAD must avoid using unstable call routes, and it must not
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make rapid spontaneous changes to its choice of call route.
Quantifying the terms "unstable" and "rapid" in the previous
sentence will require experience, but the principle is clear.
11.5 Originating TRIP routes
An LS may originate TRIP call routes by injecting call routing
information acquired by some other means (e.g. via an intra-domain
call routing protocol or through manual configuration or some
dynamic registration mechanism/protocol) into TRIP. An LS that
originates TRIP routes shall assign the degree of preference to
these call routes by passing them through the Decision Process (see
Section 11.2). These call routes may also be distributed to other
LSs within the local ITAD as part of the Internal update process
(see Section 11.3.1). The decision whether to distribute non-TRIP
acquired routes within an ITAD via TRIP or not depends on the
environment within the ITAD (e.g. type of intra-domain call routing
protocol) and should be controlled via configuration.
12. TRIP Transport
This specification defines the use of TCP as the transport layer for
TRIP. TRIP uses TCP port XXX. Running TRIP over other transport
protocols is for further study.
Editor's Note: We need to get a TCP port for TRIP.
Appendix 1. TRIP FSM State Transitions and Actions
This Appendix discusses the transitions between states in the TRIP
FSM in response to TRIP events. The following is the list of these
states and events when the negotiated Hold Time value is non-zero.
TRIP States:
1 - Idle
2 - Connect
3 - Active
4 - OpenSent
5 - OpenConfirm
6 - Established
TRIP Events:
1 - TRIP Start
2 - TRIP Stop
3 - TRIP Transport connection open
4 - TRIP Transport connection closed
5 - TRIP Transport connection open failed
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6 - TRIP Transport fatal error
7 - ConnectRetry timer expired
8 - Hold Timer expired
9 - KeepAlive timer expired
10 - Receive OPEN message
11 - Receive KEEPALIVE message
12 - Receive UPDATE messages
13 - Receive NOTIFICATION message
The following table describes the state transitions of the TRIP FSM
and the actions triggered by these transitions.
Event Actions Message Sent Next State
--------------------------------------------------------------------
Idle (1)
1 Initialize resources none 2
Start ConnectRetry timer
Initiate a transport connection
others none none 1
Connect(2)
1 none none 2
3 Complete initialization OPEN 4
Clear ConnectRetry timer
5 Restart ConnectRetry timer none 3
7 Restart ConnectRetry timer none 2
Initiate a transport connection
others Release resources none 1
Active (3)
1 none none 3
3 Complete initialization OPEN 4
Clear ConnectRetry timer
5 Close connection 3
Restart ConnectRetry timer
7 Restart ConnectRetry timer none 2
Initiate a transport connection
others Release resources none 1
OpenSent(4)
1 none none 4
4 Close transport connection none 3
Restart ConnectRetry timer
6 Release resources none 1
10 Process OPEN is OK KEEPALIVE 5
Process OPEN failed NOTIFICATION 1
others Close transport connection NOTIFICATION 1
Release resources
OpenConfirm (5)
1 none none 5
4 Release resources none 1
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6 Release resources none 1
9 Restart KeepAlive timer KEEPALIVE 5
11 Complete initialization none 6
Restart Hold Timer
13 Close transport connection 1
Release resources
others Close transport connection NOTIFICATION 1
Release resources
Established (6)
1 none none 6
4 Release resources none 1
6 Release resources none 1
9 Restart KeepAlive timer KEEPALIVE 6
11 Restart Hold Timer KEEPALIVE 6
12 Process UPDATE is OK UPDATE 6
Process UPDATE failed NOTIFICATION 1
13 Close transport connection 1
Release resources
others Close transport connection NOTIFICATION 1
Release resources
-----------------------------------------------------------------
The following is a condensed version of the above state transition
table.
Events| Idle | Connect | Active | OpenSent | OpenConfirm | Estab
| (1) | (2) | (3) | (4) | (5) | (6)
|-------------------------------------------------------------
1 | 2 | 2 | 3 | 4 | 5 | 6
| | | | | |
2 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
3 | 1 | 4 | 4 | 1 | 1 | 1
| | | | | |
4 | 1 | 1 | 1 | 3 | 1 | 1
| | | | | |
5 | 1 | 3 | 3 | 1 | 1 | 1
| | | | | |
6 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
7 | 1 | 2 | 2 | 1 | 1 | 1
| | | | | |
8 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
9 | 1 | 1 | 1 | 1 | 5 | 6
| | | | | |
10 | 1 | 1 | 1 | 1 or 5 | 1 | 1
| | | | | |
11 | 1 | 1 | 1 | 1 | 6 | 6
| | | | | |
12 | 1 | 1 | 1 | 1 | 1 | 1 or 6
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| | | | | |
13 | 1 | 1 | 1 | 1 | 1 | 1
| | | | | |
--------------------------------------------------------------
Appendix 2. Implementation Recommendations
This section presents some implementation recommendations.
A.2.1. Multiple Networks Per Message
The TRIP protocol allows for multiple address prefixes with the same
advertisement path and next-hop server to be specified in one
message. Making use of this capability is highly recommended. With
one address prefix per message there is a substantial increase in
overhead in the receiver. Not only does the system overhead increase
due to the reception of multiple messages, but the overhead of
scanning the call routing table for updates to TRIP peers is
incurred multiple times as well. One method of building messages
containing many address prefixes per advertisement path and next hop
from a call routing table that is not organized per advertisement
path is to build many messages as the call routing table is scanned.
As each address prefix is processed, a message for the associated
advertisement path and next hop is allocated, if it does not exist,
and the new address prefix is added to it. If such a message exists,
the new address prefix is just appended to it. If the message lacks
the space to hold the new address prefix, it is transmitted, a new
message is allocated, and the new address prefix is inserted into
the new message. When the entire call routing table has been
scanned, all allocated messages are sent and their resources
released. Maximum compression is achieved when all the destinations
covered by the address prefixes share a next hop server and common
attributes, making it possible to send many address prefixes in one
4096-byte message.
When peering with a TRIP implementation that does not compress
multiple address prefixes into one message, it may be necessary to
take steps to reduce the overhead from the flood of data received
when a peer is acquired or a significant network topology change
occurs. One method of doing this is to limit the rate of updates.
This will eliminate the redundant scanning of the call routing table
to provide flash updates for TRIP peers. A disadvantage of this
approach is that it increases the propagation latency of call
routing information. By choosing a minimum flash update interval
that is not much greater than the time it takes to process the
multiple messages this latency should be minimized. A better method
would be to read all received messages before sending updates.
A.2.2. Processing Messages on a Stream Protocol
Rosenberg, Salama, Squire 60
Internet Draft Telephony Routing over IP January 2000
TRIP uses TCP as a transport mechanism. Due to the stream nature of
TCP, all the data for received messages does not necessarily arrive
at the same time. This can make it difficult to process the data as
messages, especially on systems where it is not possible to
determine how much data has been received but not yet processed.
One method that can be used in this situation is to first try to
read just the message header. For the KEEPALIVE message type, this
is a complete message; for other message types, the header should
first be verified, in particular the total length. If all checks are
successful, the specified length, minus the size of the message
header is the amount of data left to read. An implementation that
would "hang" the routing information process while trying to read
from a peer could set up a message buffer (4096 bytes) per peer and
fill it with data as available until a complete message has been
received.
A.2.3. Reducing Route Flapping
To avoid excessive route flapping a n LS which needs to withdraw a
destination and send an update about a more specific or less
specific route SHOULD combine them into the same UPDATE message.
A.2.4. TRIP Timers
TRIP employs five timers: ConnectRetry, Hold Time, KeepAlive,
MinITADOriginationInterval, and MinCallRouteAdvertisementInterval
The suggested value for the ConnectRetry timer is 120 seconds. The
suggested value for the Hold Time is 90 seconds. The suggested value
for the KeepAlive timer is 30 seconds. The suggested value for the
MinITADOriginationInterval is 15 seconds. The suggested value for
the MinCallRouteAdvertisementInterval is 30 seconds.
An implementation of TRIP MUST allow these timers to be
configurable.
A.2.5. AP_SET Sorting
Another useful optimization that can be done to simplify this
situation is to sort the ITAD numbers found in an AP_SET. This
optimization is entirely optional.
Security Considerations
TBD.
References
Rosenberg, Salama, Squire 61
Internet Draft Telephony Routing over IP January 2000
[1] J. Rosenberg and H. Schulzrinne, "A Framework for a Gateway
Location Protocol" IETF Internet Draft, draft-ietf-iptel-gwloc-
framework-03.txt, Work in Progress, June 1999.
[2] Y. Rekhter and T. Li, "A Border Gateway Protocol 4 (BGP-4),"
IETF RFC 1771, March 1995.
[3] J. Moy, "Open Shortest Path First Version 2", IETF RFC 2328,
April, 1998.
[4] J. Luciani, et al, "Server Cache Synchronization Protocol
(SCSP)," IETF RFC 2334, April, 1998.
[5] International Telecommunication Union, "Visual Telephone
Systems and Equipment for Local Area Networks which Provide a
Non-Guaranteed Quality of Service," Recommendation H.323,
Telecommunication Standardization Sector of ITU, Geneva,
Switzerland, May 1996.
[6] M. Handley, H. Schulzrinne, E. Schooler, and J. Rosenberg,
"SIP: Session Initiation Protocol," IETF Internet Draft, draft-
ietf-mmusic-sip-12.txt, Work in Progress, January 1999.
Rosenberg, Salama, Squire 62
Authors' Addresses
Jonathan Rosenberg
Lucent Technologies, Bell Laboratories
101 Crawfords Corner Rd.
Holmdel, NJ 07733
Rm. 4C-526
email: jdrosen@bell-labs.com
Hussein F. Salama
Cisco Systems
Mail Stop SJ-6/3
170 W. Tasman Drive
San Jose, CA 95134
email: hsalama@cisco.com
Matt Squire
Nortel Networks
4309 Emporer Blvd
Suite 200
Durham, NC 27703
email: msquire@nortelnetworks.com
TRIP Transport August 1
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