Network Working Group T. Smith
Request for Comments: 2226 IBM Corporation
Category: Standards Track G. Armitage
LUCent Technologies
October 1997
IP Broadcast over 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.
Copyright Notice
Copyright (C) The Internet Society (1997). All Rights Reserved.
Abstract
This memo describes how the IP multicast service being developed by
the IP over ATM working group may be used to support IP broadcast
transmission. The solution revolves around treating the broadcast
problem as a special case of multicast, where every host in the
subnet or cluster is a member of the group.
An understanding of the services provided by RFC2022 is assumed.
1. Introduction.
The IETF's first step in solving the problems of running IP over
Asynchronous Transfer Mode (ATM) technology is described in RFC1577
[1]. It provides for unicast communication between hosts and routers
within Logical IP Subnets (LISs), and proposes a centralized ATM ARP
Server which provides IP to ATM address resolution services to LIS
members.
Two classes of IP service were omitted - multicast and broadcast
transmissions. Multicasting allows a single transmit operation to
cause a packet to be received by multiple remote destinations.
Broadcasting typically allows a single transmit operation to cause a
packet to be received by all IP hosts that are members of a
particular 'subnet'.
To address the need for multicast support (represented by
transmission to IP addresses in the Class D space), RFC2022
("Support for Multicast over UNI 3.0/3.1 based ATM Networks") [2] was
created. This memo creates an analog of the RFC1577 ARP Server - a
new entity known as the MARS (Multicast Address Resolution Server).
The MARS operates as a centralized registry and distribution
mechanism for mappings between IP multicast addresses and groups of
ATM unicast addresses. Host behavior is also defined for establishing
and managing point to multipoint VCs, based on the information
returned by the MARS, when hosts wish to transmit packets to a
multicast group.
This memo aims to show how RFC2022 may be used to emulate IP
broadcast within Logical IP Subnets. While the broadcast technique
does not align itself well with the underlying point-to-point nature
of ATM, clearly, some applications will still wish to use IP
broadcasts. Client-server applications where the client searches for
a server by sending out a broadcast is one scenario. Routing
protocols, most notably RIP, are other examples.
2. Review of Unicast and Multicast.
Both the unicast and multicast cases take advantage of the point-to-
point and point-to-multipoint capabilities defined in the ATM Forum
UNI 3.1 document [4]. A unicast IP address has a single ATM level
destination. Unicast transmissions occur over point to point Virtual
Channels (VCs) between the source and destination. The ARP Server
holds mappings between IP destination addresses and their associated
ATM destination address. Hosts issue an ARP_REQUEST to the ARP Server
when they wish to ascertain a particular mapping. The ARP Server
replies with either an ARP_REPLY containing the ATM address of the
destination, or an ARP_NAK when the ARP Server is unable to resolve
the address. If the request is successful the host establishes a VC
to the destination interface. This VC is then used to forward the
first (and subsequent) packets to that particular IP destination. RFC
1577 describes in further detail how hosts are administratively
grouped in to Logical IP Subnets (LISs), and how the ARP Server
establishes the initial mappings for members of the LIS it serves.
The basic host behavior for multicasting is similar - the sender must
establish and manage a point to multipoint VC whose leaf nodes are
the group's actual members. Under UNI 3.1 these VCs can only be
established and altered by the source (root) interface.
The MARS is an evolution of the ARP Server model, and performs two
key functions. The first function is the maintenance of a list of
ATM addresses corresponding to the members for each group. This list
is created by a host registration process which involves two messages
- a MARS_JOIN which declares that a host wishes to join the specified
group(s), and a MARS_LEAVE which indicates that a host wishes to
leave the specified group(s).
MARS_JOIN and MARS_LEAVE messages are also redistributed to all
members of the group so that active senders may dynamically adjust
their point to multipoint VCs accordingly.
The other major function is the retrieval of group membership from
MARS (analogous to the ARP Server providing unicast address
mappings). When faced with the need to transmit an IP packet with a
Class D destination address, a host issues a MARS_REQUEST to the
MARS. If the group has members the MARS returns a MARS_MULTI
(possibly in multiple segments) carrying a set of ATM addresses. The
host then establishes an initial point to multipoint VC using these
ATM addresses as the leaf nodes. If the MARS had no mapping it would
return a MARS_NAK.
(RFC2022 also discusses how the MARS can arrange for Class D groups
to be supported by either multicast servers, or meshes of point to
multipoint VCs from host to host. However, from the host's
perspective this is transparent, and is not central to this
discussion of IP broadcast support.)
This memo describes how a host may utilize the registration and group
management functions in an existing MARS based IP/ATM network to
emulate IP broadcasts.
3. Broadcast as a special case of Multicast.
Many of the problems that occur when implementing a broadcast
solution also occur in when implementing a multicast solution. In
fact, broadcast may be considered a special case of multicast. That
is, broadcast is a multicast group whose members include all members
in the LIS.
There are two broadcast groups which this memo addresses:
1) 255.255.255.255 - "All ones" broadcast
2) x.z - CIDR-prefix (subnet) directed broadcast
Broadcast (1) is sometimes referred to as a limited broadcast to this
physical network. Broadcast (2) can be thought of as the the
broadcast for subnets or networks in the old paradigm. As described
in [6] and [7], the notion of subnets and networks is being replaced
with a more efficient utilization of the routing address space known
as Classless Inter-Domain Routing. The CIDR-prefix (x) is the
combination of IP address and subnet mask that denotes the subnet
number. The host portion of the address (z) is all ones. One should
note that while these broadcasts have different scopes at the IP or
network layer, they have precisely the same scope at the link layer
-- namely that all members of the LIS will receive a copy.
These addresses may be used in two environments:
o Broadcasting to all members of a given LIS where
a priori knowledge of a host's IP address and
subnet mask are known (e.g. the CIDR-prefix directed
broadcast).
o Broadcasting to all members of a physical network
without knowledge of a host's IP address and
subnet mask (e.g. the all ones broadcast).
On a broadcast medium like Ethernet, these two environments result in
the same physical destination. That is, all stations on that network
will receive the broadcast even if they are on different logical
subnets, or are non-IP stations. With ATM, this may not be the case.
Because ATM is non-broadcast, a registration process must take place.
And if there are stations that register to some broadcast groups, but
not others, then the different broadcast groups will have different
memberships. The notion of broadcast becomes inconsistent.
One case that requires the use of the all ones broadcast is that of
the diskless boot, or bootp client, where the host boots up, and does
not know its own IP address or subnet mask. Clearly, the host does
not know which subnet it belongs to. So, to send a broadcast to its
bootp server, the diskless workstation must use the group which
contains no subnet information, i.e. the 255.255.255.255 broadcast
group. Carrying the example a little further, the bootp server,
after receiving the broadcast, can not send either a directed frame
nor a subnet directed broadcast to respond to the diskless
workstation. Instead, the bootp server must also use the
255.255.255.255 group to communicate with the client.
While the all ones broadcast is required at the IP layer, it also has
relevance at the link layer when deciding which broadcast group to
register with in MARS. In other Words, a bootp client wishing to
register for a link layer broadcast, can only register for
255.255.255.255 in the MARS address space because the client's subnet
is unknown at the time. Given that some applications must use the
all ones address in MARS for their broadcast group, and that we wish
to minimize the number of broadcast groups used by LIS members, the
all ones group in MARS MUST be used by all members of the LIS when
registering to receive broadcast transmissions. The VCC used for
transmitting any broadcast packet will be based on the members
registered in the MARS under the 255.255.255.255 address position.
This VCC will be referred to as the "broadcast channel" through the
remainder of this memo.
4. The MARS role in broadcast.
Many solutions have been proposed, some of which are listed in
Appendix A. This memo addresses a MARS solution which appears to do
the best job of solving the broadcast problem.
There are a number of characteristics of the MARS architecture that
should be kept intact. They include:
o MARS contains no knowledge of subnet prefixes and subnet masks.
Each group address registered with MARS is managed independently.
o A MARS may only serve one LIS. This insures that the
broadcast group 255.255.255.255 is joined by hosts from one
LIS, keeping its scope bound to conventional interpretation.
o The Multicast Server (MCS) described in [2] may be used to service
the broadcast groups defined in this memo without modification.
The MCS will reduce the number of channels used by the network.
The MARS needs no additional code or special algorithms to handle the
resolution of IP broadcast addresses. It is simply a general database
that holds {Protocol address, ATM.1, ATM.2, ... ATM.n} mappings, and
imposes no constraints on the type and length of the 'Protocol
address'. Whether the hosts view it as Class D or 'broadcast' (or
even IP) is purely a host side issue.
It is likely that end points will want to use the IP broadcast
emulation described here in order to support boot time location of
the end point's IP address. This leads to the observation that the
MARS should NOT eXPect to see both the IP source and ATM source
address fields of the MARS_JOIN filled in. This is reasonable, since
only the ATM source address is used when registering the end point as
a group member.
The MARS architecture is sufficient to insure the integrity of the
broadcast group list without any modification.
5. Host Requirements for Broadcast.
The following list of bullets describes additional characteristics of
a MARS-compliant host. These characteristics are required to take
advantage of the broadcast function.
o A host must register as a MARS client.
o A host, soon after registration MUST issue a MARS_JOIN to the
all ones broadcast address (i.e. 255.255.255.255) with the
mar$flags.layer3grp reset.
o When transmitting packets, the host should map all IP layer
broadcasts to the VCC (broadcast channel) created and maintained
based on the all ones entry in MARS.
o A host MUST monitor the MARS_JOIN/MARS_LEAVE messages
for 255.255.255.255 to keep the broadcast channel current.
o A broadcast channel should be torn down after a period of
inactivity. The corresponding timeout period MAY be specified
with a minimum value of one minute, and a RECOMMENDED
default value of 20 minutes.
One should note that while every member participating in the
broadcast MUST be a member of the all ones group, not all members
will choose to transmit broadcast information. Some members will
only elect to receive broadcast information passively. Therefore, in
a LIS with n stations, there may be less than n channels terminated
at each station for broadcast information. Further reductions may be
gained by adding a Multicast Server (MCS) to the broadcast
environment which could reduce the number of VCs to two (one
incoming, one outgoing), or one for a station that only wishes to
listen.
It is well understood that broadcasting in this environment may tax
the resources of the network and of the hosts that use it.
Therefore, an implementer MAY choose to provide a mechanism for
retracting the host's entry in the broadcast group after it has been
established or prior to joining the group. The MARS_LEAVE is used to
request withdrawal from the group if the host wishes to disable
broadcast reception after it has joined the group. The default
behavior SHALL be to join the all ones broadcast group in MARS.
6. Implications of IP broadcast on ATM level resources.
RFC2022 discusses some of the implications of large multicast groups
on the allocation of ATM level resources, both within the network and
within end station ATM interfaces.
The default mechanism is for IP multicasting to be achieved using
meshes of point to multipoint VCs, direct from source host to group
members. Under certain circumstances system administrators may, in a
manner completely transparent to end hosts, redirect multicast
traffic through ATM level Multicast Servers (MCss). This may be
performed on an individual group basis.
It is sufficient to note here that the IP broadcast 'multicast group'
will constitute the largest consumer of VCs within your ATM network
when it is active. For this reason it will probably be the first
multicast group to have one or more ATM MCSs assigned to support it.
However, there is nothing unique about an MCS assigned to support IP
broadcast traffic, so this will not be dealt with further in this
memo. RFC2022 contains further discussion on the possible
application of multiple MCSs to provide fault-tolerant architectures.
7. Further discussion.
A point of discussion on the ip-atm forum revolved around "auto
configuration" and "diskless boot". This memo describes a broadcast
solution that requires the use of the MARS. Therefore, at a minimum,
the ATM address of the MARS must be manually configured into a
diskless workstation. Suggestions such as universal channel numbers,
and universal ATM addresses have been proposed, however, no agreement
has been reached.
Another topic for discussion is multiprotocol support. MARS is
designed for protocol independence. This memo specifically addresses
the IP broadcast case, identifying which addresses are most effective
in the IP address space. However, the principles apply to any layer
3 protocol. Further work should be performed to identify suitable
addresses for other layer 3 protocols.
Finally, there has been support voiced for a link layer broadcast
that would be independent of the layer 3 protocol. Such a solution
may provide a simpler set of rules through which broadcast
applications may be used. In addition, some solutions also provide
for more efficient use of VCCs.
Security Considerations
This memo addresses a specific use of the MARS architecture and
components to provide the broadcast function. As such, the security
implications are no greater or less than the implications of using
any of the other multicast groups available in the multicast address
range. Should enhancements to security be required, they would need
to be added as an extension to the base architecture in RFC2022.
Acknowledgments
The apparent simplicity of this memo owes a lot to the services
provided in [2], which itself is the product of much discussion on
the IETF's IP-ATM working group mailing list. Grenville Armitage
worked on this document while at Bellcore.
References
[1] Laubach, M., "Classical IP and ARP over ATM", RFC1577,
December 1993.
[2] Armitage, G., "Support for Multicast over UNI 3.0/3.1 based ATM
Networks", RFC2022, November 1995.
[3] Deering, S., "Host Extensions for IP Multicasting", STD 5,
RFC1112, August 1989.
[4] ATM Forum, "ATM User-Network Interface Specification Version
3.0", Englewood Cliffs, NJ: Prentice Hall, September 1993.
[5] Perez, M., Liaw, F., Grossman, D., Mankin, A., Hoffman, E. and
A. Malis, "ATM Signaling Support for IP over ATM", RFC1755,
February 1995.
[6] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Classless Inter-
Domain Routing (CIDR): an Address Assignment and Aggregation
Strategy", RFC1519, September 1993.
[7] Baker, F., "Requirements for IP Version 4 Routers", RFC1812,
June 1995.
[8] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels, BCP 14, RFC2119, March 1997.
Authors' Addresses
Timothy J. Smith
Network Routing Systems,
International Business Machines Corporation.
N21/664
P.O.Box 12195
Research Triangle Park, NC 27709
Phone: (919) 254-4723
EMail: tjsmith@vnet.ibm.com
Grenville Armitage
Bell Labs, Lucent Technologies.
101 Crawfords Corner Rd,
Holmdel, NJ, 07733
EMail: gja@lucent.com
Appendix A. Broadcast alternatives
Throughout the development of this memo, there have been a number of
alternatives explored and discarded for one reason or another. This
appendix documents these alternatives and the reason that they were
not chosen.
A.1 ARP Server Broadcast Solutions.
The ARP Server is a good candidate to support broadcasting. There is
an ARP Server for every LIS. The ARP Server contains the entire LIS
membership. These are fundamental ingredients for the broadcast
function.
A.1.1 Base Solution without modifications to ARP Server.
One may choose as an existing starting point to use only what is
available in RFC1577. That is, a host can easily calculate the
range of members in its LIS based on its own IP address and subnet
mask. The host can then issue an ARP Request for every member of the
LIS. With this information, the host can then set up point-to-point
connections with all members, or can set up a point-to-multipoint
connection to all members. There you have it, the poor man's
broadcast.
While this solution is very straight forward, it suffers from a
number of problems.
o The load on the ARP Server is very large. If all stations on
a LIS choose to implement broadcasting, the initial surge of ARP
Requests will be huge. Some sort of slow start sequence would be
needed.
o The amount of resource required makes this a non-scalable
solution. The authors believe that broadcasting will require an
MCS to reduce the number of channel resources required to support
each broadcast 'group'. Using the ARP Server in this manner does
not allow an MCS to be transparently introduced. (Basic RFC1577
interfaces also do not implement the extended LLC/SNAP
encapsulation required to safely use more than one MCS).
o The diskless boot solution can not function in this environment
because it may be unable to determine which subnet to which it
belongs.
A.1.2 Enhanced ARP Server solution.
This solution is similar to the base solution except that it takes
some of the (MARS) multicast solution and embeds it in the ARP
Server. The first enhancement is to add the MARS_MULTI command to
the set of opcodes that the ARP Server supports. This would allow a
host to issue a single request, and to get back the list of members
in one or more MARS_REPLY packets. Rather than have a registration
mechanism, the ARP Server could simply use the list of members that
have already been registered. When a request comes in for the subnet
broadcast address, the ARP Server would aggregate the list, and send
the results to the requester.
This suffers from two drawbacks.
1) Scalability with regard to number of VCs is still an issue.
One would eventually need to add in some sort of multicast
server solution to the ARP Server.
2) The diskless boot scenario is still broken. There is no
way for a station to perform a MARS_MULTI without first
knowing its IP address and subnet mask.
The diskless boot problem could be solved by adding to the ARP Server
a registration process where anyone could register to the
255.255.255.255 address. These changes would make the ARP Server
look more and more like MARS.
A.2 MARS Solutions.
If we wish to keep the ARP Server constant as described in RFC1577,
the alternative is to use the Multicast Address Resolution Server
(MARS) described in [2].
MARS has three nice features for broadcasting.
1) It has a generalized registration approach which allows
for any address to have a group of entities registered.
So, if the subnet address is not known, a host can
register for an address that is known (e.g. 255.255.255.255).
2) The command set allows for lists of members to be passed
in a single MARS_MULTI packet. This reduces traffic.
3) MARS contains an architecture for dealing with the
scalability issues. That is, Multicast Servers (MCSs)
may be used to set up the point-to-multipoint channels
and reduce the number of channels that a host needs to
set up to one. Hosts wishing to broadcast will instead
send the packet to the MCS who will then forward it to
all members of the LIS.
A.2.1. CIDR-prefix (Subnet) Broadcast solution.
One of the earliest solutions was to simply state that broadcast
support would be implemented by using a single multicast group in the
class D address space -- namely, the CIDR-prefix (subnet) broadcast
address group. All members of a LIS would be required to register to
this address, and use it as required. A host wishing to use either
the 255.255.255.255 broadcast, or the network broadcast addresses
would internally map the VC to the subnet broadcast VC. The all ones
and network broadcast addresses would exist on MARS, but would be
unused.
The problem with this approach goes back to the diskless workstation
problem. Because the workstation may not know which subnet it
belongs to, it doesn't know which group to register with.
A.2.2. All one's first, subnet broadcast second
This solution acknowledges that the diskless boot problem requires a
generic address (one that does not contain CIDR-prefix (subnet)
information) to register with and to use until subnet knowledge is
known. In essence, all stations first register to the
255.255.255.255 group, then as they know their subnet information,
they could optionally de-register from the all one's group and
register to the CIDR-prefix (subnet) broadcast group.
This solution would appear to solve a couple of problems:
1) The bootp client can function if the server remains
registered to the all one's group continuously.
2) There will be less traffic using the all ones group
because the preferred transactions will be on the
subnet broadcast channel.
Unfortunately the first bullet contains a flaw. The server must
continually be registered to two groups -- the all ones group and the
subnet broadcast group. If this server has multiple processes that
are running different IP applications, it may be difficult for the
link layer to know which broadcast VC to use. If it always uses the
all ones, then it will be missing members that have removed
themselves from the all ones and have registered to the subnet
broadcast. If it always uses the subnet broadcast group, the
diskless boot scenario gets broken. While making the decision at the
link layer may require additional control flows be built into the
path, it may also require the rewriting of application software.
In some implementations, a simple constant is used to indicate to the
link layer that this packet is to be transmitted to the broadcast
"MAC" address. The assumption is that the physical network broadcast
and the logical protocol broadcast are one and the same. As pointed
out earlier, this is not the case with ATM. Therefore applications
would need to specifically identify the subnet broadcast group
address to take advantage of the smaller group.
These problems could be solved in a number of ways, but it was
thought that they added unnecessarily to the complexity of the
broadcast solution.
Appendix B. Should MARS Be Limited to a Single LIS?
RFC2022 explicitly states that a network administrator MUST ensure
that each LIS is served by a separate MARS, creating a one-to-one
mapping between cluster and a unicast LIS. But, it also mentions
that relaxation of this restriction MAY occur after future research
warrants it. This appendix discusses some to the potential
implications to broadcast should this restriction be removed.
The most obvious change would be that the notion of a cluster would
span more than one LIS. Therefore, the broadcast group of
255.255.255.255 would contain members from more than one LIS.
It also should be emphasized that the one LIS limitation is not a
restriction of the MARS architecture. Rather, it is only enforced if
an administrator chooses to do so.
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