RFC 2022
Support for Multicast over UNI 3.0/3.1 based ATM Networks
Pages: 82
Proposed Standard
Proposed Standard
Part 1 of 4 – Pages 1 to 16
Network Working Group G. Armitage Request for Comments: 2022 Bellcore Category: Standards Track November 1996 Support for Multicast over UNI 3.0/3.1 based ATM 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 Abstract Mapping the connectionless IP multicast service over the connection oriented ATM services provided by UNI 3.0/3.1 is a non-trivial task. This memo describes a mechanism to support the multicast needs of Layer 3 protocols in general, and describes its application to IP multicasting in particular. ATM based IP hosts and routers use a Multicast Address Resolution Server (MARS) to support RFC 1112 style Level 2 IP multicast over the ATM Forum's UNI 3.0/3.1 point to multipoint connection service. Clusters of endpoints share a MARS and use it to track and disseminate information identifying the nodes listed as receivers for given multicast groups. This allows endpoints to establish and manage point to multipoint VCs when transmitting to the group. The MARS behaviour allows Layer 3 multicasting to be supported using either meshes of VCs or ATM level multicast servers. This choice may be made on a per-group basis, and is transparent to the endpoints.
Table of Contents 1. Introduction................................................. 4 1.1 The Multicast Address Resolution Server (MARS)............. 5 1.2 The ATM level multicast Cluster............................ 5 1.3 Document overview.......................................... 6 1.4 Conventions................................................ 7 2. The IP multicast service model............................... 7 3. UNI 3.0/3.1 support for intra-cluster multicasting........... 8 3.1 VC meshes.................................................. 9 3.2 Multicast Servers.......................................... 9 3.3 Tradeoffs.................................................. 10 3.4 Interaction with local UNI 3.0/3.1 signalling entity....... 11 4. Overview of the MARS......................................... 12 4.1 Architecture............................................... 12 4.2 Control message format..................................... 12 4.3 Fixed header fields in MARS control messages............... 13 4.3.1 Hardware type.......................................... 14 4.3.2 Protocol type.......................................... 14 4.3.3 Checksum............................................... 15 4.3.4 Extensions Offset...................................... 15 4.3.5 Operation code......................................... 16 4.3.6 Reserved............................................... 16 5. Endpoint (MARS client) interface behaviour................... 16 5.1 Transmit side behaviour.................................... 17 5.1.1 Retrieving Group Membership from the MARS.............. 18 5.1.2 MARS_REQUEST, MARS_MULTI, and MARS_NAK messages........ 20 5.1.3 Establishing the outgoing multipoint VC................ 22 5.1.4 Monitoring updates on ClusterControlVC................. 24 5.1.4.1 Updating the active VCs............................ 24 5.1.4.2 Tracking the Cluster Sequence Number............... 25 5.1.5 Revalidating a VC's leaf nodes......................... 26 5.1.5.1 When leaf node drops itself........................ 27 5.1.5.2 When a jump is detected in the CSN................. 27 5.1.6 'Migrating' the outgoing multipoint VC................. 27 5.2. Receive side behaviour.................................... 29 5.2.1 Format of the MARS_JOIN and MARS_LEAVE Messages........ 30 5.2.1.1 Important IPv4 default values...................... 32 5.2.2 Retransmission of MARS_JOIN and MARS_LEAVE messages.... 33 5.2.3 Cluster member registration and deregistration......... 34 5.3 Support for Layer 3 group management....................... 34 5.4 Support for redundant/backup MARS entities................. 36 5.4.1 First response to MARS problems........................ 36 5.4.2 Connecting to a backup MARS............................ 37 5.4.3 Dynamic backup lists, and soft redirects............... 37 5.5 Data path LLC/SNAP encapsulations.......................... 40 5.5.1 Type #1 encapsulation.................................. 40 5.5.2 Type #2 encapsulation.................................. 41
5.5.3 A Type #1 example...................................... 42
6. The MARS in greater detail................................... 42
6.1 Basic interface to Cluster members......................... 43
6.1.1 Response to MARS_REQUEST............................... 43
6.1.2 Response to MARS_JOIN and MARS_LEAVE................... 43
6.1.3 Generating MARS_REDIRECT_MAP........................... 45
6.1.4 Cluster Sequence Numbers............................... 45
6.2 MARS interface to Multicast Servers (MCSs)................. 46
6.2.1 MARS_REQUESTs for MCS supported groups................. 47
6.2.2 MARS_MSERV and MARS_UNSERV messages.................... 47
6.2.3 Registering a Multicast Server (MCS)................... 49
6.2.4 Modified response to MARS_JOIN and MARS_LEAVE.......... 49
6.2.5 Sequence numbers for ServerControlVC traffic........... 51
6.3 Why global sequence numbers?............................... 52
6.4 Redundant/Backup MARS Architectures........................ 52
7. How an MCS utilises a MARS................................... 53
7.1 Association with a particular Layer 3 group................ 53
7.2 Termination of incoming VCs................................ 54
7.3 Management of outgoing VC.................................. 54
7.4 Use of a backup MARS....................................... 54
8. Support for IP multicast routers............................. 54
8.1 Forwarding into a Cluster.................................. 55
8.2 Joining in 'promiscuous' mode.............................. 55
8.3 Forwarding across the cluster.............................. 56
8.4 Joining in 'semi-promiscous' mode.......................... 56
8.5 An alternative to IGMP Queries............................. 57
8.6 CMIs across multiple interfaces............................ 58
9. Multiprotocol applications of the MARS and MARS clients...... 59
10. Supplementary parameter processing.......................... 60
10.1 Interpreting the mar$extoff field......................... 60
10.2 The format of TLVs........................................ 60
10.3 Processing MARS messages with TLVs........................ 62
10.4 Initial set of TLV elements............................... 62
11. Key Decisions and open issues............................... 62
Security Considerations......................................... 65
Acknowledgments................................................. 65
Author's Address................................................ 65
References...................................................... 66
Appendix A. Hole punching algorithms............................ 67
Appendix B. Minimising the impact of IGMP in IPv4 environments.. 69
Appendix C. Further comments on 'Clusters'...................... 71
Appendix D. TLV list parsing algorithm.......................... 72
Appendix E. Summary of timer values............................. 73
Appendix F. Pseudo code for MARS operation...................... 74
1. Introduction. Multicasting is the process whereby a source host or protocol entity sends a packet to multiple destinations simultaneously using a single, local 'transmit' operation. The more familiar cases of Unicasting and Broadcasting may be considered to be special cases of Multicasting (with the packet delivered to one destination, or 'all' destinations, respectively). Most network layer models, like the one described in RFC 1112 [1] for IP multicasting, assume sources may send their packets to abstract 'multicast group addresses'. Link layer support for such an abstraction is assumed to exist, and is provided by technologies such as Ethernet. ATM is being utilized as a new link layer technology to support a variety of protocols, including IP. With RFC 1483 [2] the IETF defined a multiprotocol mechanism for encapsulating and transmitting packets using AAL5 over ATM Virtual Channels (VCs). However, the ATM Forum's currently published signalling specifications (UNI 3.0 [8] and UNI 3.1 [4]) does not provide the multicast address abstraction. Unicast connections are supported by point to point, bidirectional VCs. Multicasting is supported through point to multipoint unidirectional VCs. The key limitation is that the sender must have prior knowledge of each intended recipient, and explicitly establish a VC with itself as the root node and the recipients as the leaf nodes. This document has two broad goals: Define a group address registration and membership distribution mechanism that allows UNI 3.0/3.1 based networks to support the multicast service of protocols such as IP. Define specific endpoint behaviours for managing point to multipoint VCs to achieve multicasting of layer 3 packets. As the IETF is currently in the forefront of using wide area multicasting this document's descriptions will often focus on IP service model of RFC 1112. A final chapter will note the multiprotocol application of the architecture. This document avoids discussion of one highly non-trivial aspect of using ATM - the specification of QoS for VCs being established in response to higher layer needs. Research in this area is still very formative [7], and so it is assumed that future documents will clarify the mapping of QoS requirements to VC establishment. The default at this time is that VCs are established with a request for
Unspecified Bit Rate (UBR) service, as typified by the IETF's use of VCs for unicast IP, described in RFC 1755 [6]. 1.1 The Multicast Address Resolution Server (MARS). The Multicast Address Resolution Server (MARS) is an extended analog of the ATM ARP Server introduced in RFC 1577 [3]. It acts as a registry, associating layer 3 multicast group identifiers with the ATM interfaces representing the group's members. MARS messages support the distribution of multicast group membership information between MARS and endpoints (hosts or routers). Endpoint address resolution entities query the MARS when a layer 3 address needs to be resolved to the set of ATM endpoints making up the group at any one time. Endpoints keep the MARS informed when they need to join or leave particular layer 3 groups. To provide for asynchronous notification of group membership changes the MARS manages a point to multipoint VC out to all endpoints desiring multicast support Valid arguments can be made for two different approaches to ATM level multicasting of layer 3 packets - through meshes of point to multipoint VCs, or ATM level multicast servers (MCS). The MARS architecture allows either VC meshes or MCSs to be used on a per- group basis. 1.2 The ATM level multicast Cluster. Each MARS manages a 'cluster' of ATM-attached endpoints. A Cluster is defined as The set of ATM interfaces choosing to participate in direct ATM connections to achieve multicasting of AAL_SDUs between themselves. In practice, a Cluster is the set of endpoints that choose to use the same MARS to register their memberships and receive their updates from. By implication of this definition, traffic between interfaces belonging to different Clusters passes through an inter-cluster device. (In the IP world an inter-cluster device would be an IP multicast router with logical interfaces into each Cluster.) This document explicitly avoids specifying the nature of inter-cluster (layer 3) routing protocols. The mapping of clusters to other constrained sets of endpoints (such as unicast Logical IP Subnets) is left to each network administrator. However, for the purposes of conformance with this document network administrators MUST ensure that each Logical IP Subnet (LIS) is
served by a separate MARS, creating a one-to-one mapping between cluster and unicast LIS. IP multicast routers then interconnect each LIS as they do with conventional subnets. (Relaxation of this restriction MAY only occur after future research on the interaction between existing layer 3 multicast routing protocols and unicast subnet boundaries.) The term 'Cluster Member' will be used in this document to refer to an endpoint that is currently using a MARS for multicast support. Thus potential scope of a cluster may be the entire membership of a LIS, while the actual scope of a cluster depends on which endpoints are actually cluster members at any given time. 1.3 Document overview. This document assumes an understanding of concepts explained in greater detail in RFC 1112, RFC 1577, UNI 3.0/3.1, and RFC 1755 [6]. Section 2 provides an overview of IP multicast and what RFC 1112 required from Ethernet. Section 3 describes in more detail the multicast support services offered by UNI 3.0/3.1, and outlines the differences between VC meshes and multicast servers (MCSs) as mechanisms for distributing packets to multiple destinations. Section 4 provides an overview of the MARS and its relationship to ATM endpoints. This section also discusses the encapsulation and structure of MARS control messages. Section 5 substantially defines the entire cluster member endpoint behaviour, on both receive and transmit sides. This includes both normal operation and error recovery. Section 6 summarises the required behaviour of a MARS. Section 7 looks at how a multicast server (MCS) interacts with a MARS. Section 8 discusses how IP multicast routers may make novel use of promiscuous and semi-promiscuous group joins. Also discussed is a mechanism designed to reduce the amount of IGMP traffic issued by routers. Section 9 discusses how this document applies in the more general (non-IP) case.
Section 10 summarises the key proposals, and identifies areas for future research that are generated by this MARS architecture. The appendices provide discussion on issues that arise out of the implementation of this document. Appendix A discusses MARS and endpoint algorithms for parsing MARS messages. Appendix B describes the particular problems introduced by the current IGMP paradigms, and possible interim work-arounds. Appendix C discusses the 'cluster' concept in further detail, while Appendix D briefly outlines an algorithm for parsing TLV lists. Appendix E summarises various timer values used in this document, and Appendix F provides example pseudo-code for a MARS entity. 1.4 Conventions. In this document the following coding and packet representation rules are used: All multi-octet parameters are encoded in big-endian form (i.e. the most significant octet comes first). In all multi-bit parameters bit numbering begins at 0 for the least significant bit when stored in memory (i.e. the n'th bit has weight of 2^n). A bit that is 'set', 'on', or 'one' holds the value 1. A bit that is 'reset', 'off', 'clear', or 'zero' holds the value 0. 2. Summary of the IP multicast service model. Under IP version 4 (IPv4), addresses in the range between 224.0.0.0 and 239.255.255.255 (224.0.0.0/4) are termed 'Class D' or 'multicast group' addresses. These abstractly represent all the IP hosts in the Internet (or some constrained subset of the Internet) who have decided to 'join' the specified group. RFC1112 requires that a multicast-capable IP interface must support the transmission of IP packets to an IP multicast group address, whether or not the node considers itself a 'member' of that group. Consequently, group membership is effectively irrelevant to the transmit side of the link layer interfaces. When Ethernet is used as the link layer (the example used in RFC1112), no address resolution is required to transmit packets. An algorithmic mapping from IP multicast address to Ethernet multicast address is performed locally before the packet is sent out the local interface in the same 'send and forget' manner as a unicast IP packet.
Joining and Leaving an IP multicast group is more explicit on the receive side - with the primitives JoinLocalGroup and LeaveLocalGroup affecting what groups the local link layer interface should accept packets from. When the IP layer wants to receive packets from a group, it issues JoinLocalGroup. When it no longer wants to receive packets, it issues LeaveLocalGroup. A key point to note is that changing state is a local issue, it has no effect on other hosts attached to the Ethernet. IGMP is defined in RFC 1112 to support IP multicast routers attached to a given subnet. Hosts issue IGMP Report messages when they perform a JoinLocalGroup, or in response to an IP multicast router sending an IGMP Query. By periodically transmitting queries IP multicast routers are able to identify what IP multicast groups have non-zero membership on a given subnet. A specific IP multicast address, 224.0.0.1, is allocated for the transmission of IGMP Query messages. Host IP layers issue a JoinLocalGroup for 224.0.0.1 when they intend to participate in IP multicasting, and issue a LeaveLocalGroup for 224.0.0.1 when they've ceased participating in IP multicasting. Each host keeps a list of IP multicast groups it has been JoinLocalGroup'd to. When a router issues an IGMP Query on 224.0.0.1 each host begins to send IGMP Reports for each group it is a member of. IGMP Reports are sent to the group address, not 224.0.0.1, "so that other members of the same group on the same network can overhear the Report" and not bother sending one of their own. IP multicast routers conclude that a group has no members on the subnet when IGMP Queries no longer elicit associated replies. 3. UNI 3.0/3.1 support for intra-cluster multicasting. For the purposes of the MARS protocol, both UNI 3.0 and UNI 3.1 provide equivalent support for multicasting. Differences between UNI 3.0 and UNI 3.1 in required signalling elements are covered in RFC 1755. This document will describe its operation in terms of 'generic' functions that should be available to clients of a UNI 3.0/3.1 signalling entity in a given ATM endpoint. The ATM model broadly describes an 'AAL User' as any entity that establishes and manages VCs and underlying AAL services to exchange data. An IP over ATM interface is a form of 'AAL User' (although the default LLC/SNAP encapsulation mode specified in RFC1755 really requires that an 'LLC entity' is the AAL User, which in turn supports the IP/ATM interface).
The most fundamental limitations of UNI 3.0/3.1's multicast support
are:
Only point to multipoint, unidirectional VCs may be established.
Only the root (source) node of a given VC may add or remove leaf
nodes.
Leaf nodes are identified by their unicast ATM addresses. UNI
3.0/3.1 defines two ATM address formats - native E.164 and NSAP
(although it must be stressed that the NSAP address is so called
because it uses the NSAP format - an ATM endpoint is NOT a Network
layer termination point). In UNI 3.0/3.1 an 'ATM Number' is the
primary identification of an ATM endpoint, and it may use either
format. Under some circumstances an ATM endpoint must be identified
by both a native E.164 address (identifying the attachment point of a
private network to a public network), and an NSAP address ('ATM
Subaddress') identifying the final endpoint within the private
network. For the rest of this document the term will be used to mean
either a single 'ATM Number' or an 'ATM Number' combined with an 'ATM
Subaddress'.
3.1 VC meshes.
The most fundamental approach to intra-cluster multicasting is the
multicast VC mesh. Each source establishes its own independent point
to multipoint VC (a single multicast tree) to the set of leaf nodes
(destinations) that it has been told are members of the group it
wishes to send packets to.
Interfaces that are both senders and group members (leaf nodes) to a
given group will originate one point to multipoint VC, and terminate
one VC for every other active sender to the group. This criss-
crossing of VCs across the ATM network gives rise to the name 'VC
mesh'.
3.2 Multicast Servers.
An alternative model has each source establish a VC to an
intermediate node - the multicast server (MCS). The multicast server
itself establishes and manages a point to multipoint VC out to the
actual desired destinations.
The MCS reassembles AAL_SDUs arriving on all the incoming VCs, and
then queues them for transmission on its single outgoing point to
multipoint VC. (Reassembly of incoming AAL_SDUs is required at the
multicast server as AAL5 does not support cell level multiplexing of
different AAL_SDUs on a single outgoing VC.)
The leaf nodes of the multicast server's point to multipoint VC must be established prior to packet transmission, and the multicast server requires an external mechanism to identify them. A side-effect of this method is that ATM interfaces that are both sources and group members will receive copies of their own packets back from the MCS (An alternative method is for the multicast server to explicitly retransmit packets on individual VCs between itself and group members. A benefit of this second approach is that the multicast server can ensure that sources do not receive copies of their own packets.) The simplest MCS pays no attention to the contents of each AAL_SDU. It is purely an AAL/ATM level device. More complex MCS architectures (where a single endpoint serves multiple layer 3 groups) are possible, but are beyond the scope of this document. More detailed discussion is provided in section 7. 3.3 Tradeoffs. Arguments over the relative merits of VC meshes and multicast servers have raged for some time. Ultimately the choice depends on the relative trade-offs a system administrator must make between throughput, latency, congestion, and resource consumption. Even criteria such as latency can mean different things to different people - is it end to end packet time, or the time it takes for a group to settle after a membership change? The final choice depends on the characteristics of the applications generating the multicast traffic. If we focussed on the data path we might prefer the VC mesh because it lacks the obvious single congestion point of an MCS. Throughput is likely to be higher, and end to end latency lower, because the mesh lacks the intermediate AAL_SDU reassembly that must occur in MCSs. The underlying ATM signalling system also has greater opportunity to ensure optimal branching points at ATM switches along the multicast trees originating on each source. However, resource consumption will be higher. Every group member's ATM interface must terminate a VC per sender (consuming on-board memory for state information, instance of an AAL service, and buffering in accordance with the vendors particular architecture). On the contrary, with a multicast server only 2 VCs (one out, one in) are required, independent of the number of senders. The allocation of VC related resources is also lower within the ATM cloud when using a multicast server. These points may be considered to have merit in environments where VCs across the UNI or within the ATM cloud are valuable (e.g. the ATM provider charges on a per VC basis), or AAL contexts are limited in the ATM interfaces of endpoints.
If we focus on the signalling load then MCSs have the advantage when faced with dynamic sets of receivers. Every time the membership of a multicast group changes (a leaf node needs to be added or dropped), only a single point to multipoint VC needs to be modified when using an MCS. This generates a single signalling event across the MCS's UNI. However, when membership change occurs in a VC mesh, signalling events occur at the UNIs of every traffic source - the transient signalling load scales with the number of sources. This has obvious ramifications if you define latency as the time for a group's connectivity to stabilise after change (especially as the number of senders increases). Finally, as noted above, MCSs introduce a 'reflected packet' problem, which requires additional per-AAL_SDU information to be carried in order for layer 3 sources to detect their own AAL_SDUs coming back. The MARS architecture allows system administrators to utilize either approach on a group by group basis. 3.4 Interaction with local UNI 3.0/3.1 signalling entity. The following generic signalling functions are presumed to be available to local AAL Users: L_CALL_RQ - Establish a unicast VC to a specific endpoint. L_MULTI_RQ - Establish multicast VC to a specific endpoint. L_MULTI_ADD - Add new leaf node to previously established VC. L_MULTI_DROP - Remove specific leaf node from established VC. L_RELEASE - Release unicast VC, or all Leaves of a multicast VC. The signalling exchanges and local information passed between AAL User and UNI 3.0/3.1 signalling entity with these functions are outside the scope of this document. The following indications are assumed to be available to AAL Users, generated by the local UNI 3.0/3.1 signalling entity: L_ACK - Succesful completion of a local request. L_REMOTE_CALL - A new VC has been established to the AAL User. ERR_L_RQFAILED - A remote ATM endpoint rejected an L_CALL_RQ, L_MULTI_RQ, or L_MULTI_ADD. ERR_L_DROP - A remote ATM endpoint dropped off an existing VC. ERR_L_RELEASE - An existing VC was terminated. The signalling exchanges and local information passed between AAL User and UNI 3.0/3.1 signalling entity with these functions are outside the scope of this document.
4. Overview of the MARS. The MARS may reside within any ATM endpoint that is directly addressable by the endpoints it is serving. Endpoints wishing to join a multicast cluster must be configured with the ATM address of the node on which the cluster's MARS resides. (Section 5.4 describes how backup MARSs may be added to support the activities of a cluster. References to 'the MARS' in following sections will be assumed to mean the acting MARS for the cluster.) 4.1 Architecture. Architecturally the MARS is an evolution of the RFC 1577 ARP Server. Whilst the ARP Server keeps a table of {IP,ATM} address pairs for all IP endpoints in an LIS, the MARS keeps extended tables of {layer 3 address, ATM.1, ATM.2, ..... ATM.n} mappings. It can either be configured with certain mappings, or dynamically 'learn' mappings. The format of the {layer 3 address} field is generally not interpreted by the MARS. A single ATM node may support multiple logical MARSs, each of which support a separate cluster. The restriction is that each MARS has a unique ATM address (e.g. a different SEL field in the NSAP address of the node on which the multiple MARSs reside). By definition a single instance of a MARS may not support more than one cluster. The MARS distributes group membership update information to cluster members over a point to multipoint VC known as the ClusterControlVC. Additionally, when Multicast Servers (MCSs) are being used it also establishes a separate point to multipoint VC out to registered MCSs, known as the ServerControlVC. All cluster members are leaf nodes of ClusterControlVC. All registered multicast servers are leaf nodes of ServerControlVC (described further in section 6). The MARS does NOT take part in the actual multicasting of layer 3 data packets. 4.2 Control message format. By default all MARS control messages MUST be LLC/SNAP encapsulated using the following codepoints: [0xAA-AA-03][0x00-00-5E][0x00-03][MARS control message] (LLC) (OUI) (PID) (This is a PID from the IANA OUI.)
MARS control messages are made up of 4 major components:
[Fixed header][Mandatory fields][Addresses][Supplementary TLVs]
[Fixed header] contains fields indicating the operation being
performed and the layer 3 protocol being referred to (e.g IPv4, IPv6,
AppleTalk, etc). The fixed header also carries checksum information,
and hooks to allow this basic control message structure to be re-used
by other query/response protocols.
The [Mandatory fields] section carries fixed width parameters that
depend on the operation type indicated in [Fixed header].
The following [Addresses] area carries variable length fields for
source and target addresses - both hardware (e.g. ATM) and layer 3
(e.g. IPv4). These provide the fundamental information that the
registrations, queries, and updates use and operate on. For the MARS
protocol fields in [Fixed header] indicate how to interpret the
contents of [Addresses].
[Supplementary TLVs] represents an optional list of TLV (type,
length, value) encoded information elements that may be appended to
provide supplementary information. This feature is described in
further detail in section 10.
MARS messages contain variable length address fields. In all cases
null addresses SHALL be encoded as zero length, and have no space
allocated in the message.
(Unique LLC/SNAP encapsulation of MARS control messages means MARS
and ARP Server functionality may be implemented within a common
entity, and share a client-server VC, if the implementor so chooses.
Note that the LLC/SNAP codepoint for MARS is different to the
codepoint used for ATMARP.)
4.3 Fixed header fields in MARS control messages.
The [Fixed header] has the following format:
Data:
mar$afn 16 bits Address Family (0x000F).
mar$pro 56 bits Protocol Identification.
mar$hdrrsv 24 bits Reserved. Unused by MARS control protocol.
mar$chksum 16 bits Checksum across entire MARS message.
mar$extoff 16 bits Extensions Offset.
mar$op 16 bits Operation code.
mar$shtl 8 bits Type & length of source ATM number. (r)
mar$sstl 8 bits Type & length of source ATM subaddress. (q)
mar$shtl and mar$sstl provide information regarding the source's hardware (ATM) address. In the MARS protocol these fields are always present, as every MARS message carries a non-null source ATM address. In all cases the source ATM address is the first variable length field in the [Addresses] section. The other fields in [Fixed header] are described in the following subsections. 4.3.1 Hardware type. mar$afn defines the type of link layer addresses being carried. The value of 0x000F SHALL be used by MARS messages generated in accordance with this document. The encoding of ATM addresses and subaddresses when mar$afn = 0x000F is described in section 5.1.2. Encodings when mar$afn != 0x000F are outside the scope of this document. 4.3.2 Protocol type. The mar$pro field is made up of two subfields: mar$pro.type 16 bits Protocol type. mar$pro.snap 40 bits Optional SNAP extension to protocol type. The mar$pro.type field is a 16 bit unsigned integer representing the following number space: 0x0000 to 0x00FF Protocols defined by the equivalent NLPIDs. 0x0100 to 0x03FF Reserved for future use by the IETF. 0x0400 to 0x04FF Allocated for use by the ATM Forum. 0x0500 to 0x05FF Experimental/Local use. 0x0600 to 0xFFFF Protocols defined by the equivalent Ethertypes. (based on the observations that valid Ethertypes are never smaller than 0x600, and NLPIDs never larger than 0xFF.) The NLPID value of 0x80 is used to indicate a SNAP encoded extension is being used to encode the protocol type. When mar$pro.type == 0x80 the SNAP extension is encoded in the mar$pro.snap field. This is termed the 'long form' protocol ID. If mar$pro.type != 0x80 then the mar$pro.snap field MUST be zero on transmit and ignored on receive. The mar$pro.type field itself identifies the protocol being referred to. This is termed the 'short form' protocol ID.
In all cases, where a protocol has an assigned number in the mar$pro.type space (excluding 0x80) the short form MUST be used when transmitting MARS messages. Additionally, where a protocol has valid short and long forms of identification, receivers MAY choose to recognise the long form. mar$pro.type values other than 0x80 MAY have 'long forms' defined in future documents. For the remainder of this document references to mar$pro SHALL be interpreted to mean mar$pro.type, or mar$pro.type in combination with mar$pro.snap as appropriate. The use of different protocol types is described further in section 9. 4.3.3 Checksum. The mar$chksum field carries a standard IP checksum calculated across the entire MARS control message (excluding the LLC/SNAP header). The field is set to zero before performing the checksum calculation. As the entire LLC/SNAP encapsulated MARS message is protected by the 32 bit CRC of the AAL5 transport, implementors MAY choose to ignore the checksum facility. If no checksum is calculated these bits MUST be reset before transmission. If no checksum is performed on reception, this field MUST be ignored. If a receiver is capable of validating a checksum it MUST only perform the validation when the received mar$chksum field is non-zero. Messages arriving with mar$chksum of 0 are always considered valid. 4.3.4 Extensions Offset. The mar$extoff field identifies the existence and location of an optional supplementary parameters list. Its use is described in section 10.
4.3.5 Operation code. The mar$op field is further subdivided into two 8 bit fields - mar$op.version (leading octet) and mar$op.type (trailing octet). Together they indicate the nature of the control message, and the context within which its [Mandatory fields], [Addresses], and [Supplementary TLVs] should be interpreted. mar$op.version 0 MARS protocol defined in this document. 0x01 - 0xEF Reserved for future use by the IETF. 0xF0 - 0xFE Allocated for use by the ATM Forum. 0xFF Experimental/Local use. mar$op.type Value indicates operation being performed, within context of the control protocol version indicated by mar$op.version. For the rest of this document references to the mar$op value SHALL be taken to mean mar$op.type, with mar$op.version = 0x00. The values used in this document are summarised in section 11. (Note this number space is independent of the ATMARP operation code number space.) 4.3.6 Reserved. mar$hdrrsv may be subdivided and assigned specific meanings for other control protocols indicated by mar$op.version != 0.
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