RFC 2814
SBM (Subnet Bandwidth Manager): A Protocol for RSVP-based Admission Control over IEEE 802-style networks
Pages: 60
Proposed Standard
Proposed Standard
Part 1 of 3 – Pages 1 to 16
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.
4.2.2.1 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).
4.2.2.2 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).4.2.2.3 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.4.2.2.4 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.
4.2.2.5 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.
4.2.2.6 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.4.2.2.7 802.1p, 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 2205.4.2.2.8 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.
(page 16 continued on part 2)