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RFC1577 - Classical IP and ARP over ATM

王朝other·作者佚名  2008-05-31
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Network Working Group M. Laubach

Request for Comments: 1577 Hewlett-Packard Laboratories

Category: Standards Track January 1994

Classical IP and ARP over ATM

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

improvements. Please refer to the current edition of the "Internet

Official Protocol Standards" (STD 1) for the standardization state

and status of this protocol. Distribution of this memo is unlimited.

Abstract

This memo defines an initial application of classical IP and ARP in

an Asynchronous Transfer Mode (ATM) network environment configured as

a Logical IP Subnetwork (LIS) as described in Section 3. This memo

does not preclude the subsequent development of ATM technology into

areas other than a LIS; specifically, as single ATM networks grow to

replace many ethernet local LAN segments and as these networks become

globally connected, the application of IP and ARP will be treated

differently. This memo considers only the application of ATM as a

direct replacement for the "wires" and local LAN segments connecting

IP end-stations ("members") and routers operating in the "classical"

LAN-based paradigm. Issues raised by MAC level bridging and LAN

emulation are beyond the scope of this paper.

This memo introdUCes general ATM technology and nomenclature.

Readers are encouraged to review the ATM Forum and ITU-TS (formerly

CCITT) references for more detailed information about ATM

implementation agreements and standards.

Acknowledgments

This memo could not have come into being without the critical review

from Jim Forster of Cisco Systems, Drew Perkins of FORE Systems, and

Bryan Lyles, Steve Deering, and Berry Kercheval of XEROX PARC. The

concepts and models presented in [1], written by Dave Piscitello and

Joseph Lawrence, laid the structural groundwork for this work. ARP

[3] written by Dave Plummer and Inverse ARP [12] written by Terry

Bradley and Caralyn Brown are the foundation of ATMARP presented in

this memo. This document could have not been completed without the

eXPertise of the IP over ATM Working Group of the IETF and the ad hoc

PVC committee at the Amsterdam IETF meeting.

1. 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.

2. Introduction

The goal of this specification is to allow compatible and

interoperable implementations for transmitting IP datagrams and ATM

Address Resolution Protocol (ATMARP) requests and replies over ATM

Adaptation Layer 5 (AAL5)[2,6].

Note: this memo defines only the operation of IP and address

resolution over ATM, and is not meant to describe the operation of

ATM networks. Any reference to virtual connections, permanent virtual

connections, or switched virtual connections applies only to virtual

channel connections used to support IP and address resolution over

ATM, and thus are assumed to be using AAL5. This memo places no

restrictions or requirements on virtual connections used for other

purposes.

Initial deployment of ATM provides a LAN segment replacement for:

1) Local area networks (e.g., Ethernets, Token Rings and FDDI).

2) Local-area backbones between existing (non-ATM) LANs.

3) Dedicated circuits or frame relay PVCs between IP routers.

Note: In 1), local IP routers with one or more ATM interfaces will be

able to connect islands of ATM networks. In 3), public or private

ATM Wide Area networks will be used to connect IP routers, which in

turn may or may not connect to local ATM networks. ATM WANs and LANs

may be interconnected.

Private ATM networks (local or wide area) will use the private ATM

address structure specified in the ATM Forum UNI specification [9].

This structure is modeled after the format of an OSI Network Service

Access Point Address. A private ATM address uniquely identifies an

ATM endpoint. Public networks will use either the address structure

specified in ITU-TS recommendation E.164 or the private network ATM

address structure. An E.164 address uniquely identifies an interface

to a public network.

The characteristics and features of ATM networks are different than

those found in LANs:

o ATM provides a Virtual Connection (VC) switched environment. VC

setup may be done on either a Permanent Virtual Connection (PVC)

or dynamic Switched Virtual Connection (SVC) basis. SVC call

management signalling is performed via implementations of the

Q.93B protocol [7,9].

o Data to be passed by a VC is segmented into 53 octet quantities

called cells (5 octets of ATM header and 48 octets of data).

o The function of mapping user Protocol Data Units (PDUs) into the

information field of the ATM cell and vice versa is performed in

the ATM Adaptation Layer (AAL). When a VC is created a specific

AAL type is associated with the VC. There are four different AAL

types, which are referred to individually as "AAL1", "AAL2",

"AAL3/4", and "AAL5". (Note: this memo concerns itself with the

mapping of IP and ATMARP over AAL5 only. The other AAL types are

mentioned for introductory purposes only.) The AAL type is known

by the VC end points via the call setup mechanism and is not

carried in the ATM cell header. For PVCs the AAL type is

administratively configured at the end points when the Connection

(circuit) is set up. For SVCs, the AAL type is communicated

along the VC path via Q.93B as part of call setup establishment

and the end points use the signaled information for

configuration. ATM switches generally do not care about the AAL

type of VCs. The AAL5 format specifies a packet format with a

maximum size of (64K - 1) octets of user data. Cells for an AAL5

PDU are transmitted first to last, the last cell indicating the

end of the PDU. ATM standards guarantee that on a given VC, cell

ordering is preserved end-to-end. NOTE: AAL5 provides a non-

assured data transfer service - it is up to higher-level

protocols to provide retransmission.

o ATM Forum signalling defines point-to-point and point-to-

multipoint Connection setup [9]. Multipoint-to-multipoint VCs

are not yet specified by ITU-TS or ATM Forum.

o An ATM Forum ATM endpoint address is either encoded as an NSAP

Address (NSAPA) or is an E.164 Public-UNI address [9]. In some

cases, both an ATM endpoint address and an E.164 Public UNI

address are needed by an ATMARP client to reach another host or

router. Since the use of ATM endpoint addresses and E.164 public

UNI addresses by ATMARP are analogous to the use of Ethernet

addresses, the notion of "hardware address" is extended to

encompass ATM addresses in the context of ATMARP, even though ATM

addresses need not have hardware significance. ATM Forum NSAPAs

use the same basic format as U.S. GOSIP NSAPAs [11]. Note: ATM

Forum addresses should not be construed as being U.S. GOSIP

NSAPAs. They are not, the administration is different, which

fields get filled out are different, etc.

This memo describes the initial deployment of ATM within "classical"

IP networks as a direct replacement for local area networks

(ethernets) and for IP links which interconnect routers, either

within or between administrative domains. The "classical" model here

refers to the treatment of the ATM host adapter as a networking

interface to the IP protocol stack operating in a LAN-based paradigm.

Characteristics of the classical model are:

o The same maximum transmission unit (MTU) size is used for all VCs

in a LIS [2]. (Refer to Section 5.)

o Default LLC/SNAP encapsulation of IP packets.

o End-to-end IP routing architecture stays the same.

o IP addresses are resolved to ATM addresses by use of an ATMARP

service within the LIS - ATMARPs stay within the LIS. From a

client's perspective, the ATMARP architecture stays faithful to

the basic ARP model presented in [3].

o One IP subnet is used for many hosts and routers. Each VC

directly connects two IP members within the same LIS.

Future memos will describe the operation of IP over ATM when ATM

networks become globally deployed and interconnected.

The deployment of ATM into the Internet community is just beginning

and will take many years to complete. During the early part of this

period, we expect deployment to follow traditional IP subnet

boundaries for the following reasons:

o Administrators and managers of IP subnetworks will tend to

initially follow the same models as they currently have deployed.

The mindset of the community will change slowly over time as ATM

increases its coverage and builds its credibility.

o Policy administration practices rely on the security, access,

routing, and filtering capability of IP Internet gateways: i.e.,

firewalls. ATM will not be allowed to "back-door" around these

mechanisms until ATM provides better management capability than

the existing services and practices.

o Standards for global IP over ATM will take some time to complete

and deploy.

This memo details the treatment of the classical model of IP and

ATMARP over ATM. This memo does not preclude the subsequent treatment

of ATM networks within the IP framework as ATM becomes globally

deployed and interconnected; this will be the subject of future

documents. This memo does not address issues related to transparent

data link layer interoperability.

3. IP Subnetwork Configuration

In the LIS scenario, each separate administrative entity configures

its hosts and routers within a closed logical IP subnetwork. Each

LIS operates and communicates independently of other LISs on the same

ATM network. Hosts connected to ATM communicate directly to other

hosts within the same LIS. Communication to hosts outside of the

local LIS is provided via an IP router. This router is an ATM

Endpoint attached to the ATM network that is configured as a member

of one or more LISs. This configuration may result in a number of

disjoint LISs operating over the same ATM network. Hosts of differing

IP subnets MUST communicate via an intermediate IP router even though

it may be possible to open a direct VC between the two IP members

over the ATM network.

The requirements for IP members (hosts, routers) operating in an ATM

LIS configuration are:

o All members have the same IP network/subnet number and address

mask [8].

o All members within a LIS are directly connected to the ATM

network.

o All members outside of the LIS are accessed via a router.

o All members of a LIS MUST have a mechanism for resolving IP

addresses to ATM addresses via ATMARP (based on [3]) and vice

versa via InATMARP (based on [12]) when using SVCs. Refer to

Section 6 "Address Resolution" in this memo.

o All members of a LIS MUST have a mechanism for resolving VCs to

IP addresses via InATMARP (based on [12]) when using PVCs. Refer

to Section 6 "Address Resolution" in this memo.

o All members within a LIS MUST be able to communicate via ATM with

all other members in the same LIS; i.e., the virtual Connection

topology underlying the intercommunication among the members is

fully meshed.

The following list identifies a set of ATM specific parameters that

MUST be implemented in each IP station connected to the ATM network:

o ATM Hardware Address (atm$ha). The ATM address of the individual

IP station.

o ATMARP Request Address (atm$arp-req). atm$arp-req is the ATM

address of an individual ATMARP server located within the LIS.

In an SVC environment, ATMARP requests are sent to this address

for the resolution of target protocol addresses to target ATM

addresses. That server MUST have authoritative responsibility

for resolving ATMARP requests of all IP members within the LIS.

Note: if the LIS is operating with PVCs only, then this parameter

may be set to null and the IP station is not required to send

ATMARP requests to the ATMARP server.

It is RECOMMENDED that routers providing LIS functionality over the

ATM network 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 these 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, it is

RECOMMENDED that a router be able to provide this multiple LIS

support with a single physical ATM interface that may have one or

more individual ATM endpoint addresses. Note: this does not

necessarily mean different End System Identifiers (ESIs) when NSAPAs

are used. The last octet of an NSAPA is the NSAPA Selector (SEL)

field which can be used to differentiate up to 256 different LISs for

the same ESI. (Refer to Section 5.1.3.1, "Private Networks" in [9].)

4. Packet Format

Implementations MUST support IEEE 802.2 LLC/SNAP encapsulation as

described in [2]. LLC/SNAP encapsulation is the default packet

format for IP datagrams.

This memo recognizes that other encapsulation methods may be used

however, in the absence of other knowledge or agreement, LLC/SNAP

encapsulation is the default.

This memo recognizes the future deployment of end-to-end signalling

within ATM that will allow negotiation of encapsulation method on a

per-VC basis. Signalling negotiations are beyond the scope of this

memo.

5. MTU Size

The default MTU size for IP members operating over the ATM network

SHALL be 9180 octets. The LLC/SNAP header is 8 octets, therefore the

default ATM AAL5 protocol data unit size is 9188 octets [2]. In

classical IP subnets, values other than the default can be used if

and only if all members in the LIS have been configured to use the

non-default value.

This memo recognizes the future deployment of end-to-end signalling

within ATM that will allow negotiation of MTU size on a per-VC basis.

Signalling negotiations are beyond the scope of this document.

6. Address Resolution

Address resolution within an ATM logical IP subnet SHALL make use of

the ATM Address Resolution Protocol (ATMARP) (based on [3]) and the

Inverse ATM Address Resolution Protocol (InATMARP) (based on [12]) as

defined in this memo. ATMARP is the same protocol as the ARP

protocol presented in [3] with extensions needed to support ARP in a

unicast server ATM environment. InATMARP is the same protocol as the

original InARP protocol presented in [12] but applied to ATM

networks. All IP stations MUST support these protocols as updated

and extended in this memo. Use of these protocols differs depending

on whether PVCs or SVCs are used.

6.1 Permanent Virtual Connections

An IP station MUST have a mechanism (eg. manual configuration) for

determining what PVCs it has, and in particular which PVCs are being

used with LLC/SNAP encapsulation. The details of the mechanism are

beyond the scope of this memo.

All IP members supporting PVCs are required to use the Inverse ATM

Address Resolution Protocol (InATMARP) (refer to [12]) on those VCs

using LLC/SNAP encapsulation. In a strict PVC environment, the

receiver SHALL infer the relevant VC from the VC on which the

InATMARP request (InARP_REQUEST) or response (InARP_REPLY) was

received. When the ATM source and/or target address is unknown, the

corresponding ATM address length in the InATMARP packet MUST be set

to zero (0) indicating a null length, otherwise the appropriate

address field should be filled in and the corresponding length set

appropriately. InATMARP packet format details are presented later in

this memo.

Directly from [12]: "When the requesting station receives the InARP

reply, it may complete the [ATM]ARP table entry and use the provided

address information. Note: as with [ATM]ARP, information learned via

In[ATM]ARP may be aged or invalidated under certain circumstances."

It is the responsibility of each IP station supporting PVCs to re-

validate [ATM]ARP table entries as part of the aging process. See

Section 6.5 on "ATMARP Table Aging".

6.2 Switched Virtual Connections

SVCs require support for ATMARP in the non-broadcast, non-multicast

environment that ATM networks currently provide. To meet this need a

single ATMARP Server MUST be located within the LIS. This server MUST

have authoritative responsibility for resolving the ATMARP requests

of all IP members within the LIS.

The server itself does not actively establish connections. It

depends on the clients in the LIS to initiate the ATMARP registration

procedure. An individual client connects to the ATMARP server using

a point-to-point VC. The server, upon the completion of an ATM

call/connection of a new VC specifying LLC/SNAP encapsulation, will

transmit an InATMARP request to determine the IP address of the

client. The InATMARP reply from the client contains the information

necessary for the ATMARP Server to build its ATMARP table cache. This

information is used to generate replies to the ATMARP requests it

receives.

The ATMARP Server mechanism requires that each client be

administratively configured with the ATM address of the ATMARP Server

atm$arp-req as defined earlier in this memo. There is to be one and

only one ATMARP Server operational per logical IP subnet. It is

RECOMMENDED that the ATMARP Server also be an IP station. This

station MUST be administratively configured to operate and recognize

itself as the ATMARP Server for a LIS. The ATMARP Server MUST be

configured with an IP address for each logical IP subnet it is

serving to support InATMARP requests.

This memo recognizes that a single ATMARP Server is not as robust as

multiple servers which synchronize their databases correctly. This

document is defining the client-server interaction by using a simple,

single server approach as a reference model, and does not prohibit

more robust approaches which use the same client-server interface.

6.3 ATMARP Server Operational Requirements

The ATMARP server accepts ATM calls/connections from other ATM end

points. At call setup and if the VC supports LLC/SNAP encapsulation,

the ATMARP server will transmit to the originating ATM station an

InATMARP request (InARP_REQUEST) for each logical IP subnet the

server is configured to serve. After receiving an InATMARP reply

(InARP_REPLY), the server will examine the IP address and the ATM

address. The server will add (or update) the <ATM address, IP

address> map entry and timestamp into its ATMARP table. If the

InATMARP IP address duplicates a table entry IP address and the

InATMARP ATM address does not match the table entry ATM address and

there is an open VC associated with that table entry, the InATMARP

information is discarded and no modifications to the table are made.

ATMARP table entries persist until aged or invalidated. VC call tear

down does not remove ATMARP table entries.

The ATMARP server, upon receiving an ATMARP request (ARP_REQUEST),

will generate the corresponding ATMARP reply (ARP_REPLY) if it has an

entry in its ATMARP table. Otherwise it will generate a negative

ATMARP reply (ARP_NAK). The ARP_NAK response is an extension to the

ARMARP protocol and is used to improve the robustness of the ATMARP

server mechanism. With ARP_NAK, a client can determine the

difference between a catastrophic server failure and an ATMARP table

lookup failure. The ARP_NAK packet format is the same as the

received ARP_REQUEST packet format with the operation code set to

ARP_NAK, i.e., the ARP_REQUEST packet data is merely copied for

transmission with the ARP_REQUEST operation code reset to ARP_NAK.

Updating the ATMARP table information timeout, the short form: when

the server receives an ATMARP request over a VC, where the source IP

and ATM address match the association already in the ATMARP table and

the ATM address matches that associated with the VC, the server may

update the timeout on the source ATMARP table entry: i.e., if the

client is sending ATMARP requests to the server over the same VC that

it used to register its ATMARP entry, the server should examine the

ATMARP requests and note that the client is still "alive" by updating

the timeout on the client's ATMARP table entry.

Adding robustness to the address resolution mechanism using ATMARP:

when the server receives an ARP_REQUEST over a VC, it examines the

source information. If there is no IP address associated with the VC

over which the ATMARP request was received and if the source IP

address is not associated with any other connection, then the server

will add the <ATM address, IP address> entry and timestamp into its

ATMARP table and associate the entry with this VC.

6.4 ATMARP Client Operational Requirements

The ATMARP client is responsible for contacting the ATMARP server to

register its own ATMARP information and to gain and refresh its own

ATMARP entry/information about other IP members. This means, as

noted above, that ATMARP clients MUST be configured with the ATM

address of the ATMARP server. ATMARP clients MUST:

1. Initiate the VC connection to the ATMARP server for

transmitting and receiving ATMARP and InATMARP packets.

2. Respond to ARP_REQUEST and InARP_REQUEST packets received on

any VC appropriately. (Refer to Section 7, "Protocol Operation"

in [12].)

3. Generate and transmit ARP_REQUEST packets to the ATMARP server

and to process ARP_REPLY and ARP_NAK packets from the server

appropriately. ARP_REPLY packets should be used to

build/refresh its own client ATMARP table entries.

4. Generate and transmit InARP_REQUEST packets as needed and to

process InARP_REPLY packets appropriately. InARP_REPLY packets

should be used to build/refresh its own client ATMARP table

entries. (Refer to Section 7, "Protocol Operation" in [12].)

5. Provide an ATMARP table aging function to remove its own old

client ATMARP tables entries after a convenient period of time.

Note: if the client does not maintain an open VC to the server, the

client MUST refresh its ATMARP information with the server at least

once every 20 minutes. This is done by opening a VC to the server

and exchanging the initial InATMARP packets.

6.5 ATMARP Table Aging

An ATMARP client or server MUST have knowledge of any open VCs it has

(permanent or switched), their association with an ATMARP table

entry, and in particular, which VCs support LLC/SNAP encapsulation.

Client ATMARP table entries are valid for a maximum time of 15

minutes.

Server ATMARP table entries are valid for a minimum time of 20

minutes.

Prior to aging an ATMARP table entry, an ATMARP server MUST generate

an InARP_REQUEST on any open VC associated with that entry. If an

InARP_REPLY is received, that table entry is updated and not deleted.

If there is no open VC associated with the table entry, the entry is

deleted.

When an ATMARP table entry ages, an ATMARP client MUST invalidate the

table entry. If there is no open VC associated with the invalidated

entry, that entry is deleted. In the case of an invalidated entry and

an open VC, the ATMARP client must revalidate the entry prior to

transmitting any non address resolution traffic on that VC. In the

case of a PVC, the client validates the entry by transmitting an

InARP_REQUEST and updating the entry on receipt of an InARP_REPLY. In

the case of an SVC, the client validates the entry by transmitting an

ARP_REQUEST to the ATMARP Server and updating the entry on receipt of

an ARP_REPLY. If a VC with an associated invalidated ATMARP table

entry is closed, that table entry is removed.

6.6 ATMARP and InATMARP Packet Format

Internet addresses are assigned independently of ATM addresses. Each

host implementation MUST know its own IP and ATM address(es) and MUST

respond to address resolution requests appropriately. IP members

MUST also use ATMARP and InATMARP to resolve IP addresses to ATM

addresses when needed.

The ATMARP and InATMARP protocols use the same hardware type

(ar$hrd), protocol type (ar$pro), and operation code (ar$op) data

formats as the ARP and InARP protocols [3,12]. The location of these

fields within the ATMARP packet are in the same byte position as

those in ARP and InARP packets. A unique hardware type value has

been assigned for ATMARP. In addition, ATMARP makes use of an

additional operation code for ARP_NAK. The remainder of the

ATMARP/InATMARP packet format is different than the ARP/InARP packet

format.

The ATMARP and InATMARP protocols have several fields that have the

following format and values:

Data:

ar$hrd 16 bits Hardware type

ar$pro 16 bits Protocol type

ar$shtl 8 bits Type & length of source ATM number (q)

ar$sstl 8 bits Type & length of source ATM subaddress (r)

ar$op 16 bits Operation code (request, reply, or NAK)

ar$spln 8 bits Length of source protocol address (s)

ar$thtl 8 bits Type & length of target ATM number (x)

ar$tstl 8 bits Type & length of target ATM subaddress (y)

ar$tpln 8 bits Length of target protocol address (z)

ar$sha qoctets source ATM number

ar$ssa roctets source ATM subaddress

ar$spa soctets source protocol address

ar$tha xoctets target ATM number

ar$tsa yoctets target ATM subaddress

ar$tpa zoctets target protocol address

Where:

ar$hrd - assigned to ATM Forum address family and is

19 decimal (0x0013) [4].

ar$pro - see Assigned Numbers for protocol type number for

the protocol using ATMARP. (IP is 0x0800).

ar$op - The operation type value (decimal):

ARP_REQUEST = 1

ARP_REPLY = 2

InARP_REQUEST = 8

InARP_REPLY = 9

ARP_NAK = 10

ar$spln - length in octets of the source protocol address. For

IP ar$spln is 4.

ar$tpln - length in octets of the target protocol address. For

IP ar$tpln is 4.

ar$sha - source ATM number (E.164 or ATM Forum NSAPA)

ar$ssa - source ATM subaddress (ATM Forum NSAPA)

ar$spa - source protocol address

ar$tha - target ATM number (E.164 or ATM Forum NSAPA)

ar$tsa - target ATM subaddress (ATM Forum NSAPA)

ar$tpa - target protocol address

The encoding of the 8-bit type and length value for ar$shtl,

ar$sstl, ar$thtl, and ar$tstl is as follows:

MSB 8 7 6 5 4 3 2 1 LSB

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

0 1/0 Octet length of address

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

Where:

bit.8 (reserved) = 0 (for future use)

bit.7 (type) = 0 ATM Forum NSAPA format

= 1 E.164 format

bit.6-1 (length) = 6 bit unsigned octet length of address

(MSB = bit.6, LSB = bit.1)

ATM addresses in Q.93B (as defined by the ATM Forum UNI 3.0

signalling specification [9]) include a "Calling Party Number

Information Element" and a "Calling Party Subaddress Information

Element". These Information Elements (IEs) SHOULD map to

ATMARP/InATMARP source ATM number and source ATM subaddress

respectively. Furthermore, ATM Forum defines a "Called Party Number

Information Element" and a "Called Party Subaddress Information

Element". These IEs map to ATMARP/InATMARP target ATM number and

target ATM subaddress respectively.

The ATM Forum defines three structures for the combined use of number

and subaddress [9]:

ATM Number ATM Subaddress

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

Structure 1 ATM Forum NSAPA null

Structure 2 E.164 null

Structure 3 E.164 ATM Forum NSAPA

IP members MUST register their ATM endpoint address with their ATMARP

server using the ATM address structure appropriate for their ATM

network connection: i.e., LISs implemented over ATM LANs following

ATM Forum UNI 3.0 should register using Structure 1; LISs implemented

over an E.164 "public" ATM network should register using Structure 2.

A LIS implemented over a combination of ATM LANs and public ATM

networks may need to register using Structure 3. Implementations

based on this memo MUST support all three ATM address structures.

ATMARP and InATMARP requests and replies for ATM address structures 1

and 2 MUST indicate a null ATM subaddress; i.e., ar$sstl.type = 1 and

ar$sstl.length = 0 and ar$tstl.type = 1 and ar$tstl.length = 0. When

ar$sstl.length and ar$tstl.length =0, the ar$tsa and ar$ssa fields

are not present.

Note: the ATMARP packet format presented in this memo is general in

nature in that the ATM number and ATM subaddress fields SHOULD map

directly to the corresponding Q.93B fields used for ATM

call/connection setup signalling messages. The IP over ATM Working

Group expects ATM Forum NSAPA numbers (Structure 1) to predominate

over E.164 numbers (Structure 2) as ATM endpoint identifiers within

ATM LANs. The ATM Forum's VC Routing specification is not complete

at this time and therefore its impact on the operational use of ATM

Address Structure 3 is undefined. The ATM Forum will be defining this

relationship in the future. It is for this reason that IP members

need to support all three ATM address structures.

6.7 ATMARP/InATMARP Packet Encapsulation

ATMARP and InATMARP packets are to be encoded in AAL5 PDUs using

LLC/SNAP encapsulation. The format of the AAL5 CPCS-SDU payload field

for ATMARP/InATMARP PDUs is:

Payload Format for ATMARP/InATMARP PDUs:

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

LLC 0xAA-AA-03

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

OUI 0x00-00-00

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

Ethertype 0x08-06

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

ATMARP/InATMARP Packet

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

The LLC value of 0xAA-AA-03 (3 octets) indicates the presence of a

SNAP header.

The OUI value of 0x00-00-00 (3 octets) indicates that the following

two-bytes is an ethertype.

The Ethertype value of 0x08-06 (2 octets) indicates ARP [4].

The total size of the LLC/SNAP header is fixed at 8-octets. This

aligns the start of the ATMARP packet on a 64-bit boundary relative

to the start of the AAL5 CPCS-SDU.

The LLC/SNAP encapsulation for ATMARP/InATMARP presented here is

consistent with the treatment of multiprotocol encapsulation of IP

over ATM AAL5 as specified in [2] and in the format of ATMARP over

IEEE 802 networks as specified in [5].

Traditionally, address resolution requests are broadcast to all

directly connected IP members within a LIS. It is conceivable in the

future that larger scaled ATM networks may handle ATMARP requests to

destinations outside the originating LIS, perhaps even globally;

issues raised by ATMARP'ing outside the LIS or by a global ATMARP

mechanism are beyond the scope of this memo.

7. IP Broadcast Address

ATM does not support broadcast addressing, therefore there are no

mappings available from IP broadcast addresses to ATM broadcast

services. Note: this lack of mapping does not restrict members from

transmitting or receiving IP datagrams specifying any of the four

standard IP broadcast address forms as described in [8]. Members,

upon receiving an IP broadcast or IP subnet broadcast for their LIS,

MUST process the packet as if addressed to that station.

8. IP Multicast Address

ATM does not support multicast address services, therefore there are

no mappings available from IP multicast addresses to ATM multicast

services. Current IP multicast implementations (i.e., MBONE and IP

tunneling, see [10]) will continue to operate over ATM based logical

IP subnets if operated in the WAN configuration.

This memo recognizes the future development of ATM multicast service

addressing by the ATM Forum. When available and widely implemented,

the roll-over from the current IP multicast architecture to this new

ATM architecture will be straightforward.

9. Security

Not all of the security issues relating to IP over ATM are clearly

understood at this time, due to the fluid state of ATM

specifications, newness of the technology, and other factors.

It is believed that ATM and IP facilities for authenticated call

management, authenticated end-to-end communications, and data

encryption will be needed in globally connected ATM networks. Such

future security facilities and their use by IP networks are beyond

the scope of this memo.

There are known security issues relating to host impersonation via

the address resolution protocols used in the Internet [13]. No

special security mechanisms have been added to the address resolution

mechanism defined here for use with networks using IP over ATM.

10. Open Issues

o Interim Local Management Interface (ILMI) services will not be

generally implemented initially by some providers and vendors and

will not be used to oBTain the ATM address network prefix from

the network [9]. Meta-signalling does provide some of this

functionality and in the future we need to document the options.

o Well known ATM address(es) for ATMARP servers? It would be very

handy if a mechanism were available for determining the "well

known" ATM address(es) for the client's ATMARP server in the LIS.

o There are many VC management issues which have not yet been

addressed by this specification and which await the unwary

implementor. For example, one problem that has not yet been

resolved is how two IP members decide which of duplicate VCs can

be released without causing VC thrashing. If two IP stations

simultaneously established VCs to each other, it is tempting to

allow only one of these VCs to be established, or to release one

of these VCs immediately after it is established. If both IP

stations simultaneously decide to release opposite VCs, a

thrashing effect can be created where VCs are repeatedly

established and immediately released. For the time being, the

safest strategy is to allow duplicate VCs to be established and

simply age them like any other VCs.

References

[1] Piscitello, D., and J. Lawrence, "IP and ARP over the SMDS

Service", RFC1209, Bell Communications Research, March 1991.

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

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

[3] Plummer, D., "An Ethernet Address Resolution Protocol - or -

Converting Network Addresses to 48.bit Ethernet Address for

Transmission on Ethernet Hardware", STD 37, RFC826, MIT,

November 1982.

[4] Reynolds, J., and J. Postel, "Assigned Numbers", STD 2, RFC1340,

USC/Information Sciences Institute, July 1992.

[5] Postel, J., and J. Reynolds, "A Standard for the Transmission of

IP Datagrams over IEEE 802 Networks", STD 43, RFC1042,

USC/Information Sciences Institute, February 1988.

[6] CCITT, "Draft Recommendation I.363", CCITT Study Group XVIII,

Geneva, 19-29 January 1993.

[7] CCITT, "Draft text for Q.93B", CCITT Study Group XI, 23 September

- 2 October 1992.

[8] Braden, R., "Requirements for Internet Hosts -- Communication

Layers", STD 3, RFC1122, USC/Information Sciences Institute,

October 1989.

[9] ATM Forum, "ATM User-Network Interface Specification Version

3.0.", ATM Forum, 480 San Antonio Road, Suite 100, Mountain View,

CA 94040, June 1993.

[10] Deering, S., "Host Extensions for IP Multicasting", STD 5, RFC

1112, Stanford University, August 1989.

[11] Colella, R., and Gardner, E., and R. Callon, "Guidelines for OSI

NSAP Allocation in the Internet", RFC1237, NIST, Mitre, DEC,

July 1991.

[12] Bradely, T., and C. Brown, "Inverse Address Resolution Protocol",

RFC1293, Wellfleet Communications, Inc., January 1992.

[13] Bellovin, S., "Security Problems in the TCP/IP Protocol Suite",

ACM Computer Communications Review, Vol. 19, Issue 2, pp. 32-48,

1989.

Security Considerations

Security issues are discussed in Section 9.

Author's Address

Mark Laubach

Hewlett-Packard Laboratories

1501 Page Mill Road

Palo Alto, CA 94304

Phone: 415-857-3513

Fax: 415-857-8526

EMail: laubach@hpl.hp.com

 
 
 
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