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Network Working Group G. White
INTERNET DRAFT Bay Networks, Inc
Expires 26 February 1998
<draft-ietf-ipcdn-ip-over-mcns-00.txt> 26 August 1997
Logical IP Subnetworks over MCNS Data Link Services
Status of this Memo
This document is an Internet-Draft. Internet Drafts are working
documents of the Internet Engineering Task Force (IETF), its areas,
and its working groups. Note that other groups may also distribute
working documents as Internet Drafts.
Internet-Drafts draft documents are valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
To learn the current status of any Internet-Draft, please check the
"1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
Directories on ftp.is.co.za (Africa), nic.nordu.net (Europe),
munnari.oz.au (Pacific Rim), ds.internic.net (US East Coast), or
ftp.isi.edu (US West Coast).
Work In Progress
This memo is work in progress in support of the activities of the IP
over Cable Data Networks (ipcdn) Working Group of the IETF.
Table of Contents
1. ABSTRACT . . . . . . . . . . . . . . . . . . . . . . . . . . 2
2. ACKNOWLEDGMENT . . . . . . . . . . . . . . . . . . . . . . . 2
3. CONVENTIONS . . . . . . . . . . . . . . . . . . . . . . . . . 2
4. INTRODUCTION . . . . . . . . . . . . . . . . . . . . . . . . 3
5. IP SUBNETWORK CONFIGURATION . . . . . . . . . . . . . . . . . 5
5.1 Background . . . . . . . . . . . . . . . . . . . . . . . . 5
5.2 LIS Configuration Requirements . . . . . . . . . . . . . . 5
5.3 LIS Router Additional Configuration . . . . . . . . . . . . 6
6. IP PACKET FORMAT . . . . . . . . . . . . . . . . . . . . . . 6
7. IP BROADCAST ADDRESS . . . . . . . . . . . . . . . . . . . . 7
8. IP MULTICAST ADDRESS . . . . . . . . . . . . . . . . . . . . 7
9. IP INTEGRATED SERVICES SUPPORT . . . . . . . . . . . . . . . 8
10. FILTERING . . . . . . . . . . . . . . . . . . . . . . . . . 9
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11 CM REQUIREMENTS . . . . . . . . . . . . . . . . . . . . . . 9
12. SECURITY . . . . . . . . . . . . . . . . . . . . . . . . . . 9
13. MIB SPECIFICATION . . . . . . . . . . . . . . . . . . . . . 10
14. OPEN ISSUES . . . . . . . . . . . . . . . . . . . . . . . . 10
15. REFERENCES . . . . . . . . . . . . . . . . . . . . . . . . . 10
16. AUTHOR . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
1. ABSTRACT
This memo defines an initial application of IP and ARP in an MCNS
Community Access Television (CATV) Residential Access Network
environment where the cable system is used for bi-directional data
transfer. The MCNS network provides a traditional Ethernet or 802.2
link layer service between a cable system head end and the customer
premises using the cable television system infrastructure. This
interface is available via IEEE 802.1D layer services to support IP
residential access networking services.
In this memo, the term Logical IP Subnetwork (LIS) is defined to
apply to traditional IP over Ethernet operating over MCNS services.
The recommendations in this draft rely on existing IETF standards for
IP and ARP over Broadcast Ethernet networks.
2. ACKNOWLEDGMENT
The author would like to thank the efforts of the MCNS working group
and the efforts of the IETF ipcdn working group. The basis of this
document was shamelessly stolen from Mark Laubach's draft for IP over
802.14. Wilson Sawyer of Bay Networks provided valuable early review
of the document.
3. CONVENTIONS
The following language conventions are used in the items of
specification in this document:
o MUST, SHALL, or MANDATORY -- the item is an absolute requirement
of the specification.
o SHOULD or RECOMMEND -- this item should generally be followed for
all but exceptional circumstances.
o MAY or OPTIONAL -- the item is truly optional and may be followed
or ignored according to the needs of the implementor.
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4. INTRODUCTION
The goal of this specification is to allow compatible and
interoperable implementations of Logical IP Subnetworks over MCNS
services [MCNS1->4], including the transmission of IP datagrams and
Address Resolution Protocol (ARP) requests and replies.
It refers only to an MCNS network based on a two way capable cable
plant [MCNS4] in which the cable system is used for data transport
in both upstream and downstream directions. MCNS networks using non
cable system based return paths such as the telephone network are not
considered.
This memo specifies the default operational model which will always
be available in IP over MCNS implementations. Subsequent memos will
build upon and refine this model, however, in the absence or failure
of those extensions, operations will default to the specifications
contained in this memo. Consequently, this memo will not reference
these other extensions.
The MCNS subnetwork consists of a Cable Modem Termination System
(CMTS) typically located at the cable system head end and one or more
cable modems (CM). The CMTS and each CM are MCNS entities. The CMTS
is responsible for all aspects of packet processing, resource
sharing, and management of the MCNS Media Access Control (MAC) and
Physical (PHY) functions. A cable modem is essentially a slave of
the CMTS.
The organization of the CMTS to each CM is a strict rooted hierarchy:
i.e., it is a two-level tree where the CMTS is the root and CM's are
the children. In the downstream direction, a MCNS MAC Protocol Data
Unit (PDU) may be sent to an individual CM (unicast) or a group of
CM's (multicast and broadcast). In the upstream direction, all MAC
PDUs (individual or group addressed) are sent from the CM to the
CMTS.
The CMTS is active and originates and terminates all upstream MAC PDU
flows; that is, the CMTS processes the MAC PDUs and does not merely
repeat upstream MAC PDUs back on the downstream for station to
station communication. The CMTS MAC layer service function
determines whether information will be forwarded back on the
downstream as defined in [MCNS4]. This may be based on data-link-
layer (bridging) semantics, network-layer (routing) semantics or some
combination of the two.
The specific format of the MCNS MAC PDU is transparent to higher
level services, e.g. IP datagrams, and therefore not of specific
interest to this draft. However, it is useful to note that MCNS CMTS
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and CM entities support a variable length MAC PDU which encapsulates
an Ethernet frame for transmission on the CATV network. The MCNS MAC
PDU is not presented to the IP layer. For the purposes of protocol
specification, IP may only access MCNS services via the 802.2 (LLC) /
802.1D (bridge) MAC frame interface, hereafter called the Ethernet
interface or Ethernet frame interface.
Note: the MCNS Ethernet interface provides
1) A frame based packet interface for the transmission of IP
datagrams and ARP packets via the MCNS link services.
2) An emulated Ethernet service between all CM's and the CMTS.
3) Subject to CMTS forwarding rules a bridged Ethernet service
between all CM's.
The MCNS system employs a link layer encryption mechanism to provide
basic data privacy. This is transparent to higher layer services.
Basic traffic management support and Quality of Service (QoS) support
is provided by the MCNS system. These mechanisms can be exploited to
provide for IP integrated services support.
The MCNS system specifications provide options to support
IEEE802.1D/p, and IEEE802.1Q Virtual LAN (VLAN) extensions. These
mechanisms may be used to facilitate support for IP integrated
services.
The characteristics for residential LISs using MCNS Ethernet frame
service interface are:
o Supports default IP and ARP over Ethernet services.
o Other IETF standards can be used to extend these services; e.g.
integrated services over 802.1D/p/Q.
o More than one LIS may be in operation over the same MCNS
subnetwork (MAC domain) .
o An MCNS host system may be a member of more than one LIS.
o Layer management services are available to the frame service
layer for the purposes of managing point-to-point services on the
downstream and upstream, and point-to-multipoint services on the
downstream.
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o Layer management services are available to the frame service
layer for traffic management and Quality of Service (QoS)
control.
The scope of this specification covers implementation,
interoperability, and operational extension issues for delivering
Logical IP Subnetwork services via a residential access network
implemented via the MCNS standard. Due to the flexibility provided
by the MCNS system features, other IETF standards will be relied on
when appropriate to do so.
For the purposes of this memo, the MCNS subnetwork is intended to
support residential Logical IP Subnetwork services. This
specification does not preclude the operation of other multiple non-
IP services which may be in simultaneous service over the MCNS
subnetwork: e.g., voice or video integrated services.
5. IP SUBNETWORK CONFIGURATION
5.1 Background
The MCNS subnetwork can support multiple simultaneously operating
disjoint LISs over the same MAC domain. For each LIS a separate
administrative entity configures its hosts and routers within the
LIS. Each LIS operates and communicates independently of other LISs
on the same MCNS network.
In the classical model, hosts communicate directly via MCNS to other
hosts within the same LIS using the appropriate address resolution
service. In this case the ARP service is the mechanism for resolving
target IP addresses to target 48-bit IEEE MAC addresses.
As LISs are independent, inter-LIS protocol translation or address
resolution conversion services are beyond the scope of this memo.
Communication to hosts located outside of a LIS is provided via an IP
router.
The scope of an Ethernet LIS can span beyond an individual MCNS
subnetwork using traditional frame-based bridging; e.g., IEEE 802.1D
transparent bridging services.
5.2 Residential LIS Configuration Requirements
The requirements for IP members (hosts, routers) operating in an
MCNS-based LIS are:
o All members of the LIS MUST have the same IP network/subnet
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number and address mask [IP4].
o All members are directly connected
to the MCNS subnetwork or to an IEEE 802.1 bridged network
communicating with the MCNS subnetwork.
o All members of a LIS MUST use ARP as the mechanism for
resolving IP addresses to link addresses.
o All LIS members connected to the MCNS subnetwork via an
MCNS CM MUST be able to communicate via the MCNS
subnetwork to or beyond the MCNS CMTS.
The MCNS CMTS may be configured to not forward upstream
communications from one station to another downstream station
in the LIS; in this case, an IP router attached to or
colocated with the CMTS should provide the forwarding from
upstream to downstream.
o All LIS members connected to the MCNS subnetwork via an
MCNS CMTS MUST be able to communicate via the MCNS
subnetwork to or beyond any downstream MCNS station in the
LIS.
o A LIS MAY span more than one MCNS subnetwork.
Conventional Layer 2 bridging/switching MAY
interconnect more than one CMTS.
5.3 LIS Router Additional Configuration
Routers providing LIS functionality over the MCNS subnetwork MAY also
support the ability to interconnect multiple LISs. Routers that wish
to provide interconnection of differing LISs MUST be able to support
multiple sets of parameters (one set for each connected LIS) and be
able to associate each set of parameters to a specific IP
network/subnet number. In addition, a router MAY be able to provide
this multiple LIS support with a single physical MCNS interface with
a different link address per LIS.
6. IP PACKET FORMAT
Implementations built using the MCNS data link layer services MUST
support IP over Ethernet as described in [IP1]. The MTU of this
interface is 1500 octets.
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7. IP BROADCAST ADDRESS
The MCNS downstream MAC PDU supports point-to-multipoint addressing.
For each LIS, the IP layer service support at the MCNS CMTS MUST
create a single downstream point-to-multipoint group whose membership
contains all MCNS CM's participating in that LIS. By default, all
downstream IP datagrams whose destination address specifies one of
the four forms of IP broadcast addresses mited, directed, subnet
directed, or all-subnets directed) [IP4] or an IP multicast address
MUST be sent to members of the LIS using this MAC address group.
Note: By default, all upstream IP datagrams are sent from the MCNS CM
to the CMTS on the single point-to-point connection.
Note: the above defaults do not preclude the use of additional
downstream point-to-point or point-to-multipoint, or additional
upstream point-to-point connections for handling of specific IP flows
for integrated-services or multicast distribution support; e.g.,
mapping IP flows to specific additional connections.
In general, it is preferred that downstream data bandwidth resources
be used in an efficient manner. Therefore, IP over MCNS
implementations SHOULD only send one copy of a packet downstream per
IP broadcast transmission or IP multicast transmission.
8. IP MULTICAST ADDRESS
The MCNS downstream MAC service supports point-to-multipoint
addressing. MAC point-to-multipoint addresses can span LISs.
For efficiency reasons, a separate point-to-multipoint group MAY be
used to support downstream IP multicast datagram distribution. The
specific implementation is beyond the scope of this memo, however it
can be noted that single or multiple IP multicast groups MAY be
mapped to a MAC point-to-multipoint group subject to the abilities of
the MCNS CMTS and participating CM's.
Note: By default, all upstream IP datagrams are sent from the MCNS CM
to the CMTS on the single point-to-point connection.
Note: the above defaults do not preclude the use of additional
downstream point-to-multipoint or additional upstream point-to-point
connections for handling of specific IP flows for integrated-services
or multicast distribution support; e.g., mapping IP flows to specific
additional connections.
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It is preferred that downstream data bandwidth resources be used in
an efficient manner, therefore IP over MCNS implementations MUST only
send one copy of a packet downstream per IP multicast transmission.
Specially, MAC point-to-multipoint groups used for IP multicast
datagram distribution may span LISs.
For example, there may be two LISs operating via an MCNS subnetwork,
LIS-1 and LIS-2. LIS1 may have station members ST-A, ST- B, and ST-
C. and LIS-2 may have station members ST-X, ST-Y, and ST-Z. The
Ethernet layer management services at the CMTS would have created two
point-to-multipoint groups PTM-1 and PTM-2 used for default
downstream broadcast and multicast transmission. The membership of
PTM-1 would be ST-A, ST-B, and ST-C. The membership of PTM-2 would
be ST-X, ST-Y, ST-Z. There may be another point-to-multipoint group
for distributing a specific IP multicast group, call this PTM-3. The
members of PTM-3 might be ST-B, ST-C, and ST-X therefore PTM-3 spans
LIS-1 and LIS-2.
The coupling of the MCNS layer management services responsible for
group management with that of IP Internet Group Management Protocol
(IGMP) is TBD.
9. IP INTEGRATED SERVICES SUPPORT
By default, the MCNS service delivers IP traffic on a best effort
basis.
The underlying protocol of MCNS is designed to support multiple
service classes with their associated Quality of Service requirements
(maximum data rate, guaranteed data rate, priority) subject to the
characteristics of the downstream and upstream channel rates.
Mappings from IP integrated services to IP over MCNS can be exploited
to provide traffic management and Quality of Service (QoS) on a per
IP flow basis, for unicast and multicast traffic. As such, these
capabilities are available for the support of integrated services and
RSVP over MCNS.
The MCNS MAC protocol contains the concept of Service IDs which
provide both device identification and quality-of-service management.
A Service ID defines a particular mapping between a CM and the CMTS.
This mapping is the basis on which bandwidth is allocated to the CM
by the CMTS and hence by which quality of service is implemented.
The CMTS MAY assign one or more Service IDs (SIDs) to each CM,
corresponding to the classes of service required by the CM. This
mapping MUST be negotiated between the CMTS and the CM during CM
registration.
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In a basic CM implementation a single Service ID MAY be used to offer
best-effort IP service. However, the Service ID concept allows for
CMs to be developed with support for multiple service classes. This
allows for a service ID to be provided for each "data flow" on which
protocols such as RSVP and RTP are based.
The mechanism to achieve this is TBD.
10. FILTERING
The MCNS system provides for the use of filtering for IP and ARP
protocol packets. This filtering is available for use by the network
operator or the end user (subject to the operator's policy).
The MCNS system permits filters to be placed in the upstream and
downstream at the CM and the CMTS and independently for point-to-
point and point-to-multipoint connections. In addition, filters may
be placed at the CMTS in the service function responsible for
forwarding packets from upstream to downstream.
11. CM REQUIREMENTS
The IP over MCNS cable modem MUST be able to separately and
simultaneously reassemble or reconstruct packets for each point-to-
point or point-to-multipoint downstream connection being used for IP
LIS or IP Multicast services.
By default, all unicast, broadcast, and multicast communications from
an IP over MCNS CM MUST be sent using the point-to-point connection
to the MCNS CMTS. It is noted that the default behavior MAY be
modified in the future by providing additional connections for IP
traffic from the CM to the CMTS.
12. SECURITY
The MCNS system employs a DES based link security system between the
CMTS and all CM's to protect the confidentiality of communications
over the RF channels. The specific mechanisms are beyond the scope
of this memo, however it should be noted that
1) the security system is transparent to any higher layer protocol
(including IP).
2) the security system does not preclude the use of IPSEC methods for
providing additional security.
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3) each MAC point-to-point connection is managed using different keys
making it difficult to snoop on another station's unicast MAC
traffic.
4) each MAC point-to-multipoint connection is managed using different
keys (stations only have keys for the MAC multicast groups of which
they are a member).
13. MIB SPECIFICATION
TBD.
14. OPEN ISSUES
o IEEE 802.1D/p and Q extensions, including GARP will be mentioned
in a future revision of this memo.
o RSVP coupling to MCNS service id is TBD.
o IGMP coupling to MCNS layer management is TBD.
15. REFERENCES
[MCNS1] MCNS Data Over Cable Service Interface Specification
Request for Proposals, December 11, 1995.
[MCNS2] MCNS Cable Modem Termination System
- Network-Side Interface Specification
SP-CMTS-NSID04-960409 (CMTS-NSI), April 9, 1996.
[MCNS3] MCNS Cable Modem to Customer Premise Equipment Interface
Specification SP-CMCID04-960409 (CMCI), April 9, 1996
[MCNS4] MCNS Radio Frequency Interface
Specification SP-RFII01-970326, March 26, 1997
[IP1] Hornig, C.., "A Standard for the Transmission for IP Datagrams
over Ethernet Networks", RFC-894, Symbolics Cambridge Research
Center, April, 1984.
[IP2] Plummer, D., "An Ethernet Address Resolution Protocol - or -
Converting Network Addresses to 48.bit Ethernet Address for
Transmission on Ethernet Hardware",
RFC-826, MIT, November 1982.
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[IP3] Postel, J., and Reynolds, J., "A Standard for the Transmission
of IP Datagrams over IEEE 802 Networks", RFC-1042,
USC/Information Sciences Institute, February 1988.
[IP4] Braden, R., "Requirements for Internet Hosts -- Communication
Layers", RFC-1122, USC/Information Sciences Institute, October
1989.
[IP5] Deering, S, "Host Extensions for IP Multicasting", RFC-1112,
USC/Information Sciences Institute, August 1989.
16. AUTHOR
Gerry White
Broadband Technology Division
Bay Networks, Inc.
200 Bulfinch Drive
Andover, MA 01810
Phone: 508.692.1600
FAX: 508.692.3200
EMail: gwhite@baynetworks.com
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