Network Working Group R. Yavatkar Request for Comments: 2814 Intel Category: Standards Track D. Hoffman Teledesic Y. Bernet Microsoft F. Baker Cisco M. Speer Sun Microsystems May 2000 SBM (Subnet Bandwidth Manager): A Protocol for RSVP-based Admission Control over IEEE 802-style networks Status of this Memo This document specifies an Internet standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "Internet Official Protocol Standards" (STD 1) for the standardization state and status of this protocol. Distribution of this memo is unlimited. Copyright Notice Copyright (C) The Internet Society (2000). All Rights Reserved. Abstract This document describes a signaling method and protocol for RSVP- based admission control over IEEE 802-style LANs. The protocol is designed to work both with the current generation of IEEE 802 LANs as well as with the recent work completed by the IEEE 802.1 committee. 1. Introduction New extensions to the Internet architecture and service models have been defined for an integrated services Internet [RFC-1633, RFC-2205, RFC-2210] so that applications can request specific qualities or levels of service from an internetwork in addition to the current IP best-effort service. These extensions include RSVP, a resource reservation setup protocol, and definition of new service classes to be supported by Integrated Services routers. RSVP and service class definitions are largely independent of the underlying networking technologies and it is necessary to define the mapping of RSVP and Integrated Services specifications onto specific subnetwork technologies. For example, a definition of service mappings and
reservation setup protocols is needed for specific link-layer technologies such as shared and switched IEEE-802-style LAN technologies. This document defines SBM, a signaling protocol for RSVP-based admission control over IEEE 802-style networks. SBM provides a method for mapping an internet-level setup protocol such as RSVP onto IEEE 802 style networks. In particular, it describes the operation of RSVP-enabled hosts/routers and link layer devices (switches, bridges) to support reservation of LAN resources for RSVP-enabled data flows. A framework for providing Integrated Services over shared and switched IEEE-802-style LAN technologies and a definition of service mappings have been described in separate documents [RFC- FRAME, RFC-MAP]. 2. Goals and Assumptions The SBM (Subnet Bandwidth Manager) protocol and its use for admission control and bandwidth management in IEEE 802 level-2 networks is based on the following architectural goals and assumptions: I. Even though the current trend is towards increased use of switched LAN topologies consisting of newer switches that support the priority queuing mechanisms specified by IEEE 802.1p, we assume that the LAN technologies will continue to be a mix of legacy shared/ switched LAN segments and newer switched segments based on IEEE 802.1p specification. Therefore, we specify a signaling protocol for managing bandwidth over both legacy and newer LAN topologies and that takes advantage of the additional functionality (such as an explicit support for different traffic classes or integrated service classes) as it becomes available in the new generation of switches, hubs, or bridges. As a result, the SBM protocol would allow for a range of LAN bandwidth management solutions that vary from one that exercises purely administrative control (over the amount of bandwidth consumed by RSVP-enabled traffic flows) to one that requires cooperation (and enforcement) from all the end-systems or switches in a IEEE 802 LAN. II. This document specifies only a signaling method and protocol for LAN-based admission control over RSVP flows. We do not define here any traffic control mechanisms for the link layer; the protocol is designed to use any such mechanisms defined by IEEE 802. In addition, we assume that the Layer 3 end-systems (e.g., a host or a router) will exercise traffic control by policing Integrated Services traffic flows to ensure that each flow stays within its traffic specifications stipulated in an earlier reservation request submitted for admission control. This then
allows a system using SBM admission control combined with per flow shaping at end systems and IEEE-defined traffic control at link layer to realize some approximation of Controlled Load (and even Guaranteed) services over IEEE 802-style LANs. III. In the absence of any link-layer traffic control or priority queuing mechanisms in the underlying LAN (such as a shared LAN segment), the SBM-based admission control mechanism only limits the total amount of traffic load imposed by RSVP-enabled flows on a shared LAN. In such an environment, no traffic flow separation mechanism exists to protect the RSVP-enabled flows from the best- effort traffic on the same shared media and that raises the question of the utility of such a mechanism outside a topology consisting only of 802.1p-compliant switches. However, we assume that the SBM-based admission control mechanism will still serve a useful purpose in a legacy, shared LAN topology for two reasons. First, assuming that all the nodes that generate Integrated Services traffic flows utilize the SBM-based admission control procedure to request reservation of resources before sending any traffic, the mechanism will restrict the total amount of traffic generated by Integrated Services flows within the bounds desired by a LAN administrator (see discussion of the NonResvSendLimit parameter in Appendix C). Second, the best-effort traffic generated by the TCP/IP-based traffic sources is generally rate adaptive (using a TCP-style "slow start" congestion avoidance mechanism or a feedback-based rate adaptation mechanism used by audio/video streams based on RTP/RTCP protocols) and adapts to stay within the available network bandwidth. Thus, the combination of admission control and rate adaptation should avoid persistent traffic congestion. This does not, however, guarantee that non-Integrated-Services traffic will not interfere with the Integrated Services traffic in the absence of traffic control support in the underlying LAN infrastructure. 3. Organization of the rest of this document The rest of this document provides a detailed description of the SBM-based admission control procedure(s) for IEEE 802 LAN technologies. The document is organized as follows: * Section 4 first defines the various terms used in the document and then provides an overview of the admission control procedure with an example of its application to a sample network. * Section 5 describes the rules for processing and forwarding PATH (and PATH_TEAR) messages at DSBMs (Designated Subnet Bandwidth Managers), SBMs, and DSBM clients.
* Section 6 addresses the inter-operability issues when a DSBM may operate in the absence of RSVP signaling at Layer 3 or when another signaling protocol (such as SNMP) is used to reserve resources on a LAN segment. * Appendix A describes the details of the DSBM election algorithm used for electing a designated SBM on a LAN segment when more than one SBM is present. It also describes how DSBM clients discover the presence of a DSBM on a managed segment. * Appendix B specifies the formats of SBM-specific messages used and the formats of new RSVP objects needed for the SBM operation. * Appendix C describes usage of the DSBM to distribute configuration information to senders on a managed segment. 4. Overview 4.1. Definitions - Link Layer or Layer 2 or L2: We refer to data-link layer technologies such as IEEE 802.3/Ethernet as L2 or layer 2. - Link Layer Domain or Layer 2 domain or L2 domain: a set of nodes and links interconnected without passing through a L3 forwarding function. One or more IP subnets can be overlaid on a L2 domain. - Layer 2 or L2 devices: We refer to devices that only implement Layer 2 functionality as Layer 2 or L2 devices. These include 802.1D bridges or switches. - Internetwork Layer or Layer 3 or L3: Layer 3 of the ISO 7 layer model. This document is primarily concerned with networks that use the Internet Protocol (IP) at this layer. - Layer 3 Device or L3 Device or End-Station: these include hosts and routers that use L3 and higher layer protocols or application programs that need to make resource reservations. - Segment: A L2 physical segment that is shared by one or more senders. Examples of segments include (a) a shared Ethernet or Token-Ring wire resolving contention for media access using CSMA or token passing ("shared L2 segment"), (b) a half duplex link between two stations or switches, (c) one direction of a switched full-duplex link.
- Managed segment: A managed segment is a segment with a DSBM present and responsible for exercising admission control over requests for resource reservation. A managed segment includes those interconnected parts of a shared LAN that are not separated by DSBMs. - Traffic Class: An aggregation of data flows which are given similar service within a switched network. - User_priority: User_priority is a value associated with the transmission and reception of all frames in the IEEE 802 service model: it is supplied by the sender that is using the MAC service. It is provided along with the data to a receiver using the MAC service. It may or may not be actually carried over the network: Token-Ring/802.5 carries this value (encoded in its FC octet), basic Ethernet/802.3 does not, 802.12 may or may not depending on the frame format in use. 802.1p defines a consistent way to carry this value over the bridged network on Ethernet, Token Ring, Demand-Priority, FDDI or other MAC-layer media using an extended frame format. The usage of user_priority is fully described in section 2.5 of 802.1D [IEEE8021D] and 802.1p [IEEE8021P] "Support of the Internal Layer Service by Specific MAC Procedures". - Subnet: used in this memo to indicate a group of L3 devices sharing a common L3 network address prefix along with the set of segments making up the L2 domain in which they are located. - Bridge/Switch: a layer 2 forwarding device as defined by IEEE 802.1D. The terms bridge and switch are used synonymously in this document. - DSBM: Designated SBM (DSBM) is a protocol entity that resides in a L2 or L3 device and manages resources on a L2 segment. At most one DSBM exists for each L2 segment. - SBM: the SBM is a protocol entity that resides in a L2 or L3 device and is capable of managing resources on a segment. However, only a DSBM manages the resources for a managed segment. When more than one SBM exists on a segment, one of the SBMs is elected to be the DSBM. - Extended segment: An extended segment includes those parts of a network which are members of the same IP subnet and therefore are not separated by any layer 3 devices. Several managed segments, interconnected by layer 2 devices, constitute an extended segment.
- Managed L2 domain: An L2 domain consisting of managed segments is referred to as a managed L2 domain to distinguish it from a L2 domain with no DSBMs present for exercising admission control over resources at segments in the L2 domain. - DSBM clients: These are entities that transmit traffic onto a managed segment and use the services of a DSBM for the managed segment for admission control over a LAN segment. Only the layer 3 or higher layer entities on L3 devices such as hosts and routers are expected to send traffic that requires resource reservations, and, therefore, DSBM clients are L3 entities. - SBM transparent devices: A "SBM transparent" device is unaware of SBMs or DSBMs (though it may or may not be RSVP aware) and, therefore, does not participate in the SBM-based admission control procedure over a managed segment. Such a device uses standard forwarding rules appropriate for the device and is transparent with respect to SBM. An example of such a L2 device is a legacy switch that does not participate in resource reservation. - Layer 3 and layer 2 addresses: We refer to layer 3 addresses of L3/L2 devices as "L3 addresses" and layer 2 addresses as "L2 addresses". This convention will be used in the rest of the document to distinguish between Layer 3 and layer 2 addresses used to refer to RSVP next hop (NHOP) and previous hop (PHOP) devices. For example, in conventional RSVP message processing, RSVP_HOP object in a PATH message carries the L3 address of the previous hop device. We will refer to the address contained in the RSVP_HOP object as the RSVP_HOP_L3 address and the corresponding MAC address of the previous hop device will be referred to as the RSVP_HOP_L2 address. 4.2. Overview of the SBM-based Admission Control Procedure A protocol entity called "Designated SBM" (DSBM) exists for each managed segment and is responsible for admission control over the resource reservation requests originating from the DSBM clients in that segment. Given a segment, one or more SBMs may exist on the segment. For example, many SBM-capable devices may be attached to a shared L2 segment whereas two SBM-capable switches may share a half- duplex switched segment. In that case, a single DSBM is elected for the segment. The procedure for dynamically electing the DSBM is described in Appendix A. The only other approved method for specifying a DSBM for a managed segment is static configuration at SBM-capable devices.
The presence of a DSBM makes the segment a "managed segment". Sometimes, two or more L2 segments may be interconnected by SBM transparent devices. In that case, a single DSBM will manage the resources for those segments treating the collection of such segments as a single managed segment for the purpose of admission control. 4.2.1. Basic Algorithm Figure 1 - An Example of a Managed Segment. +-------+ +-----+ +------+ +-----+ +--------+ |Router | | Host| | DSBM | | Host| | Router | | R2 | | C | +------+ | B | | R3 | +-------+ +-----+ / +-----+ +--------+ | | / | | | | / | | ==============================================================LAN | | | | +------+ +-------+ | Host | | Router| | A | | R1 | +------+ +-------+ Figure 1 shows an example of a managed segment in a L2 domain that interconnects a set of hosts and routers. For the purpose of this discussion, we ignore the actual physical topology of the L2 domain (assume it is a shared L2 segment and a single managed segment represents the entire L2 domain). A single SBM device is designated to be the DSBM for the managed segment. We will provide examples of operation of the DSBM over switched and shared segments later in the document. The basic DSBM-based admission control procedure works as follows: 1. DSBM Initialization: As part of its initial configuration, DSBM obtains information such as the limits on fraction of available resources that can be reserved on each managed segment under its control. For instance, bandwidth is one such resource. Even though methods such as auto-negotiation of link speeds and knowledge of link topology allow discovery of link capacity, the configuration may be necessary to limit the fraction of link capacity that can be reserved on a link. Configuration is likely to be static with the current L2/L3 devices. Future work may allow for dynamic discovery of this information. This document does not specify the configuration mechanism.
2. DSBM Client Initialization: For each interface attached, a DSBM client determines whether a DSBM exists on the interface. The procedure for discovering and verifying the existence of the DSBM for an attached segment is described in Appendix A. If the client itself is capable of serving as the DSBM on the segment, it may choose to participate in the election to become the DSBM. At the start, a DSBM client first verifies that a DSBM exists in its L2 domain so that it can communicate with the DSBM for admission control purposes. In the case of a full-duplex segment, an election may not be necessary as the SBM at each end will typically act as the DSBM for outgoing traffic in each direction. 3. DSBM-based Admission Control: To request reservation of resources (e.g., LAN bandwidth in a L2 domain), DSBM clients (RSVP-capable L3 devices such as hosts and routers) follow the following steps: a) When a DSBM client sends or forwards a RSVP PATH message over an interface attached to a managed segment, it sends the PATH message to the segment's DSBM instead of sending it to the RSVP session destination address (as is done in conventional RSVP processing). After processing (and possibly updating an ADSPEC), the DSBM will forward the PATH message toward its destination address. As part of its processing, the DSBM builds and maintains a PATH state for the session and notes the previous L2/L3 hop that sent it the PATH message. Let us consider the managed segment in Figure 1. Assume that a sender to a RSVP session (session address specifies the IP address of host A on the managed segment in Figure 1) resides outside the L2 domain of the managed segment and sends a PATH message that arrives at router R1 which is on the path towards host A. DSBM client on Router R1 forwards the PATH message from the sender to the DSBM. The DSBM processes the PATH message and forwards the PATH message towards the RSVP receiver (Detailed message processing and forwarding rules are described in Section 5). In the process, the DSBM builds the PATH state, remembers the router R1 (its L2 and l3 addresses) as the previous hop for the session, puts its own L2 and L3 addresses in the PHOP objects (see explanation later), and effectively inserts itself as an intermediate node between the sender (or R1 in Figure 1) and the receiver (host A) on the managed segment.
b) When an application on host A wishes to make a reservation for the RSVP session, host A follows the standard RSVP message processing rules and sends a RSVP RESV message to the previous hop L2/L3 address (the DSBMs address) obtained from the PHOP object(s) in the previously received PATH message. c) The DSBM processes the RSVP RESV message based on the bandwidth available and returns an RESV_ERR message to the requester (host A) if the request cannot be granted. If sufficient resources are available and the reservation request is granted, the DSBM forwards the RESV message towards the PHOP(s) based on its local PATH state for the session. The DSBM merges reservation requests for the same session as and when possible using the rules similar to those used in the conventional RSVP processing (except for an additional criterion described in Section 5.8). d) If the L2 domain contains more than one managed segment, the requester (host A) and the forwarder (router R1) may be separated by more than one managed segment. In that case, the original PATH message would propagate through many DSBMs (one for each managed segment on the path from R1 to A) setting up PATH state at each DSBM. Therefore, the RESV message would propagate hop-by-hop in reverse through the intermediate DSBMs and eventually reach the original forwarder (router R1) on the L2 domain if admission control at all DSBMs succeeds. 4.2.2. Enhancements to the conventional RSVP operation (D)SBMs and DSBM clients implement minor additions to the standard RSVP protocol. These are summarized in this section. A detailed description of the message processing and forwarding rules follows in section 5. 126.96.36.199 Sending PATH Messages to the DSBM on a Managed Segment Normal RSVP forwarding rules apply at a DSBM client when it is not forwarding an outgoing PATH message over a managed segment. However, outgoing PATH messages on a managed segment are sent to the DSBM for the corresponding managed segment (Section 5.2 describes how the PATH messages are sent to the DSBM on a managed segment). 188.8.131.52 The LAN_NHOP Objects In conventional RSVP processing over point-to-point links, RSVP nodes (hosts/routers) use RSVP_HOP object (NHOP and PHOP info) to keep track of the next hop (downstream node in the path of data packets in a traffic flow) and the previous hop (upstream nodes with respect to
the data flow) nodes on the path between a sender and a receiver. Routers along the path of a PATH message forward the message towards the destination address based on the L3 routing (packet forwarding) tables. For example, consider the L2 domain in Figure 1. Assume that both the sender (some host X) and the receiver (some host Y) in a RSVP session reside outside the L2 domain shown in the Figure, but PATH messages from the sender to its receiver pass through the routers in the L2 domain using it as a transit subnet. Assume that the PATH message from the sender X arrives at the router R1. R1 uses its local routing information to decide which next hop router (either router R2 or router R3) to use to forward the PATH message towards host Y. However, when the path traverses a managed L2 domain, we require the PATH and RESV messages to go through a DSBM for each managed segment. Such a L2 domain may span many managed segments (and DSBMs) and, typically, SBM protocol entities on L2 devices (such as a switch) will serve as the DSBMs for the managed segments in a switched topology. When R1 forwards the PATH message to the DSBM (an L2 device), the DSBM may not have the L3 routing information necessary to select the egress router (between R2 and R3) before forwarding the PATH message. To ensure correct operation and routing of RSVP messages, we must provide additional forwarding information to DSBMs. For this purpose, we introduce new RSVP objects called LAN_NHOP address objects that keep track of the next L3 hop as the PATH message traverses an L2 domain between two L3 entities (RSVP PHOP and NHOP nodes). 184.108.40.206 Including Both Layer-2 and Layer-3 Addresses in the LAN_NHOP When a DSBM client (a host or a router acting as the originator of a PATH message) sends out a PATH message to the DSBM, it must include LAN_NHOP information in the message. In the case of a unicast destination, the LAN_NHOP address specifies the destination address (if the destination is local to its L2 domain) or the address of the next hop router towards the destination. In our example of an RSVP session involving the sender X and receiver Y with L2 domain in Figure 1 acting as the transit subnet, R1 is the ingress node that receives the PATH message. R1 first determines that R2 is the next hop router (or the egress node in the L2 domain for the session address) and then inserts a LAN_NHOP object that specifies R2's IP address. When a DSBM receives a PATH message, it can now look at the address in the LAN_NHOP object and forward the PATH message towards the egress node after processing the PATH message. However, we expect the L2 devices (such as switches) to act as DSBMs on the path within the L2 domain and it may not be reasonable to expect these devices to have an ARP capability to determine the MAC address (we
call it L2ADDR for Layer 2 address) corresponding to the IP address in the LAN_NHOP object. Therefore, we require that the LAN_NHOP information (generated by the L3 device) include both the IP address (LAN_NHOP_L3 address) and the corresponding MAC address (LAN_NHOP_L2 address ) for the next L3 hop over the L2 domain. The LAN_NHOP_L3 address is used by SBM protocol entities on L3 devices to forward the PATH message towards its destination whereas the L2 address is used by the SBM protocol entities on L2 devices to determine how to forward the PATH message towards the L3 NHOP (egress point from the L2 domain). The exact format of the LAN_NHOP information and relevant objects is described later in Appendix B. 220.127.116.11 Similarities to Standard RSVP Message Processing - When a DSBM receives a RSVP PATH message, it processes the PATH message according to the PATH processing rules described in the RSVP specification. In particular, the DSBM retrieves the IP address of the previous hop from the RSVP_HOP object in the PATH message and stores the PHOP address in its PATH state. It then forwards the PATH message with the PHOP (RSVP_HOP) object modified to reflect its own IP address (RSVP_HOP_L3 address). Thus, the DSBM inserts itself as an intermediate hop in the chain of nodes in the path between two L3 nodes across the L2 domain. - The PATH state in a DSBM is used for forwarding subsequent RESV messages as per the standard RSVP message processing rules. When the DSBM receives a RESV message, it processes the message and forwards it to appropriate PHOP(s) based on its PATH state. - Because a DSBM inserts itself as a hop between two RSVP nodes in the path of a RSVP flow, all RSVP related messages (such as PATH, PATH_TEAR, RESV, RESV_CONF, RESV_TEAR, and RESV_ERR) now flow through the DSBM. In particular, a PATH_TEAR message is routed exactly through the intermediate DSBM(s) as its corresponding PATH message and the local PATH state is first cleaned up at each intermediate hop before the PATH_TEAR message gets forwarded. - So far, we have described how the PATH message propagates through the L2 domain establishing PATH state at each DSBM along the managed segments in the path. The layer 2 address (LAN_NHOP_L2 address) in the LAN_NHOP object should be used by the L2 devices along the path to decide how to forward the PATH message toward the next L3 hop. Such devices will apply the standard IEEE 802.1D forwarding rules (e.g., send it on a single port based on its filtering database, or flood it on all ports active in the spanning tree if the L2 address does not appear in the filtering
database) to the LAN_NHOP_L2 address as are applied normally to data packets destined to the address. 18.104.22.168 Including Both Layer-2 and Layer-3 Addresses in the RSVP_HOP Objects In the conventional RSVP message processing, the PATH state established along the nodes on a path is used to route the RESV message from a receiver to a sender in an RSVP session. As each intermediate node builds the path state, it remembers the previous hop (stores the PHOP IP address available in the RSVP_HOP object of an incoming message) that sent it the PATH message and, when the RESV message arrives, the intermediate node simply uses the stored PHOP address to forward the RESV after processing it successfully. In our case, we expect the SBM entities residing at L2 devices to act as DSBMs (and, therefore, intermediate RSVP hops in an L2 domain) along the path between a sender (PHOP) and receiver (NHOP). Thus, when a RESV message arrives at a DSBM, it must use the stored PHOP IP address to forward the RESV message to its previous hop. However, it may not be reasonable to expect the L2 devices to have an ARP cache or the ARP capability to map the PHOP IP address to its corresponding L2 address before forwarding the RESV message. To obviate the need for such address mapping at L2 devices, we use a RSVP_HOP_L2 object in the PATH message. The RSVP_HOP_L2 object includes the Layer 2 address (L2ADDR) of the previous hop and complements the L3 address information included in the RSVP_HOP object (RSVP_HOP_L3 address). When a L3 device constructs and forwards a PATH message over a managed segment, it includes its IP address (IP address of the interface over which PATH is sent) in the RSVP_HOP object and adds a RSVP_HOP_L2 object that includes the corresponding L2 address for the interface. When a device in the L2 domain receives such a PATH message, it remembers the addresses in the RSVP_HOP and RSVP_HOP_L2 objects in its PATH state and then overwrites the RSVP_HOP and RSVP_HOP_L2 objects with its own addresses before forwarding the PATH message over a managed segment. The exact format of RSVP_HOP_L2 object is specified in Appendix B. 22.214.171.124 Loop Detection When an RSVP session address is a multicast address and a SBM, DSBM, and DSBM clients share the same L2 segment (a shared segment), it is possible for a SBM or a DSBM client to receive one or more copies of a PATH message that it forwarded earlier when a DSBM on the same wire
forwards it (See Section 5.7 for an example of such a case). To facilitate detection of such loops, we use a new RSVP object called the LAN_LOOPBACK object. DSBM clients or SBMs (but not the DSBMs reflecting a PATH message onto the interface over which it arrived earlier) must overwrite (or add if the PATH message does NOT already include a LAN_LOOPBACK object) the LAN_LOOPBACK object in the PATH message with their own unicast IP address. Now, a SBM or a DSBM client can easily detect and discard the duplicates by checking the contents of the LAN_LOOPBACK object (a duplicate PATH message will list a device's own interface address in the LAN_LOOPBACK object). Appendix B specifies the exact format of the LAN_LOOPBACK object. 126.96.36.199 802.1p, User Priority and TCLASS The model proposed by the Integrated Services working group requires isolation of traffic flows from each other during their transit across a network. The motivation for traffic flow separation is to provide Integrated Services flows protection from misbehaving flows and other best-effort traffic that share the same path. The basic IEEE 802.3/Ethernet networks do not provide any notion of traffic classes to discriminate among different flows that request different services. However, IEEE 802.1p defines a way for switches to differentiate among several "user_priority" values encoded in packets representing different traffic classes (see [IEEE802Q, IEEE8021p] for further details). The user_priority values can be encoded either in native LAN packets (e.g., in IEEE 802.5's FC octet) or by using an encapsulation above the MAC layer (e.g., in the case of Ethernet, the user_priority value assigned to each packet will be carried in the frame header using the new, extended frame format defined by IEEE 802.1Q [IEEE8021Q]. IEEE, however, makes no recommendations about how a sender or network should use the user_priority values. An accompanying document makes recommendations on the usage of the user_priority values (see [RFC-MAP] for details). Under the Integrated Services model, L3 (or higher) entities that transmit traffic flows onto a L2 segment should perform per-flow policing to ensure that the flows do not exceed their traffic specification as specified during admission control. In addition, L3 devices may label the frames in such flows with a user_priority value to identify their service class. For the purpose of this discussion, we will refer to the user_priority value carried in the extended frame header as the "traffic class" of a packet. Under the ISSLL model, the L3 entities, that send traffic and that use the SBM protocol, may select the appropriate traffic class of outgoing packets [RFC-MAP]. This
selection may be overridden by DSBM devices, in the following manner. once a sender sends a PATH message, downstream DSBMs will insert a new traffic class object (TCLASS object) in the PATH message that travels to the next L3 device (L3 NHOP for the PATH message). To some extent, the TCLASS object contents are treated like the ADSPEC object in the RSVP PATH messages. The L3 device that receives the PATH message must remove and store the TCLASS object as part of its PATH state for the session. Later, when the same L3 device needs to forward a RSVP RESV message towards the original sender, it must include the TCLASS object in the RESV message. When the RESV message arrives at the original sender, the sender must use the user_priority value from the TCLASS object to override its selection for the traffic class marked in outgoing packets. The format of the TCLASS object is specified in Appendix B. Note that TCLASS and other SBM-specific objects are carried in a RSVP message in addition to all the other, normal RSVP objects per RFC 2205. 188.8.131.52 Processing the TCLASS Object In summary, use of TCLASS objects requires following additions to the conventional RSVP message processing at DSBMs, SBMs, and DSBM clients: * When a DSBM receives a PATH message over a managed segment and the PATH message does not include a TCLASS object, the DSBM MAY add a TCLASS object to the PATH message before forwarding it. The DSBM determines the appropriate user_priority value for the TCLASS object. A mechanism for selecting the appropriate user_priority value is described in an accompanying document [RFC-MAP]. * When SBM or DSBM receives a PATH message with a TCLASS object over a managed segment in a L2 domain and needs to forward it over a managed segment in the same L2 domain, it will store it in its path state and typically forward the message without changing the contents of the TCLASS object. However, if the DSBM/SBM cannot support the service class represented by the user_priority value specified by the TCLASS object in the PATH message, it may change the priority value in the TCLASS to a semantically "lower" service value to reflect its capability and store the changed TCLASS value in its path state.
[NOTE: An accompanying document defines the int-serv mappings over IEEE 802 networks [RFC-MAP] provides a precise definition of user_priority values and describes how the user_priority values are compared to determine "lower" of the two values or the "lowest" among all the user_priority values.] * When a DSBM receives a RESV message with a TCLASS object, it may use the traffic class information (in addition to the usual flowspec information in the RSVP message) for its own admission control for the managed segment. Note that this document does not specify the actual algorithm or policy used for admission control. At one extreme, a DSBM may use per-flow reservation request as specified by the flowspec for a fine grain admission control. At the other extreme, a DSBM may only consider the traffic class information for a very coarse- grain admission control based on some static allocation of link capacity for each traffic class. Any combination of the options represented by these two extremes may also be used. * When a DSBM (at an L2 or L3) device receives a RESV message without a TCLASS object and it needs to forward the RESV message over a managed segment within the same L2 domain, it should first check its path state and check whether it has stored a TCLASS value. If so, it should include the TCLASS object in the outgoing RESV message after performing its own admission control. If no TCLASS value is stored, it must forward the RESV message without inserting a TCLASS object. * When a DSBM client (residing at an L3 device such as a host or an edge router) receives the TCLASS object in a PATH message that it accepts over an interface, it should store the TCLASS object as part of its PATH state for the interface. Later, when the client forwards a RESV message for the same session on the interface, the client must include the TCLASS object (unchanged from what was received in the previous PATH message) in the RESV message it forwards over the interface. * When a DSBM client receives a TCLASS object in an incoming RESV message over a managed segment and local admission control succeeds for the session for the outgoing interface over the managed segment, the client must pass the user_priority value in the TCLASS object to its local packet classifier. This will ensure that the data packets in the admitted RSVP flow that are subsequently forwarded over the outgoing interface will contain the appropriate value encoded in their frame header.
* When an L3 device receives a PATH or RESV message over a managed segment in one L2 domain and it needs to forward the PATH/RESV message over an interface outside that domain, the L3 device must remove the TCLASS object (along with LAN_NHOP, RSVP_HOP_L2, and LAN_LOOPBACK objects in the case of the PATH message) before forwarding the PATH/RESV message. If the outgoing interface is on a separate L2 domain, these objects may be regenerated according to the processing rules applicable to that interface.