rfc2816.txt

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RFC 2816        Framework for Int-Serv Over IEEE 802 LAN        May 2000


   NOTE: The original IEEE 802.1D standard [2] contains the
   specifications for the operation of MAC bridges.  This has recently
   been extended to include support for traffic classes and dynamic
   multicast filtering [3].  In this document, the reader should be
   aware that references to the IEEE 802.1D standard refer to [3],
   unless explicitly noted otherwise.

   IEEE 802.1D [3] defines a consistent way for carrying the value of
   the user_priority over a bridged network consisting of Ethernet,
   Token Ring, Demand Priority, FDDI or other MAC layer media using an
   extended frame format.  The usage of user_priority is summarized
   below.  We refer the interested reader to the IEEE 802.1D
   specification for further information.

   If the user_priority is carried explicitly in packets, its utility is
   as a simple label enabling packets within a data stream in different
   classes to be discriminated easily by downstream nodes without having
   to parse the packet in more detail.

   Apart from making the job of desktop or wiring closet switches
   easier, an explicit field means they do not have to change hardware
   or software as the rules for classifying packets evolve; e.g.  based
   on new protocols or new policies.  More sophisticated Layer 3
   switches, perhaps deployed in the core of a network, may be able to
   provide added value by performing packet classification more
   accurately and, hence, utilizing network resources more efficiently
   and providing better isolation between flows.  This appears to be a
   good economic choice since there are likely to be very many more
   desktop/wiring closet switches in a network than switches requiring
   Layer 3 functionality.

   The IEEE 802 specifications make no assumptions about how
   user_priority is to be used by end stations or by the network.
   Although IEEE 802.1D defines static priority queuing as the default
   mode of operation of switches that implement multiple queues, the
   user_priority is really a priority only in a loose sense since it
   depends on the number of traffic classes actually implemented by a
   switch.  The user_priority is defined as a 3 bit quantity with a
   value of 7 representing the highest priority and a value of 0 as the
   lowest.  The general switch algorithm is as follows.  Packets are
   queued within a particular traffic class based on the received
   user_priority, the value of which is either obtained directly from
   the packet if an IEEE 802.1Q header or IEEE 802.5 network is used, or
   is assigned according to some local policy.  The queue is selected
   based on a mapping from user_priority (0 through 7) onto the number
   of available traffic classes.  A switch may implement one or more
   traffic classes.  The advertised IntServ parameters and the switch's
   admission control behavior may be used to determine the mapping from



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RFC 2816        Framework for Int-Serv Over IEEE 802 LAN        May 2000


   user_priority to traffic classes within the switch.  A switch is not
   precluded from implementing other scheduling algorithms such as
   weighted fair queuing and round robin.

   IEEE 802.1D makes no recommendations about how a sender should select
   the value for user_priority.  One of the primary purposes of this
   document is to propose such usage rules, and to discuss the
   communication of the semantics of these values between switches and
   end stations.  In the remainder of this document we use the term
   traffic class synonymously with user_priority.

4.2. Ethernet/IEEE 802.3

   There is no explicit traffic class or user_priority field carried in
   Ethernet packets.  This means that user_priority must be regenerated
   at a downstream receiver or switch according to some defaults or by
   parsing further into higher layer protocol fields in the packet.
   Alternatively, IEEE 802.1Q encapsulation [4] may be used which
   provides an explicit user_priority field on top of the basic MAC
   frame format.

   For the different IP packet encapsulations used over Ethernet/IEEE
   802.3, it will be necessary to adjust any admission control
   calculations according to the framing and padding requirements as
   shown in Table 1.  Here, "ip_len" refers to the length of the IP
   packet including its headers.

                    Table 1: Ethernet encapsulations

   ---------------------------------------------------------------
   Encapsulation                          Framing Overhead  IP MTU
                                             bytes/pkt       bytes
   ---------------------------------------------------------------
   IP EtherType (ip_len<=46 bytes)             64-ip_len    1500
                (1500>=ip_len>=46 bytes)         18         1500

   IP EtherType over 802.1D/Q (ip_len<=42)     64-ip_len    1500*
                (1500>=ip_len>=42 bytes)         22         1500*

   IP EtherType over LLC/SNAP (ip_len<=40)     64-ip_len    1492
                (1500>=ip_len>=40 bytes)         24         1492
   ---------------------------------------------------------------

   *Note that the packet length of an Ethernet frame using the IEEE
   802.1Q specification exceeds the current IEEE 802.3 maximum packet
   length values by 4 bytes.  The change of maximum MTU size for IEEE
   802.1Q frames is being accommodated by IEEE 802.3ac [21].




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RFC 2816        Framework for Int-Serv Over IEEE 802 LAN        May 2000


4.3. Token Ring/IEEE 802.5

   The Token Ring standard [6] provides a priority mechanism that can be
   used to control both the queuing of packets for transmission and the
   access of packets to the shared media.  The priority mechanisms are
   implemented using bits within the Access Control (AC) and the Frame
   Control (FC) fields of a LLC frame.  The first three bits of the AC
   field, the Token Priority bits, together with the last three bits of
   the AC field, the Reservation bits, regulate which stations get
   access to the ring.  The last three bits of the FC field of a LLC
   frame, the User Priority bits, are obtained from the higher layer in
   the user_priority parameter when it requests transmission of a
   packet.  This parameter also establishes the Access Priority used by
   the MAC. The user_priority value is conveyed end-to-end by the User
   Priority bits in the FC field and is typically preserved through
   Token Ring bridges of all types.  In all cases, 0 is the lowest
   priority.

   Token Ring also uses a concept of Reserved Priority which relates to
   the value of priority which a station uses to reserve the token for
   its next transmission on the ring.  When a free token is circulating,
   only a station having an Access Priority greater than or equal to the
   Reserved Priority in the token will be allowed to seize the token for
   transmission.  Readers are referred to [14] for further discussion of
   this topic.

   A Token Ring station is theoretically capable of separately queuing
   each of the eight levels of requested user_priority and then
   transmitting frames in order of priority.  A station sets Reservation
   bits according to the user_priority of frames that are queued for
   transmission in the highest priority queue.  This allows the access
   mechanism to ensure that the frame with the highest priority
   throughout the entire ring will be transmitted before any lower
   priority frame.  Annex I to the IEEE 802.5 Token Ring standard
   recommends that stations send/relay frames as follows.
















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RFC 2816        Framework for Int-Serv Over IEEE 802 LAN        May 2000


          Table 2: Recommended use of Token Ring User Priority

            -------------------------------------
            Application             User Priority
            -------------------------------------
            Non-time-critical data      0
                  -                     1
                  -                     2
                  -                     3
            LAN management              4
            Time-sensitive data         5
            Real-time-critical data     6
            MAC frames                  7
            -------------------------------------

   To reduce frame jitter associated with high priority traffic, the
   annex also recommends that only one frame be transmitted per token
   and that the maximum information field size be 4399 octets whenever
   delay sensitive traffic is traversing the ring.  Most existing
   implementations of Token Ring bridges forward all LLC frames with a
   default access priority of 4.  Annex I recommends that bridges
   forward LLC frames that have a user_priority greater than 4 with a
   reservation equal to the user_priority (although IEEE 802.1D [3]
   permits network management override this behavior).  The capabilities
   provided by the Token Ring architecture, such User Priority and
   Reserved Priority, can provide effective support for Integrated
   Services flows that require QoS guarantees.

   For the different IP packet encapsulations used over Token Ring/IEEE
   802.5, it will be necessary to adjust any admission control
   calculations according to the framing requirements as shown in Table
   3.

                   Table 3: Token Ring encapsulations

   ---------------------------------------------------------------
   Encapsulation                          Framing Overhead  IP MTU
                                             bytes/pkt       bytes
   ---------------------------------------------------------------
   IP EtherType over 802.1D/Q                    29          4370*
   IP EtherType over LLC/SNAP                    25          4370*
   ---------------------------------------------------------------

   *The suggested MTU from RFC 1042 [13] is 4464 bytes but there are
   issues related to discovering the maximum supported MTU between any
   two points both within and between Token Ring subnets.  The MTU
   reported here is consistent with the IEEE 802.5 Annex I
   recommendation.



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RFC 2816        Framework for Int-Serv Over IEEE 802 LAN        May 2000


4.4. Fiber Distributed Data Interface

   The Fiber Distributed Data Interface (FDDI) standard [16] provides a
   priority mechanism that can be used to control both the queuing of
   packets for transmission and the access of packets to the shared
   media.  The priority mechanisms are implemented using similar
   mechanisms to Token Ring described above.  The standard also makes
   provision for "Synchronous" data traffic with strict media access and
   delay guarantees.  This mode of operation is not discussed further
   here and represents area within the scope of the ISSLL working group
   that requires further work.  In the remainder of this document, for
   the discussion of QoS mechanisms, FDDI is treated as a 100 Mbps Token
   Ring technology using a service interface compatible with IEEE 802
   networks.

4.5. Demand Priority/IEEE 802.12

   IEEE 802.12 [19] is a standard for a shared 100 Mbps LAN. Data
   packets are transmitted using either the IEEE 802.3 or IEEE 802.5
   frame format.  The MAC protocol is called Demand Priority.  Its main
   characteristics with respect to QoS are the support of two service
   priority levels, normal priority and high priority, and the order of
   service for each of these.  Data packets from all network nodes (end
   hosts and bridges/switches) are served using a simple round robin
   algorithm.

   If the IEEE 802.3 frame format is used for data transmission then the
   user_priority is encoded in the starting delimiter of the IEEE 802.12
   data packet.  If the IEEE 802.5 frame format is used then the
   user_priority is additionally encoded in the YYY bits of the FC field
   in the IEEE 802.5 packet header (see also Section 4.3).  Furthermore,
   the IEEE 802.1Q encapsulation with its own user_priority field may
   also be applied in IEEE 802.12 networks.  In all cases, switches are
   able to recover any user_priority supplied by a sender.

   The same rules apply for IEEE 802.12 user_priority mapping in a
   bridge as with other media types.  The only additional information is
   that normal priority is used by default for user_priority values 0
   through 4 inclusive, and high priority is used for user_priority
   levels 5 through 7.  This ensures that the default Token Ring
   user_priority level of 4 for IEEE 802.5 bridges is mapped to normal
   priority on IEEE 802.12 segments.