分享
 
 
 

RFC1932 - IP over ATM: A Framework Document

王朝other·作者佚名  2008-05-31
窄屏简体版  字體: |||超大  

Network Working Group R. Cole

Request for Comments: 1932 D. Shur

Category: Informational AT&T Bell Laboratories

C. Villamizar

ANS

April 1996

IP over ATM: A Framework Document

Status of this Memo

This memo provides information for the Internet community. This memo

does not specify an Internet standard of any kind. Distribution of

this memo is unlimited.

Abstract

The discussions of the IP over ATM working group over the last

several years have prodUCed a diverse set of proposals, some of which

are no longer under active consideration. A categorization is

provided for the purpose of focusing discussion on the various

proposals for IP over ATM deemed of primary interest by the IP over

ATM working group. The intent of this framework is to help clarify

the differences between proposals and identify common features in

order to promote convergence to a smaller and more mutually

compatible set of standards. In summary, it is hoped that this

document, in classifying ATM approaches and issues will help to focus

the IP over ATM working group's direction.

1. Introduction

The IP over ATM Working Group of the Internet Engineering Task Force

(IETF) is chartered to develop standards for routing and forwarding

IP packets over ATM sub-networks. This document provides a

classification/taxonomy of IP over ATM options and issues and then

describes various proposals in these terms.

The remainder of this memorandum is organized as follows:

o Section 2 defines several terms relating to networking and

internetworking.

o Section 3 discusses the parameters for a taxonomy of the

different ATM models under discussion.

o Section 4 discusses the options for low level encapsulation.

o Section 5 discusses tradeoffs between connection oriented and

connectionless approaches.

o Section 6 discusses the various means of providing direct

connections across IP subnet boundaries.

o Section 7 discusses the proposal to extend IP routing to better

accommodate direct connections across IP subnet boundaries.

o Section 8 identifies several prominent IP over ATM proposals that

have been discussed within the IP over ATM Working Group and

their relationship to the framework described in this document.

o Section 9 addresses the relationship between the documents

developed in the IP over ATM and related working groups and the

various models discussed.

2. Definitions and Terminology

We define several terms:

A Host or End System: A host delivers/receives IP packets to/from

other systems, but does not relay IP packets.

A Router or Intermediate System: A router delivers/receives IP

packets to/from other systems, and relays IP packets among

systems.

IP Subnet: In an IP subnet, all members of the subnet are able to

transmit packets to all other members of the subnet directly,

without forwarding by intermediate entities. No two subnet

members are considered closer in the IP topology than any other.

From an IP routing and IP forwarding standpoint a subnet is

atomic, though there may be repeaters, hubs, bridges, or switches

between the physical interfaces of subnet members.

Bridged IP Subnet: A bridged IP subnet is one in which two or

more physically disjoint media are made to appear as a single IP

subnet. There are two basic types of bridging, media Access

control (MAC) level, and proxy ARP (see section 6).

A Broadcast Subnet: A broadcast network supports an arbitrary

number of hosts and routers and additionally is capable of

transmitting a single IP packet to all of these systems.

A Multicast Capable Subnet: A multicast capable subnet supports

a facility to send a packet which reaches a subset of the

destinations on the subnet. Multicast setup may be sender

initiated, or leaf initiated. ATM UNI 3.0 [4] and UNI 3.1

support only sender initiated while IP supports leaf initiated

join. UNI 4.0 will support leaf initiated join.

A Non-Broadcast Multiple Access (NBMA) Subnet: An NBMA supports

an arbitrary number of hosts and routers but does not

natively support a convenient multi-destination connectionless

transmission facility, as does a broadcast or multicast capable

subnetwork.

An End-to-End path: An end-to-end path consists of two hosts which

can communicate with one another over an arbitrary number of

routers and subnets.

An internetwork: An internetwork (small "i") is the concatenation

of networks, often of various different media and lower level

encapsulations, to form an integrated larger network supporting

communication between any of the hosts on any of the component

networks. The Internet (big "I") is a specific well known

global concatenation of (over 40,000 at the time of writing)

component networks.

IP forwarding: IP forwarding is the process of receiving a packet

and using a very low overhead decision process determining how

to handle the packet. The packet may be delivered locally

(for example, management traffic) or forwarded externally. For

traffic that is forwarded externally, the IP forwarding process

also determines which interface the packet should be sent out on,

and if necessary, either removes one media layer encapsulation

and replaces it with another, or modifies certain fields in the

media layer encapsulation.

IP routing: IP routing is the exchange of information that takes

place in order to have available the information necessary to

make a correct IP forwarding decision.

IP address resolution: A quasi-static mapping exists between IP

address on the local IP subnet and media address on the local

subnet. This mapping is known as IP address resolution.

An address resolution protocol (ARP) is a protocol supporting

address resolution.

In order to support end-to-end connectivity, two techniques are used.

One involves allowing direct connectivity across classic IP subnet

boundaries supported by certain NBMA media, which includes ATM. The

other involves IP routing and IP forwarding. In essence, the former

technique is extending IP address resolution beyond the boundaries of

the IP subnet, while the latter is interconnecting IP subnets.

Large internetworks, and in particular the Internet, are unlikely to

be composed of a single media, or a star topology, with a single

media at the center. Within a large network supporting a common

media, typically any large NBMA such as ATM, IP routing and IP

forwarding must always be accommodated if the internetwork is larger

than the NBMA, particularly if there are multiple points of

interconnection with the NBMA and/or redundant, diverse

interconnections.

Routing information exchange in a very large internetwork can be

quite dynamic due to the high probability that some network elements

are changing state. The address resolution space consumption and

resource consumption due to state change, or maintenance of state

information is rarely a problem in classic IP subnets. It can become

a problem in large bridged networks or in proposals that attempt to

extend address resolution beyond the IP subnet. Scaling properties

of address resolution and routing proposals, with respect to state

information and state change, must be considered.

3. Parameters Common to IP Over ATM Proposals

In some discussion of IP over ATM distinctions have made between

local area networks (LANs), and wide area networks (WANs) that do not

necessarily hold. The distinction between a LAN, MAN and WAN is a

matter of geographic dispersion. Geographic dispersion affects

performance due to increased propagation delay.

LANs are used for network interconnections at the the major Internet

traffic interconnect sites. Such LANs have multiple administrative

authorities, currently exclusively support routers providing transit

to multihomed internets, currently rely on PVCs and static address

resolution, and rely heavily on IP routing. Such a configuration

differs from the typical LANs used to interconnect computers in

corporate or campus environments, and emphasizes the point that prior

characterization of LANs do not necessarily hold. Similarly, WANs

such as those under consideration by numerous large IP providers, do

not conform to prior characterizations of ATM WANs in that they have

a single administrative authority and a small number of nodes

aggregating large flows of traffic onto single PVCs and rely on IP

routers to avoid forming congestion bottlenecks within ATM.

The following characteristics of the IP over ATM internetwork may be

independent of geographic dispersion (LAN, MAN, or WAN).

o The size of the IP over ATM internetwork (number of nodes).

o The size of ATM IP subnets (LIS) in the ATM Internetwork.

o Single IP subnet vs multiple IP subnet ATM internetworks.

o Single or multiple administrative authority.

o Presence of routers providing transit to multihomed internets.

o The presence or absence of dynamic address resolution.

o The presence or absence of an IP routing protocol.

IP over ATM should therefore be characterized by:

o Encapsulations below the IP level.

o Degree to which a connection oriented lower level is available

and utilized.

o Type of address resolution at the IP subnet level (static or

dynamic).

o Degree to which address resolution is extended beyond the IP

subnet boundary.

o The type of routing (if any) supported above the IP level.

ATM-specific attributes of particular importance include:

o The different types of services provided by the ATM Adaptation

Layers (AAL). These specify the Quality-of-Service, the

connection-mode, etc. The models discussed within this document

assume an underlying connection-oriented service.

o The type of virtual circuits used, i.e., PVCs versus SVCs. The

PVC environment requires the use of either static tables for

ATM-to-IP address mapping or the use of inverse ARP, while the

SVC environment requires ARP functionality to be provided.

o The type of support for multicast services. If point-to-point

services only are available, then a server for IP multicast is

required. If point-to-multipoint services are available, then

IP multicast can be supported via meshes of point-to-multipoint

connections (although use of a server may be necessary due to

limits on the number of multipoint VCs able to be supported or to

maintain the leaf initiated join semantics).

o The presence of logical link identifiers (VPI/VCIs) and the

various information element (IE) encodings within the ATM SVC

signaling specification, i.e., the ATM Forum UNI version 3.1.

This allows a VC originator to specify a range of "layer"

entities as the destination "AAL User". The AAL specifications

do not prohibit any particular "layer X" from attaching

directly to a local AAL service. Taken together these points

imply a range of methods for encapsulation of upper layer

protocols over ATM. For example, while LLC/SNAP encapsulation is

one approach (the default), it is also possible to bind virtual

circuits to higher level entities in the TCP/IP protocol stack.

Some examples of the latter are single VC per protocol binding,

TULIP, and TUNIC, discussed further in Section 4.

o The number and type of ATM administrative domains/networks, and

type of addressing used within an administrative domain/network.

In particular, in the single domain/network case, all attached

systems may be safely assumed to be using a single common

addressing format, while in the multiple domain case, attached

stations may not all be using the same common format,

with corresponding implications on address resolution. (See

Appendix A for a discussion of some of the issues that arise

when multiple ATM address formats are used in the same logical

IP subnet (LIS).) Also security/authentication is much more of a

concern in the multiple domain case.

IP over ATM proposals do not universally accept that IP routing over

an ATM network is required. Certain proposals rely on the following

assumptions:

o The widespread deployment of ATM within premises-based networks,

private wide-area networks and public networks, and

o The definition of interfaces, signaling and routing protocols

among private ATM networks.

The above assumptions amount to ubiquitous deployment of a seamless

ATM fabric which serves as the hub of a star topology around which

all other media is attached. There has been a great deal of

discussion over when, if ever, this will be a realistic assumption

for very large internetworks, such as the Internet. Advocates of

such approaches point out that even if these are not relevant to very

large internetworks such as the Internet, there may be a place for

such models in smaller internetworks, such as corporate networks.

The NHRP protocol (Section 8.2), not necessarily specific to ATM,

would be particularly appropriate for the case of ubiquitous ATM

deployment. NHRP supports the establishment of direct connections

across IP subnets in the ATM domain. The use of NHRP does not

require ubiquitous ATM deployment, but currently imposes topology

constraints to avoid routing loops (see Section 7). Section 8.2

describes NHRP in greater detail.

The Peer Model assumes that internetwork layer addresses can be

mapped onto ATM addresses and vice versa, and that reachability

information between ATM routing and internetwork layer routing can be

exchanged. This approach has limited applicability unless ubiquitous

deployment of ATM holds. The peer model is described in Section 8.4.

The Integrated Model proposes a routing solution supporting an

exchange of routing information between ATM routing and higher level

routing. This provides timely external routing information within

the ATM routing and provides transit of external routing information

through the ATM routing between external routing domains. Such

proposals may better support a possibly lengthy transition during

which assumptions of ubiquitous ATM access do not hold. The

Integrated Model is described in Section 8.5.

The Multiprotocol over ATM (MPOA) Sub-Working Group was formed by the

ATM Forum to provide multiprotocol support over ATM. The MPOA effort

is at an early stage at the time of this writing. An MPOA baseline

document has been drafted, which provides terminology for further

discussion of the architecture. This document is available from the

FTP server ftp.atmforum.com in pub/contributions as the file atm95-

0824.ps or atm95-0824.txt.

4. Encapsulations and Lower Layer Identification

Data encapsulation, and the identification of VC endpoints,

constitute two important issues that are somewhat orthogonal to the

issues of network topology and routing. The relationship between

these two issues is also a potential sources of confusion. In

conventional LAN technologies the 'encapsulation' wrapped around a

packet of data typically defines the (de)multiplexing path within

source and destination nodes (e.g. the Ethertype field of an

Ethernet packet). Choice of the protocol endpoint within the

packet's destination node is essentially carried 'in-band'.

As the multiplexing is pushed towards ATM and away from LLC/SNAP

mechanism, a greater burden will be placed upon the call setup and

teardown capacity of the ATM network. This may result in some

questions being raised regarding the scalability of these lower level

multiplexing options.

With the ATM Forum UNI version 3.1 service the choice of endpoint

within a destination node is made 'out of band' - during the Call

Setup phase. This is quite independent of any in-band encapsulation

mechanisms that may be in use. The B-LLI Information Element allows

Layer 2 or Layer 3 entities to be specified as a VC's endpoint. When

faced with an incoming SETUP message the Called Party will search

locally for an AAL User that claims to provide the service of the

layer specified in the B-LLI. If one is found then the VC will be

accepted (assuming other conditions such as QoS requirements are also

met).

An obvious approach for IP environments is to simply specify the

Internet Protocol layer as the VCs endpoint, and place IP packets

into AAL--SDUs for transmission. This is termed 'VC multiplexing' or

'Null Encapsulation', because it involves terminating a VC (through

an AAL instance) directly on a layer 3 endpoint. However, this

approach has limitations in environments that need to support

multiple layer 3 protocols between the same two ATM level endpoints.

Each pair of layer 3 protocol entities that wish to exchange packets

require their own VC.

RFC-1483 [6] notes that VC multiplexing is possible, but focuses on

describing an alternative termed 'LLC/SNAP Encapsulation'. This

allows any set of protocols that may be uniquely identified by an

LLC/SNAP header to be multiplexed onto a single VC. Figure 1 shows

how this works for IP packets - the first 3 bytes indicate that the

payload is a Routed Non-ISO PDU, and the Organizationally Unique

Identifier (OUI) of 0x00-00-00 indicates that the Protocol Identifier

(PID) is derived from the EtherType associated with IP packets

(0x800). ARP packets are multiplexed onto a VC by using a PID of

0x806 instead of 0x800.

.---------------.

: :

: IP Packet :

: :

---------------

: :

: :

8 byte header V V

.-------------.-------------.------------.---------------.

: : : : :

: : : : Encapsulated :

: 0xAA-AA-03 : 0x00-00-00 : 0x08-00 : Payload :

: : : : :

-------------^-------------^------------^---------------

: : : :

: (LLC) (OUI) (PID) : : :

V V V V

.----------------------------------------------------------.

: :

: AAL SDU :

: :

----------------------------------------------------------

Figure 1: IP packet encapsulated in an AAL5 SDU

.----------. .----------. .---------. .----------.

: : : : : : : :

: IP : : ARP : :AppleTalk: : etc... :

: : : : : : : :

---------- ---------- --------- ----------

^ : ^ : ^ : ^ :

: : : : : : : :

: V : V : V : V

.-----------------------------------------------------------.

: :

: 0x800 0x806 0x809 other :

: :

: Instance of layer using LLC/SNAP header to :

: perform multiplexing/demultiplexing :

: :

-----------------------------------------------------------

^ :

: :

: V

.------------------.

: :

: Instance of AAL5 :

: terminating :

: one VCC :

: :

------------------

Figure 2: LLC/SNAP encapsulation allows more than just

IP or ARP per VC.

Whatever layer terminates a VC carrying LLC/SNAP encapsulated traffic

must know how to parse the AAL--SDUs in order to retrieve the

packets. The recently approved signalling standards for IP over ATM

are more eXPlicit, noting that the default SETUP message used to

establish IP over ATM VCs must carry a B-LLI specifying an ISO 8802/2

Layer 2 (LLC) entity as each VCs endpoint. More significantly, there

is no information carried within the SETUP message about the identity

of the layer 3 protocol that originated the request - until the

packets begin arriving the terminating LLC entity cannot know which

one or more higher layers are packet destinations.

Taken together, this means that hosts require a protocol entity to

register with the host's local UNI 3.1 management layer as being an

LLC entity, and this same entity must know how to handle and generate

LLC/SNAP encapsulated packets. The LLC entity will also require

mechanisms for attaching to higher layer protocols such as IP and

ARP. Figure 2 attempts to show this, and also highlights the fact

that such an LLC entity might support many more than just IP and ARP.

In fact the combination of RFC1483 LLC/SNAP encapsulation, LLC

entities terminating VCs, and suitable choice of LLC/SNAP values, can

go a long way towards providing an integrated approach to building

multiprotocol networks over ATM.

The processes of actually establishing AAL Users, and identifying

them to the local UNI 3.1 management layers, are still undefined and

are likely to be very dependent on operating system environments.

Two encapsulations have been discussed within the IP over ATM working

group which differ from those given in RFC-1483 [6]. These have the

characteristic of largely or totally eliminating IP header overhead.

These models were discussed in the July 1993 IETF meeting in

Amsterdam, but have not been fully defined by the working group.

TULIP and TUNIC assume single hop reachability between IP entities.

Following name resolution, address resolution, and SVC signaling, an

implicit binding is established between entities in the two hosts.

In this case full IP headers (and in particular source and

destination addresses) are not required in each data packet.

o The first model is "TCP and UDP over Lightweight IP" (TULIP)

in which only the IP protocol field is carried in each packet,

everything else being bound at call set-up time. In this

case the implicit binding is between the IP entities in each

host. Since there is no further routing problem once the binding

is established, since AAL5 can indicate packet size, since

fragmentation cannot occur, and since ATM signaling will handle

exception conditions, the absence of all other IP header fields

and of ICMP should not be an issue. Entry to TULIP mode would

occur as the last stage in SVC signaling, by a simple extension

to the encapsulation negotiation described in RFC-1755 [10].

TULIP changes nothing in the abstract architecture of the IP

model, since each host or router still has an IP address which is

resolved to an ATM address. It simply uses the point-to-point

property of VCs to allow the elimination of some per-packet

overhead. The use of TULIP could in principle be negotiated on a

per-SVC basis or configured on a per-PVC basis.

o The second model is "TCP and UDP over a Nonexistent IP

Connection" (TUNIC). In this case no network-layer information

is carried in each packet, everything being bound at virtual

circuit set-up time. The implicit binding is between two

applications using either TCP or UDP directly over AAL5 on a

dedicated VC. If this can be achieved, the IP protocol field has

no useful dynamic function. However, in order to achieve binding

between two applications, the use of a well-known port number

in classical IP or in TULIP mode may be necessary during call

set-up. This is a subject for further study and would require

significant extensions to the use of SVC signaling described in

RFC-1755 [10].

Encapsulation In setup message Demultiplexing

-------------+--------------------------+------------------------

SNAP/LLC _ nothing _ source and destination

_ _ address, protocol

_ _ family, protocol, ports

_ _

NULL encaps _ protocol family _ source and destination

_ _ address, protocol, ports

_ _

TULIP _ source and destination _ protocol, ports

_ address, protocol family _

_ _

TUNIC - A _ source and destination _ ports

_ address, protocol family _

_ protocol _

_ _

TUNIC - B _ source and destination _ nothing

_ address, protocol family _

_ protocol, ports _

Table 1: Summary of Encapsulation Types

TULIP/TUNIC can be presented as being on one end of a continuum opposite

the SNAP/LLC encapsulation, with various forms of null encapsulation

somewhere in the middle. The continuum is simply a matter of how much

is moved from in-stream demultiplexing to call setup demultiplexing.

The various encapsulation types are presented in Table 1.

Encapsulations such as TULIP and TUNIC make assumptions with regard to

the desirability to support connection oriented flow. The tradeoffs

between connection oriented and connectionless are discussed in Section

5.

5. Connection Oriented and Connectionless Tradeoffs

The connection oriented and connectionless approaches each offer

advantages and disadvantages. In the past, strong advocates of pure

connection oriented and pure connectionless architectures have argued

intensely. IP over ATM does not need to be purely connectionless or

purely connection oriented.

APPLICATION Pure Connection Oriented Approach

----------------+-------------------------------------------------

General _ Always set up a VC

_

Short Duration _ Set up a VC. Either hold the packet during VC

UDP (DNS) _ setup or drop it and await a retransmission.

_ Teardown on a timer basis.

_

Short Duration _ Set up a VC. Either hold packet(s) during VC

TCP (SMTP) _ setup or drop them and await retransmission.

_ Teardown on detection of FIN-ACK or on a timer

_ basis.

_

Elastic (TCP) _ Set up a VC same as above. No clear method to

Bulk Transfer _ set QoS parameters has emerged.

_

Real Time _ Set up a VC. QoS parameters are assumed to

(audio, video) _ precede traffic in RSVP or be carried in some

_ form within the traffic itself.

Table 2: Connection Oriented vs. Connectionless - a) a pure

connection oriented approach

ATM with basic AAL 5 service is connection oriented. The IP layer

above ATM is connectionless. On top of IP much of the traffic is

supported by TCP, a reliable end-to-end connection oriented protocol.

A fundamental question is to what degree is it beneficial to map

different flows above IP into separate connections below IP. There is

a broad spectrum of opinion on this.

As stated in section 4, at one end of the spectrum, IP would remain

highly connectionless and set up single VCs between routers which are

adjacent on an IP subnet and for which there was active traffic flow.

All traffic between the such routers would be multiplexed on a single

ATM VC. At the other end of the spectrum, a separate ATM VC would be

created for each identifiable flow. For every unique TCP or UDP

address and port pair encountered a new VC would be required. Part of

the intensity of early arguments has been over failure to recognize

that there is a middle ground.

ATM offers QoS and traffic management capabilities that are well

suited for certain types of services. It may be advantageous to use

separate ATM VC for such services. Other IP services such as DNS, are

ill suited for connection oriented delivery, due to their normal very

short duration (typically one packet in each direction). Short

duration transactions, even many using TCP, may also be poorly suited

for a connection oriented model due to setup and state overhead. ATM

QoS and traffic management capabilities may be poorly suited for

elastic traffic.

APPLICATION Middle Ground

----------------+-------------------------------------------------

General _ Use RSVP or other indication which clearly

_ indicate a VC is needed and what QoS parameters

_ are appropriate.

_

Short Duration _ Forward hop by hop. RSVP is unlikely to precede

UDP (DNS) _ this type of traffic.

_

Short Duration _ Forward hop by hop unless RSVP indicates

TCP (SMTP) _ otherwise. RSVP is unlikely to precede this

_ type of traffic.

_

Elastic (TCP) _ By default hop by hop forwarding is used.

Bulk Transfer _ However, RSVP information, local configuration

_ about TCP port number usage, or a locally

_ implemented method for passing QoS information

_ from the application to the IP/ATM driver may

_ allow/suggest the establishment of direct VCs.

_

Real Time _ Forward hop by hop unless RSVP indicates

(audio, video) _ otherwise. RSVP will indicate QoS requirements.

_ It is assumed RSVP will generally be used for

_ this case. A local decision can be made as to

_ whether the QoS is better served by a separate

_ VC.

Table 3: Connection Oriented vs. Connectionless - b) a middle ground

approach

APPLICATION Pure Connectionless Approach

----------------+-------------------------------------------------

General _ Always forward hop by hop. Use queueing

_ algorithms implemented at the IP layer to

_ support reservations such as those specified by

_ RSVP.

_

Short Duration _ Forward hop by hop.

UDP (DNS) _

_

Short Duration _ Forward hop by hop.

TCP (SMTP) _

_

Elastic (TCP) _ Forward hop by hop. Assume ability of TCP to

Bulk Transfer _ share bandwidth (within a VBR VC) works as well

_ or better than ATM traffic management.

_

Real Time _ Forward hop by hop. Assume that queueing

(audio, video) _ algorithms at the IP level can be designed to

_ work with sufficiently good performance

_ (e.g., due to support for predictive

_ reservation).

Table 4: Connection Oriented vs. Connectionless - c) a pure

connectionless approach

Work in progress is addressing how QoS requirements might be

expressed and how the local decisions might be made as to whether

those requirements are best and/or most cost effectively accomplished

using ATM or IP capabilities. Table 2, Table 3, and Table 4 describe

typical treatment of various types of traffic using a pure connection

oriented approach, middle ground approach, and pure connectionless

approach.

The above qualitative description of connection oriented vs

connectionless service serve only as examples to illustrate differing

approaches. Work in the area of an integrated service model, QoS and

resource reservation are related to but outside the scope of the IP

over ATM Work Group. This work falls under the Integrated Services

Work Group (int-serv) and Reservation Protocol Work Group (rsvp), and

will ultimately determine when direct connections will be

established. The IP over ATM Work Group can make more rapid progress

if concentrating solely on how direct connections are established.

6. Crossing IP Subnet Boundaries

A single IP subnet will not scale well to a large size. Techniques

which extend the size of an IP subnet in other media include MAC

layer bridging, and proxy ARP bridging.

MAC layer bridging alone does not scale well. Protocols such as ARP

rely on the media broadcast to exchange address resolution

information. Most bridges improve scaling characteristics by

capturing ARP packets and retaining the content, and distributing the

information among bridging peers. The ARP information gathered from

ARP replies is broadcast only where explicit ARP requests are made.

This technique is known as proxy ARP.

Proxy ARP bridging improves scaling by reducing broadcast traffic,

but still suffers scaling problems. If the bridged IP subnet is part

of a larger internetwork, a routing protocol is required to indicate

what destinations are beyond the IP subnet unless a statically

configured default route is used. A default route is only applicable

to a very simple topology with respect to the larger internet and

creates a single point of failure. Because internets of enormous

size create scaling problems for routing protocols, the component

networks of such large internets are often partitioned into areas,

autonomous systems or routing domains, and routing confederacies.

The scaling limits of the simple IP subnet require a large network to

be partitioned into smaller IP subnets. For NBMA media like ATM,

there are advantages to creating direct connections across the entire

underlying NBMA network. This leads to the need to create direct

connections across IP subnet boundaries.

.----------.

---------< Non-ATM :

.-------. / /-< Subnet >- :Sub-ES >--/ : ---------- :

------- : :

: :

.--^---. .--^---.

:Router: :Router:

-v-v-- -v-v--

: : : :

.--------. : : .--------. : : .--------.

.-------. : >-/ \-< >-/ \-< : .-------.

:Sub-ES :---: Subnet :-----: Subnet :-----: Subnet :---:Sub-ES :

------- : : : : : : -------

-------- ---v---- --------

:

.--^----.

:Sub-ES :

-------

Figure 3: A configuration with both ATM-based and non-ATM based

subnets.

For example, figure 3 shows an end-to-end configuration consisting of

four components, three of which are ATM technology based, while the

fourth is a standard IP subnet based on non-ATM technology. End-

systems (either hosts or routers) attached to the ATM-based networks

may communicate either using the Classical IP model or directly via

ATM (subject to policy constraints). Such nodes may communicate

directly at the IP level without necessarily needing an intermediate

router, even if end-systems do not share a common IP-level network

prefix. Communication with end-systems on the non-ATM-based

Classical IP subnet takes place via a router, following the Classical

IP model (see Section 8.1 below).

Many of the problems and issues associated with creating such direct

connections across subnet boundaries were originally being addressed

in the IETF's IPLPDN working group and the IP over ATM working group.

This area is now being addressed in the Routing over Large Clouds

working group. Examples of work performed in the IPLPDN working

group include short-cut routing (proposed by P. Tsuchiya) and

directed ARP RFC-1433 [5] over SMDS networks. The ROLC working group

has produced the distributed ARP server architectures and the NBMA

Address Resolution Protocol (NARP) [7]. The Next Hop Resolution

Protocol (NHRP) is still work in progress, though the ROLC WG is

considering advancing the current document. Questions/issues

specifically related to defining a capability to cross IP subnet

boundaries include:

o How can routing be optimized across multiple logical IP subnets

over both a common ATM based and a non-ATM based infrastructure.

For example, in Figure 3, there are two gateways/routers between

the non-ATM subnet and the ATM subnets. The optimal path

from end-systems on any ATM-based subnet to the non ATM-based

subnet is a function of the routing state information of the two

routers.

o How to incorporate policy routing constraints.

o What is the proper coupling between routing and address

resolution particularly with respect to off-subnet communication.

o What are the local procedures to be followed by hosts and

routers.

o Routing between hosts not sharing a common IP-level (or L3)

network prefix, but able to be directly connected at the NBMA

media level.

o Defining the details for an efficient address resolution

architecture including defining the procedures to be followed by

clients and servers (see RFC-1433 [5], RFC-1735 [7] and NHRP).

o How to identify the need for and accommodate special purpose SVCs

for control or routing and high bandwidth data transfers.

For ATM (unlike other NBMA media), an additional complexity in

supporting IP routing over these ATM internets lies in the

multiplicity of address formats in UNI 3.0 [4]. NSAP modeled address

formats only are supported on "private ATM" networks, while either 1)

E.164 only, 2) NSAP modeled formats only, or 3) both are supported on

"public ATM" networks. Further, while both the E.164 and NSAP

modeled address formats are to be considered as network points of

attachment, it seems that E.164 only networks are to be considered as

subordinate to "private networks", in some sense. This leads to some

confusion in defining an ARP mechanism in supporting all combinations

of end-to-end scenarios (refer to the discussion in Appendix A on the

possible scenarios to be supported by ARP).

7. Extensions to IP Routing

RFC-1620 [3] describes the problems and issues associated with direct

connections across IP subnet boundaries in greater detail, as well as

possible solution approaches. The ROLC WG has identified persistent

routing loop problems that can occur if protocols which lose

information critical to path vector routing protocol loop suppression

are used to accomplish direct connections across IP subnet

boundaries.

The problems may arise when a destination network which is not on the

NBMA network is reachable via different routers attached to the NBMA

network. This problem occurs with proposals that attempt to carry

reachability information, but do not carry full path attributes (for

path vector routing) needed for inter-AS path suppression, or full

metrics (for distance vector or link state routing even if path

vector routing is not used) for intra-AS routing.

For example, the NHRP protocol may be used to support the

establishment of direct connections across subnetwork boundaries.

NHRP assumes that routers do run routing protocols (intra and/or

inter domain) and/or static routing. NHRP further assumes that

forwarding tables constructed by these protocols result in a steady

state loop-free forwarding. Note that these two assumptions do not

impose any additional requirements on routers, beyond what is

required in the absence of NHRP.

NHRP runs in addition to routing protocols, and provides the

information that allows the elimination of multiple IP hops (the

multiple IP hops result from the forwarding tables constructed by the

routing protocols) when traversing an NBMA network. The IPATM and

ROLC WGs have both expended considerable effort in discussing and

coming to understand these limitations.

It is well-known that truncating path information in Path Vector

protocols (e.g., BGP) or losing metric information in Distance Vector

protocols (e.g., RIP) could result in persistent forwarding loops.

These loops could occur without ATM and without NHRP.

The combination of NHRP and static routing alone cannot be used in

some topologies where some of the destinations are served by multiple

routers on the NBMA. The combination of NHRP and an intra-AS routing

protocol that does not carry inter-AS routing path attributes alone

cannot be used in some topologies in which the NBMA will provide

inter-AS transit connectivity to destinations from other AS served by

multiple routers on the NBMA.

Figure 4 provides an example of the routing loops that may be formed

in these circumstances. The example illustrates how the use of NHRP

in the environment where forwarding loops could exist even without

NHRP (due to either truncated path information or loss of metric

information) would still produce forwarding loops.

There are many potential scenarios for routing loops. An example is

given in Figure 4. It is possible to produce a simpler example where

a loop can form. The example in Figure 4 illustrates a loop which

will persist even if the protocol on the NBMA supports redirects or

can invalidate any route which changes in any way, but does not

support the communication of full metrics or path attributes.

.----. .----.

: H1 >----< S1 : Notes:

---- vvvv H#n == host #n

/ : \ R#n == router #n

/ : \ S#n == subnet #n

/------/ : : : \ S2 to R3 breaks

.--^---. .----. .-^--.

: : : R4 : : R6 :

: NBMA : --v- --v- See the text for

: : : : details of the

-v--v- = = looping conditions

: \ = SLOW = and mechanisms

: .-^--. = LINK =

: : R2 : = =

: --v- : :

: : .--^-. .--^-.

.-^--. : : R5 : : R7 :

: R8 : : --v- --v-

--v- \ : :

: \ / :

\ .-^^-. .--^-.

\ : S2 : : S4 :

\ --v- --v-

\ \ /

\ \ /

\ .^--^.

\ : R3 : path before the break is

\ -v-- H1->S1->R1->NBMA->R2->S2->R3->H2

\ /

.----. .-^^-. path after the break is

: H2 >---< S3 : H1->S1->R1->NBMA->R2->S2->R5->R4->S1

---- ---- \------<--the-loop--<-------/

Figure 4: A Routing Loop Due to Lost PV Routing Attributes.

In the example in Figure 4, Host 1 is sending traffic toward Host 2.

In practice, host routes would not be used, so the destination for

the purpose of routing would be Subnet 3. The traffic travels by way

of Router 1 which establishes a "cut-through" SVC to the NBMA next-

hop, shown here as Router 2. Router 2 forwards traffic destined for

Subnet 3 through Subnet 2 to Router 3. Traffic from Host 1 would

then reach Host 2.

Router 1's cut-through routing implementation caches an association

between Host 2's IP address (or more likely all of Subnet 3) and

Router 2's NBMA address. While the cut-through SVC is still up, Link

1 fails. Router 5 loses it's preferred route through Router 3 and

must direct traffic in the other direction. Router 2 loses a route

through Router 3, but picks up an alternate route through Router 5.

Router 1 is still directing traffic toward Router 2 and advertising a

means of reaching Subnet 3 to Subnet 1. Router 5 and Router 2 will

see a route, creating a loop.

This loop would not form if path information normally carried by

interdomain routing protocols such as BGP and IDRP were retained

across the NBMA. Router 2 would reject the initial route from Router

5 due to the path information. When Router 2 declares the route to

Subnet 3 unreachable, Router 1 withdraws the route from routing at

Subnet 1, leaving the route through Router 4, which would then reach

Router 5, and would reach Router 2 through both Router 1 and Router

5. Similarly, a link state protocol would not form such a loop.

Two proposals for breaking this form of routing loop have been

discussed. Redirect in this example would have no effect, since

Router 2 still has a route, just has different path attributes. A

second proposal is that is that when a route changes in any way, the

advertising NBMA cut-through router invalidates the advertisement for

some time period. This is similar to the notion of Poison Reverse in

distance vector routing protocols. In this example, Router 2 would

eventually readvertise a route since a route through Router 6 exists.

When Router 1 discovers this route, it will advertise it to Subnet 1

and form the loop. Without path information, Router 1 cannot

distinguish between a loop and restoration of normal service through

the link L1.

The loop in Figure 4 can be prevented by configuring Router 4 or

Router 5 to refuse to use the reverse path. This would break backup

connectivity through Router 8 if L1 and L3 failed. The loop can also

be broken by configuring Router 2 to refuse to use the path through

Router 5 unless it could not reach the NBMA. Special configuration of

Router 2 would work as long as Router 2 was not distanced from Router

3 and Router 5 by additional subnets such that it could not determine

which path was in use. If Subnet 1 is in a different AS or RD than

Subnet 2 or Subnet 4, then the decision at Router 2 could be based on

path information.

.--------. .--------.

: Router : : Router :

--v-v--- ---v-v--

: : : :

.--------. .--------. : : .--------. : : .--------. .--------.

: Sub-ES :---: Subnet :-/ \-: Subnet :-/ \-: Subnet :---: Sub-ES :

-------- -------- -------- -------- --------

Figure 5: The Classical IP model as a concatenation of three separate

ATM IP subnets.

In order for loops to be prevented by special configuration at the

NBMA border router, that router would need to know all paths that

could lead back to the NBMA. The same argument that special

configuration could overcome loss of path information was posed in

favor of retaining the use of the EGP protocol defined in the now

historic RFC-904 [11]. This turned out to be unmanageable, with

routing problems occurring when topology was changed elsewhere.

8. IP Over ATM Proposals

8.1 The Classical IP Model

The Classical IP Model was suggested at the Spring 1993 IETF meeting

[8] and retains the classical IP subnet architecture. This model

simply consists of cascading instances of IP subnets with IP-level

(or L3) routers at IP subnet borders. An example realization of this

model consists of a concatenation of three IP subnets. This is shown

in Figure 5. Forwarding IP packets over this Classical IP model is

straight forward using already well established routing techniques

and protocols.

SVC-based ATM IP subnets are simplified in that they:

o limit the number of hosts which must be directly connected at any

given time to those that may actually exchange traffic.

o The ATM network is capable of setting up connections between

any pair of hosts. Consistent with the standard IP routing

algorithm [2] connectivity to the "outside" world is achieved

only through a router, which may provide firewall functionality

if so desired.

o The IP subnet supports an efficient mechanism for address

resolution.

Issues addressed by the IP Over ATM Working Group, and some of the

resolutions, for this model are:

o Methods of encapsulation and multiplexing. This issue is

addressed in RFC-1483 [6], in which two methods of encapsulation

are defined, an LLC/SNAP and a per-VC multiplexing option.

o The definition of an address resolution server (defined in

RFC-1577).

o Defining the default MTU size. This issue is addressed in

RFC-1626 [1] which proposes the use of the MTU discovery

protocol (RFC-1191 [9]).

o Support for IP multicasting. In the summer of 1994, work began

on the issue of supporting IP multicasting over the SVC LATM

model. The proposal for IP multicasting is currently defined by

a set of IP over ATM WG Works in Progress, referred to collectively

as the IPMC documents. In order to support IP multicasting the

ATM subnet must either support point-to- multipoint SVCs, or

multicast servers, or both.

o Defining interim SVC parameters, such as QoS parameters and

time-out values.

o Signaling and negotiations of parameters such as MTU size

and method of encapsulation. RFC-1755 [10] describes an

implementation agreement for routers signaling the ATM network

to establish SVCs initially based upon the ATM Forum's UNI

version 3.0 specification [4], and eventually to be based

upon the ATM Forum's UNI version 3.1 and later specifications.

Topics addressed in RFC-1755 include (but are not limited to)

VC management procedures, e.g., when to time-out SVCs, QOS

parameters, service classes, explicit setup message formats for

various encapsulation methods, node (host or router) to node

negotiations, etc.

RFC-1577 is also applicable to PVC-based subnets. Full mesh PVC

connectivity is required.

For more information see RFC-1577 [8].

8.2 The ROLC NHRP Model

The Next Hop Resolution Protocol (NHRP), currently a work in progress

defined by the Routing Over Large Clouds Working Group (ROLC),

performs address resolution to accomplish direct connections across

IP subnet boundaries. NHRP can supplement RFC-1577 ARP. There has

been recent discussion of replacing RFC-1577 ARP with NHRP. NHRP can

also perform a proxy address resolution to provide the address of the

border router serving a destination off of the NBMA which is only

served by a single router on the NBMA. NHRP as currently defined

cannot be used in this way to support addresses learned from routers

for which the same destinations may be heard at other routers,

without the risk of creating persistent routing loops.

8.3 "Conventional" Model

The "Conventional Model" assumes that a router can relay IP packets

cell by cell, with the VPI/VCI identifying a flow between adjacent

routers rather than a flow between a pair of nodes. A latency

advantage can be provided if cell interleaving from multiple IP

packets is allowed. Interleaving frames within the same VCI requires

an ATM AAL such as AAL3/4 rather than AAL5. Cell forwarding is

accomplished through a higher level mapping, above the ATM VCI layer.

The conventional model is not under consideration by the IP/ATM WG.

The COLIP WG has been formed to develop protocols based on the

conventional model.

8.4 The Peer Model

The Peer Model places IP routers/gateways on an addressing peer basis

with corresponding entities in an ATM cloud (where the ATM cloud may

consist of a set of ATM networks, inter-connected via UNI or P-NNI

interfaces). ATM network entities and the attached IP hosts or

routers exchange call routing information on a peer basis by

algorithmically mapping IP addressing into the NSAP space. Within

the ATM cloud, ATM network level addressing (NSAP-style), call

routing and packet formats are used.

In the Peer Model no provision is made for selection of primary path

and use of alternate paths in the event of primary path failure in

reaching multihomed non-ATM destinations. This will limit the

topologies for which the peer model alone is applicable to only those

topologies in which non-ATM networks are singly homed, or where loss

of backup connectivity is not an issue. The Peer Model may be used

to avoid the need for an address resolution protocol and in a proxy-

ARP mode for stub networks, in conjunction with other mechanisms

suitable to handle multihomed destinations.

During the discussions of the IP over ATM working group, it was felt

that the problems with the end-to-end peer model were much harder

than any other model, and had more unresolved technical issues.

While encouraging interested individuals/companies to research this

area, it was not an initial priority of the working group to address

these issues. The ATM Forum Network Layer Multiprotocol Working

Group has reached a similar conclusion.

8.5 The PNNI and the Integrated Models

The Integrated model (proposed and under study within the

Multiprotocol group of ATM Forum) considers a single routing protocol

to be used for both IP and for ATM. A single routing information

exchange is used to distribute topological information. The routing

computation used to calculate routes for IP will take into account

the topology, including link and node characteristics, of both the IP

and ATM networks and calculates an optimal route for IP packets over

the combined topology.

The PNNI is a hierarchical link state routing protocol with multiple

link metrics providing various available QoS parameters given current

loading. Call route selection takes into account QoS requirements.

Hysteresis is built into link metric readvertisements in order to

avoid computational overload and topological hierarchy serves to

subdivide and summarize complex topologies, helping to bound

computational requirements.

Integrated Routing is a proposal to use PNNI routing as an IP routing

protocol. There are several sets of technical issues that need to be

addressed, including the interaction of multiple routing protocols,

adaptation of PNNI to broadcast media, support for NHRP, and others.

These are being investigated. However, the ATM Forum MPOA group is

not currently performing this investigation. Concerned individuals

are, with an expectation of bringing the work to the ATM Forum and

the IETF.

PNNI has provisions for carrying uninterpreted information. While

not yet defined, a compatible extension of the base PNNI could be

used to carry external routing attributes and avoid the routing loop

problems described in Section 7.

++++++++++++++++++++++++++++++++++++++++++

+ .------------. .------------. +

.---------. + .-: :-. .-: :-. +

: Host or >-+-< : Single ATM : >--< : Single ATM : >-+----- : Router : + : : Domain : : : : Domain : : + :

--------- + -: :- -: :- + .---^----.

+ ------------ ------------ + : Router :

+ .------------. + ---v----

.---------. + .-: :-. + :

: Host or >-+- ... ... --< : Single ATM : >-+-----/

: Router : + : : Domain : : +

--------- + ATM Cloud -: :- +

+ ------------ +

++++++++++++++++++++++++++++++++++++++++++

Note: IS within ATM cloud are ATM IS

Figure 6: The ATM transition model assuming the presence of gateways

or routers between the ATM networks and the ATM peer networks.

8.6 Transition Models

Finally, it is useful to consider transition models, lying somewhere

between the Classical IP Models and the Peer and Integrated Models.

Some possible architectures for transition models have been suggested

by Fong Liaw. Others are possible, for example Figure 6 showing a

Classical IP transition model which assumes the presence of gateways

between ATM networks and ATM Peer networks.

Some of the models described in the prior sections, most notably the

Integrated Model, anticipate the need for mixed environment with

complex routing topologies. These inherently support transition

(possibly with an indefinite transition period). Models which

provide no transition support are primarily of interest to new

deployments which make exclusive, or near exclusive use of ATM or

deployments capable of wholesale replacement of existing networks or

willing to retain only non-ATM stub networks.

For some models, most notably the Peer Model, the ability to attach

to a large non-ATM or mixed internetwork is infeasible without

routing support at a higher level, or at best may pose

interconnection topology constraints (for example: single point of

attachment and a static default route). If a particular model

requires routing support at a higher level a large deployment will

need to be subdivided to provide scalability at the higher level,

which for some models degenerates back to the Classical model.

9. Application of the Working Group's and Related Documents

The IP Over ATM Working Group has generated several Works in Progress

and RFCs. This section identifies the relationship of these and

other related documents to the various IP Over ATM Models identified

in this document. The documents and RFCs produced to date are the

following references, RFC-1483 [6], RFC-1577 [8], RFC-1626 [1], RFC-

1755 [10] and the IPMC documents. The ROLC WG has produced the NHRP

document. Table 5 gives a summary of these documents and their

relationship to the various IP Over ATM Models.

Acknowledgments

This memo is the direct result of the numerous discussions of the IP

over ATM Working Group of the Internet Engineering Task Force. The

authors also had the benefit of several private discussions with H.

Nguyen of AT&T Bell Laboratories. Brian Carpenter of CERN was kind

enough to contribute the TULIP and TUNIC sections to this memo.

Grenville Armitage of Bellcore was kind enough to contribute the

sections on VC binding, encapsulations and the use of B-LLI

information elements to signal such bindings. The text of Appendix A

was pirated liberally from Anthony Alles' of Cisco posting on the IP

over ATM discussion list (and modified at the authors' discretion).

M. Ohta provided a description of the Conventional Model (again which

the authors modified at their discretion). This memo also has

benefitted from numerous suggestions from John T. Amenyo of ANS, Joel

Halpern of Newbridge, and Andy Malis of Ascom-Timplex. Yakov Rekhter

of Cisco provided valuable comments leading to the clarification of

normal loop free NHRP operation and the potential for routing loop

problems only with the improper use of NHRP.

Documents Summary

----------------+-------------------------------------------------

RFC-1483 _ How to identify/label multiple

_ packet/frame-based protocols multiplexed over

_ ATM AAL5. Applies to any model dealing with IP

_ over ATM AAL5.

_

RFC-1577 _ Model for transporting IP and ARP over ATM AAL5

_ in an IP subnet where all nodes share a common

_ IP network prefix. Includes ARP server/Inv-ARP

_ packet formats and procedures for SVC/PVC

_ subnets.

_

RFC-1626 _ Specifies default IP MTU size to be used with

_ ATM AAL5. Requires use of PATH MTU discovery.

_ Applies to any model dealing with IP over ATM

_ AAL5

_

RFC-1755 _ Defines how implementations of IP over ATM

_ should use ATM call control signaling

_ procedures, and recommends values of mandatory

_ and optional IEs focusing particularly on the

_ Classical IP model.

_

IPMC _ Defines how to support IP multicast in Classical

_ IP model using either (or both) meshes of

_ point-to-multipoint ATM VCs, or multicast

_ server(s). IPMC is work in progress.

_

NHRP _ Describes a protocol that can be used by hosts

_ and routers to determine the NBMA next hop

_ address of a destination in "NBMA

_ connectivity"

_ of the sending node. If the destination is not

_ connected to the NBMA fabric, the IP and NBMA

_ addresses of preferred egress points are

_ returned. NHRP is work in progress (ROLC WG).

Table 5: Summary of WG Documents

References

[1] Atkinson, R., "Default IP MTU for use over ATM AAL5", RFC1626,

Naval Research Laboratory, May 1994.

[2] Braden, R., and J. Postel, "Requirements for Internet Gateways",

STD 4, RFC1009, USC/Information Sciences Institute, June 1987.

[3] Braden, R., Postel, J., and Y. Rekhter, "Internet Architecture

Extensions for Shared Media", RFC1620, USC/Information Sciences

Institute, IBM Research, May 1994.

[4] ATM Forum, "ATM User-Network Interface Specification", Prentice

Hall, September 1993.

[5] Garrett, J., Hagan, J., and J. Wong, "Directed ARP", RFC1433,

AT&T Bell Labs, University of Pennsylvania, March 1993.

[6] Heinanen, J., "Multiprotocol Encapsulation over ATM Adaptation

Layer 5", RFC1483, Telecom Finland, July 1993.

[7] Heinanen, J., and R. Govindan, "NBMA Address Resolution Protocol

(NARP)", RFC1735, Telecom Finland, USC/Information Sciences

Institute, December 1994.

[8] Laubach, M., "Classical IP and ARP over ATM", RFC1577,

Hewlett-Packard Laboratories, January 1994.

[9] Mogul, J., and S. Deering, "Path MTU Discovery", RFC1191,

DECWRL, Stanford University, November 1990.

[10] Perez, M., Liaw, F., Grossman, D., Mankin, A., and A. Hoffman,

"ATM signalling support for IP over ATM", RFC1755,

USC/Information Sciences Institute, FORE Systems, Inc., Motorola

Codex, Ascom Timeplex, Inc., January 1995.

[11] Mills, D., "Exterior Gateway Protocol Formal Specification",

STD 18, RFC904, BBN, April 1984.

A Potential Interworking Scenarios to be Supported by ARP

The architectural model of the VC routing protocol, being defined by

the Private Network-to-Network Interface (P-NNI) working group of the

ATM Forum, categorizes ATM networks into two types:

o Those that participate in the VC routing protocols and use NSAP

modeled addresses UNI 3.0 [4] (referred to as private networks,

for short), and

o Those that do not participate in the VC routing protocol.

Typically, but possibly not in all cases, public ATM networks

that use native mode E.164 addresses UNI 3.0 [4] will fall into

this later category.

The issue for ARP, then is to know what information must be returned

to allow such connectivity. Consider the following scenarios:

o Private host to Private Host, no intervening public transit

network(s): Clearly requires that ARP return only the NSAP

modeled address format of the end host.

o Private host to Private host, through intervening public

networks: In this case, the connection setup from host A to host

B must transit the public network(s). This requires that at

each ingress point to the public network that a routing decision

be made as to which is the correct egress point from that public

network to the next hop private ATM switch, and that the native

E.164 address of that egress point be found (finding this is a VC

routing problem, probably requiring configuration of the public

network links and connectivity information). ARP should return,

at least, the NSAP address of the endpoint in which case the

mapping of the NSAP addresses to the E.164 address, as specified

in [4], is the responsibility of ingress switch to the public

network.

o Private Network Host to Public Network Host: To get connectivity

between the public node and the private nodes requires the

same kind of routing information discussed above - namely, the

directly attached public network needs to know the (NSAP format)

ATM address of the private station, and the native E.164 address

of the egress point from the public network to that private

network (or to that of an intervening transit private network

etc.). There is some argument, that the ARP mechanism could

return this egress point native E.164 address, but this may

be considered inconsistent for ARP to return what to some is

clearly routing information, and to others is required signaling

information.

In the opposite direction, the private network node can use, and

should only get, the E.164 address of the directly attached public

node. What format should this information be carried in? This

question is clearly answered, by Note 9 of Annex A of UNI 3.0 [4],

vis:

"A call originated on a Private UNI destined for an host which

only has a native (non-NSAP) E.164 address (i.e. a system

directly attached to a public network supporting the native E.164

format) will code the Called Party number information element in

the (NSAP) E.164 private ATM Address Format, with the RD, AREA,

and ESI fields set to zero. The Called Party Subaddress

information element is not used."

Hence, in this case, ARP should return the E.164 address of the

public ATM station in NSAP format. This is essentially implying an

algorithmic resolution between the native E.164 and NSAP addresses of

directly attached public stations.

o Public network host to Public network host, no intervening

private network: In this case, clearly the Q.2931 requests would

use native E.164 address formats.

o Public network host to Public network host, intervening private

network: same as the case immediately above, since getting

to and through the private network is a VC routing, not an

addressing issue.

So several issues arise for ARP in supporting arbitrary connections

between hosts on private and public network. One is how to

distinguish between E.164 address and E.164 encoded NSAP modeled

address. Another is what is the information to be supplied by ARP,

e.g., in the public to private scenario should ARP return only the

private NSAP modeled address or both an E.164 address, for a point of

attachment between the public and private networks, along with the

private NSAP modeled address.

Authors' Addresses

Robert G. Cole

AT&T Bell Laboratories

101 Crawfords Corner Road, Rm. 3L-533

Holmdel, NJ 07733

Phone: (908) 949-1950

Fax: (908) 949-8887

EMail: rgc@qsun.att.com

David H. Shur

AT&T Bell Laboratories

101 Crawfords Corner Road, Rm. 1F-338

Holmdel, NJ 07733

Phone: (908) 949-6719

Fax: (908) 949-5775

EMail: d.shur@att.com

Curtis Villamizar

ANS

100 Clearbrook Road

Elmsford, NY 10523

EMail: curtis@ans.net

 
 
 
免责声明:本文为网络用户发布,其观点仅代表作者个人观点,与本站无关,本站仅提供信息存储服务。文中陈述内容未经本站证实,其真实性、完整性、及时性本站不作任何保证或承诺,请读者仅作参考,并请自行核实相关内容。
2023年上半年GDP全球前十五强
 百态   2023-10-24
美众议院议长启动对拜登的弹劾调查
 百态   2023-09-13
上海、济南、武汉等多地出现不明坠落物
 探索   2023-09-06
印度或要将国名改为“巴拉特”
 百态   2023-09-06
男子为女友送行,买票不登机被捕
 百态   2023-08-20
手机地震预警功能怎么开?
 干货   2023-08-06
女子4年卖2套房花700多万做美容:不但没变美脸,面部还出现变形
 百态   2023-08-04
住户一楼被水淹 还冲来8头猪
 百态   2023-07-31
女子体内爬出大量瓜子状活虫
 百态   2023-07-25
地球连续35年收到神秘规律性信号,网友:不要回答!
 探索   2023-07-21
全球镓价格本周大涨27%
 探索   2023-07-09
钱都流向了那些不缺钱的人,苦都留给了能吃苦的人
 探索   2023-07-02
倩女手游刀客魅者强控制(强混乱强眩晕强睡眠)和对应控制抗性的关系
 百态   2020-08-20
美国5月9日最新疫情:美国确诊人数突破131万
 百态   2020-05-09
荷兰政府宣布将集体辞职
 干货   2020-04-30
倩女幽魂手游师徒任务情义春秋猜成语答案逍遥观:鹏程万里
 干货   2019-11-12
倩女幽魂手游师徒任务情义春秋猜成语答案神机营:射石饮羽
 干货   2019-11-12
倩女幽魂手游师徒任务情义春秋猜成语答案昆仑山:拔刀相助
 干货   2019-11-12
倩女幽魂手游师徒任务情义春秋猜成语答案天工阁:鬼斧神工
 干货   2019-11-12
倩女幽魂手游师徒任务情义春秋猜成语答案丝路古道:单枪匹马
 干货   2019-11-12
倩女幽魂手游师徒任务情义春秋猜成语答案镇郊荒野:与虎谋皮
 干货   2019-11-12
倩女幽魂手游师徒任务情义春秋猜成语答案镇郊荒野:李代桃僵
 干货   2019-11-12
倩女幽魂手游师徒任务情义春秋猜成语答案镇郊荒野:指鹿为马
 干货   2019-11-12
倩女幽魂手游师徒任务情义春秋猜成语答案金陵:小鸟依人
 干货   2019-11-12
倩女幽魂手游师徒任务情义春秋猜成语答案金陵:千金买邻
 干货   2019-11-12
 
推荐阅读
 
 
 
>>返回首頁<<
 
靜靜地坐在廢墟上,四周的荒凉一望無際,忽然覺得,淒涼也很美
© 2005- 王朝網路 版權所有