be called an aggregated trunk.  Two trunks can be aggregated if they   share a portion of their path.  There is no requirement on the exact   length of the common portion of the path, and thus the exact   requirements for forming an aggregated trunk are beyond the scope of   this document.  Note that traffic class (i.e., QoS indication) is   propagated when an additional label is added to a trunk, so trunks of   different classes may be aggregated.   Trunks can be terminated at any point, resulting in a deaggregation   of traffic.  The obvious consequence is that there needs to be   sufficient switching capacity at the point of deaggregation to deal   with the resultant traffic.   High reliability for a trunk can be provided through the use of one   or more backup trunks.  Backup trunks can be initiated either by the   same router that would initiate the primary trunk or by another   backup router.  The status of the primary trunk can be ascertained by   the router that initiated the backup trunk (note that this may be   either the same or a different router as the router that initiated   the primary trunk) through out of band information, such as the IGP.   If a backup trunk is established and the primary trunk returns to   service, the backup trunk can be deactivated and the primary trunk   used instead.4.3 RSVP   Originally RSVP was designed as a protocol to install state   associated with resource reservations for individual flows   originated/destined to hosts, where path was determined by   destination-based routing. Quoting directly from the RSVP   specifications, "The RSVP protocol is used by a host, on behalf of an   application data stream, to request a specific quality of service   (QoS) from the network for particular data streams or flows"   [RFC2205].Li & Rekhter                 Informational                      [Page 6]RFC 2430                         PASTE                      October 1998   The usage of RSVP in PASTE is quite different from the usage of RSVP   as it was originally envisioned by its designers.  The first   difference is that RSVP is used in PASTE to install state that   applies to a collection of flows that all share a common path and   common pool of reserved resources.  The second difference is that   RSVP is used in PASTE to install state related to forwarding,   including label switching information, in addition to resource   reservations.  The third difference is that the path that this state   is installed along is no longer constrained by the destination-based   routing.   The key factor that makes RSVP suitable for PASTE is the set of   mechanisms provided by RSVP. Quoting from the RSVP specifications,   "RSVP protocol mechanisms provide a general facility for creating and   maintaining distributed reservation state across a mesh of multicast   or unicast delivery paths." Moreover, RSVP provides a straightforward   extensibility mechanism by allowing for the creation of new RSVP   Objects. This flexibility allows us to also use the mechanisms   provided by RSVP to create and maintain distributed state for   information other than pure resource reservation, as well as allowing   the creation of forwarding state in conjunction with resource   reservation state.   The original RSVP design, in which "RSVP itself transfers and   manipulates QoS control parameters as opaque data, passing them to   the appropriate traffic control modules for interpretation" can thus   be extended to include explicit route parameters and label binding   parameters. Just as with QoS parameters, RSVP can transfer and   manipulate explicit route parameters and label binding parameters as   opaque data, passing explicit route parameters to the appropriate   forwarding module, and label parameters to the appropriate MPLS   module.   Moreover, an RSVP session in PASTE is not constrained to be only   between a pair of hosts, but is also used between pairs of routers   that act as the originator and the terminator of a traffic trunk.   Using RSVP in PASTE helps consolidate procedures for several tasks:   (a) procedures for establishing forwarding along an explicit route,   (b) procedures for establishing a label switched path, and (c) RSVP's   existing procedures for resource reservation.  In addition, these   functions can be cleanly combined in any manner.  The main advantage   of this consolidation comes from an observation that the above three   tasks are not independent, but inter-related. Any alternative that   accomplished each of these functions via independent sets of   procedures, would require additional coordination between functions,   adding more complexity to the system.Li & Rekhter                 Informational                      [Page 7]RFC 2430                         PASTE                      October 19984.4 Traffic Engineering   The purpose of traffic engineering is to give the ISP precise control   over the flow of traffic within its network.  Traffic engineering is   necessary because standard IGPs compute the shortest path across the   ISP's network based solely on the metric that has been   administratively assigned to each link.  This computation does not   take into account the loading of each link.  If the ISP's network is   not a full mesh of physical links, the result is that there may not   be an obvious way to assign metrics to the existing links such that   no congestion will occur given known traffic patterns.  Traffic   engineering can be viewed as assistance to the routing infrastructure   that provides additional information in routing traffic along   specific paths, with the end goal of more efficient utilization of   networking resources.   Traffic engineering is performed by directing trunks along explicit   paths within the ISP's topology.  This diverts the traffic away from   the shortest path computed by the IGP and presumably onto uncongested   links, eventually arriving at the same destination.  Specification of   the explicit route is done by enumerating an explicit list of the   routers in the path.  Given this list, traffic engineering trunks can   be constructed in a variety of ways.  For example, a trunk could be   manually configured along the explicit path.  This would involve   configuring each router along the path with state information for   forwarding the particular label.  Such techniques are currently used   for traffic engineering in some ISPs today.   Alternately, a protocol such as RSVP can be used with an Explicit   Route Object (ERO) so that the first router in the path can establish   the trunk.  The computation of the explicit route is beyond the scope   of this document but may include considerations of policy, static and   dynamic bandwidth allocation, congestion in the topology and manually   configured alternatives.4.5 Resource reservation   Priority traffic has certain requirements on capacity and traffic   handling.  To provide differentiated services, the ISP's   infrastructure must know of, and support these requirements.  The   mechanism used to communicate these requirements dynamically is RSVP.   The flow specification within RSVP can describe many characteristics   of the flow or trunk.  An LSR receiving RSVP information about a flow   or trunk has the ability to look at this information and either   accept or reject the reservation based on its local policy.  This   policy is likely to include constraints about the traffic handling   functions that can be supported by the network and the aggregate   capacity that the network is willing to provide for Priority traffic.Li & Rekhter                 Informational                      [Page 8]RFC 2430                         PASTE                      October 19984.6 Inter-Provider SLAs (IPSs)   Trunks that span multiple ISPs are likely to be based on legal   agreements and some other external considerations.  As a result, one   of the common functions that we would expect to see in this type of   architecture is a bilateral agreement between ISPs to support   differentiated services.  In addition to the obvious compensation,   this agreement is likely to spell out the acceptable traffic handling   policies and capacities to be used by both parties.   Documents similar to this exist today on behalf of Best Effort   traffic and are known as peering agreements.  Extending a peering   agreement to support differentiated services would effectively create   an Inter-Provider SLA (IPS).  Such agreements may include the types   of differentiated services that one ISP provides to the other ISP, as   well as the upper bound on the amount of traffic associated with each   such service that the ISP would be willing to accept and carry from   the other ISP.  Further, an IPS may limit the types of differentiated   services and an upper bound on the amount of traffic that may   originate from a third party ISP and be passed from one signer of the   IPS to the other.   If the expected costs associated with the IPS are not symmetric, the   parties may agree that one ISP will provide the other ISP with   appropriate compensation.  Such costs may be due to inequality of   traffic exchange, costs in delivering the exchanged traffic, or the   overhead involved in supporting the protocols exchanged between the   two ISPs.   Note that the PASTE architecture provides a technical basis to   establish IPSs, while the procedures necessary to create such IPSs   are outside the scope of PASTE.4.7 Traffic shaping and policing   To help support IPSs, special facilities must be available at the   interconnect between ISPs.  These mechanisms are necessary to insure   that the network transmitting a trunk of Priority traffic does so   within the agreed traffic characterization and capacity.  A   simplistic example of such a mechanism might be a token bucket   system, implemented on a per-trunk basis.  Similarly, there need to   be mechanisms to insure, on a per trunk basis, that an ISP receiving   a trunk receives only the traffic that is in compliance with the   agreement between ISPs.Li & Rekhter                 Informational                      [Page 9]RFC 2430                         PASTE                      October 19984.8 Multilateral IPSs   Trunks may span multiple ISPs.  As a result, establishing a   particular trunk may require more than two ISPs.  The result would be   a multilateral IPS.  This type of agreement is unusual with respect   to existing Internet business practices in that it requires multiple   participating parties for a useful result.  This is also challenging   because without a commonly accepted service level definition, there   will need to be a multilateral definition, and this definition may   not be compatible used in IPSs between the same parties.   Because this new type of agreement may be a difficulty, it may in   some cases be simpler for certain ISPs to establish aggregated trunks   through other ISPs and then contract with customers to aggregate   their trunks.  In this way, trunks can span multiple ISPs without   requiring multilateral IPSs.   Either or both of these two alternatives are possible and acceptable   within this architecture, and the choice is left for the the   participants to make on a case-by-case basis.5.0 The Provider Architecture for differentiated Services and Traffic    Engineering (PASTE)   The Provider Architecture for differentiated Services and Traffic   Engineering (PASTE) is based on the usage of MPLS and RSVP as   mechanisms to establish differentiated service connections across   ISPs.  This is done in a scalable way by aggregating differentiated   flows into traffic class specific MPLS tunnels, also known as traffic   trunks.   Such trunks can be given an explicit route by an ISP to define the   placement of the trunk within the ISP's infrastructure, allowing the   ISP to traffic engineer its own network.  Trunks can also be   aggregated and merged, which helps the scalability of the   architecture by minimizing the number of individual trunks that   intermediate systems must support.   Special traffic handling operations, such as specific queuing   algorithms or drop computations, can be supported by a network on a   per-trunk basis, allowing these services to scale with the number of   trunks in the network.   Agreements for handling of trunks between ISPs require both legal   documentation and conformance mechanisms on both sides of the   agreement.  As a trunk is unidirectional, it is sufficient for the   transmitter to monitor and shape outbound traffic, while the receiver   polices the traffic profile.Li & Rekhter                 Informational                     [Page 10]RFC 2430                         PASTE                      October 1998   Trunks can either be aggregated across other ISPs or can be the   subject of a multilateral agreement for the carriage of the trunk.   RSVP information about individual flows is tunneled in the trunk to   provide an end-to-end reservation.  To insure that the return RSVP   traffic is handled properly, each trunk must also have another tunnel   running in the opposite direction.  Note that the reverse tunnel may   be a different trunk or it may be an independent tunnel terminating   at the same routers as the trunk.  Routing symmetry between a trunk   and its return is not assumed.   RSVP already contains the ability to do local path repair.  In the   event of a trunk failure, this capability, along with the ability to   specify abstractions in the ERO, allows RSVP to re-establish the   trunk in many failure scenarios.6.0 Traffic flow in the PASTE architecture   As an example of the operation of this architecture, we consider an   example of a single differentiated flow.  Suppose that a user wishes   to make a telephone call using a Voice over IP service.  While this   call is full duplex, we can consider the data flow in each direction   in a half duplex fashion because the architecture operates   symmetrically.   Suppose that the data packets for this voice call are created at a   node S and need to traverse to node D.  Because this is a voice call,   the data packets are encoded as Priority packets.  If there is more   granularity within the traffic classes, these packets might be   encoded as wanting low jitter and having low drop preference.   Initially this is encoded into the precedence bits of the IPv4 ToS   byte.6.1 Propagation of RSVP messages