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RFC3140 - Per Hop Behavior Identification Codes

王朝other·作者佚名  2008-05-31
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Network Working Group D. Black

Request for Comments: 3140 S. Brim

Obsoletes: 2836 B. Carpenter

Category: Standards Track F. Le FaUCheur

June 2001

Per Hop Behavior Identification Codes

Status of this Memo

This document specifies an Internet standards track protocol for the

Internet community, and requests discussion and suggestions for

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

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

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

Copyright Notice

Copyright (C) The Internet Society (2001). All Rights Reserved.

Abstract

This document defines a 16 bit encoding mechanism for the

identification of differentiated services Per Hop Behaviors in

protocol messages. It replaces RFC2836.

Table of Contents

1. Introduction.................................................2

1.1. Usage Scenarios............................................2

2. Encoding.....................................................3

3. Signalling the Class Selector Codepoints.....................4

4. IANA Considerations..........................................5

5. Security Considerations......................................5

Changes from RFC2836...........................................5

Acknowledgements................................................6

References......................................................6

Authors' Addresses..............................................6

Intellectual Property...........................................7

Full Copyright Statement........................................8

1. Introduction

Differentiated Services [RFC2474, RFC2475] introduces the notion of

Per Hop Behaviors (PHBs) that define how traffic belonging to a

particular behavior aggregate is treated at an individual network

node. In IP packet headers, PHBs are not indicated as such; instead

Differentiated Services Codepoint (DSCP) values are used. There are

only 64 possible DSCP values, but there is no such limit on the

number of PHBs. In a given network domain, there is a locally

defined mapping between DSCP values and PHBs. Standardized PHBs

recommend a DSCP mapping, but network operators may choose

alternative mappings.

In some cases it is necessary or desirable to identify a particular

PHB in a protocol message, such as a message negotiating bandwidth

management or path selection, especially when such messages pass

between management domains. Examples where work is in progress

include communication between bandwidth brokers, and MPLS support of

diffserv.

In certain cases, what needs to be identified is not an individual

PHB, but a set of PHBs. One example is a set of PHBs that must

follow the same physical path to prevent re-ordering. An instance of

this is the set of three PHBs belonging to a single Assured

Forwarding class, such as the PHBs AF11, AF12 and AF13 [RFC2597].

This document defines a binary encoding to uniquely identify PHBs

and/or sets of PHBs in protocol messages. This encoding MUST be used

when such identification is required.

This document replaces RFC2836, which omitted considerations for the

Class Selector codepoints.

The key Words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this

document are to be interpreted as described in [RFC2119].

1.1. Usage Scenarios

Diffserv services are eXPected to be supported over various

underlying technologies which we broadly refer to as "link layers"

for the purpose of this discussion. For the transport of IP packets,

some of these link layers make use of connections or logical

connections where the forwarding behavior supported by each link

layer device is a property of the connection. In particular, within

the link layer domain, each link layer node will schedule traffic

depending on which connection the traffic is transported in.

Examples of such "link layers" include ATM and MPLS.

For efficient support of diffserv over these link layers, one model

is for different Behavior Aggregates (BAs) (or sets of Behavior

Aggregates) to be transported over different connections so that they

are granted different (and appropriate) forwarding behaviors inside

the link layer cloud. When those connections are dynamically

established for the transport of diffserv traffic, it is very useful

to communicate at connection establishment time what forwarding

behavior(s) is (are) to be granted to each connection by the link

layer device so that the BAs transported experience consistent

forwarding behavior inside the link layer cloud. This can be

achieved by including in the connection establishment signaling

messages the encoding of the corresponding PHB, or set of PHBs, as

defined in this document. Details on proposed usage of PHB encodings

by some MPLS label distribution protocols (RSVP and LDP) for support

of Diff-Serv over MPLS, can be found in [MPLS-DS].

In another approach, the ATM Forum has a requirement to indicate

desired IP QOS treatments in ATM signaling, so that ATM switches can

be just as supportive of the desired service as are IP forwarders.

To do so the Forum is defining a new VC call setup information

element is which will carry PHB identification codes (although will

be generalized to do more if needed).

2. Encoding

PHBs and sets of PHBs are encoded in an unsigned 16 bit binary field.

The 16 bit field is arranged as follows:

Case 1: PHBs defined by standards action, as per [RFC2474].

The encoding for a single PHB is the recommended DSCP value for that

PHB, left-justified in the 16 bit field, with bits 6 through 15 set

to zero. Note that the recommended DSCP value MUST be used, even if

the network in question has chosen a different mapping.

The encoding for a set of PHBs is the numerically smallest of the set

of encodings for the various PHBs in the set, with bit 14 set to 1.

(Thus for the AF1x PHBs, the encoding is that of the AF11 PHB, with

bit 14 set to 1.)

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

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

DSCP 0 0 0 0 0 0 0 0 X 0

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

Case 2: PHBs not defined by standards action, i.e., experimental or

local use PHBs as allowed by [RFC2474]. In this case an arbitrary

12 bit PHB identification code, assigned by the IANA, is placed

left-justified in the 16 bit field. Bit 15 is set to 1, and bit 14

is zero for a single PHB or 1 for a set of PHBs. Bits 12 and 13 are

zero.

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

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

PHB id code 0 0 X 1

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

Bits 12 and 13 are reserved either for expansion of the PHB

identification code, or for other use, at some point in the future.

In both cases, when a single PHBID is used to identify a set of PHBs

(i.e., bit 14 is set to 1), that set of PHBs MUST constitute a PHB

Scheduling Class (i.e., use of PHBs from the set MUST NOT cause

intra-microflow traffic reordering when different PHBs from the set

are applied to traffic in the same microflow). The set of AF1x PHBs

[RFC2597] is an example of a PHB Scheduling Class. Sets of PHBs

that do not constitute a PHB Scheduling Class can be identified by

using more than one PHBID.

3. Signalling the Class Selector Codepoints

[RFC2474] defines the eight DS codepoint values of the form 'xxx000'

(where x may be '0' or '1') as the Class Selector Codepoints.

Codepoint 000000 is the recommended DSCP value for the Default PHB,

and hence the Case 1 PHBID constructed from that codepoint is used to

signal the Default PHB (see Section 2 above).

For convenience and consistent operation with networks that employ IP

Precedence [RFC1812], the Case 1 format PHBIDs constructed from the

other seven Class Selector Codepoints may also be used to signal

PHBs. In each case, the PHB signaled by such a PHBID is the PHB to

which the embedded class selector codepoint (or IP Precedence value

that corresponds to it in non-diffserv domains) is mapped in the

recipient's network. Note that different networks will employ

different mappings; see Section 4 of [RFC2474] for further

discussion.

Any specified use of PHBIDs SHOULD allow the use of the eight Case 1

PHBIDs constructed from the Class Selector Codepoints.

4. IANA Considerations

IANA is requested to create a new assignment registry for "Per-Hop

Behavior Identification Codes", initially allowing values in the

range 0 to 4095 decimal.

Assignment of values in this field require:

- the identity of the assignee

- a brief description of the new PHB, with enough detail to

distinguish it from existing standardized and non-standardized

PHBs. In the case of a set of PHBs, this description should

cover all PHBs in the set.

- a reference to a stable document describing the PHB in detail.

During the first year of existence of this registry, IANA is

requested to refer all requests to the IETF diffserv WG for review.

Subsequently, requests should be reviewed by the IETF Transport Area

Directors or by an expert that they designate.

If the number of assignments begins to approach 4096, the Transport

Area Directors should be alerted.

5. Security Considerations

This encoding in itself raises no security issues. However, users of

this encoding should consider that modifying a PHB identification

code may constitute theft or denial of service, so protocols using

this encoding must be adequately protected.

Just signalling a PHBID SHOULD NOT be sufficient to grant the sender

Access to a PHB that it would otherwise not be able to use. In cases

where this is an issue, receivers SHOULD treat received PHBIDs as

requests for service, and use local policy to determine whether to

grant or deny such requests.

Changes from RFC2836

[RFC2836] did not consider the Class Selector code points, which are

covered by section 3 of the present document. A clarification has

been added at the end of section 2 for the case of PHB Scheduling

Classes. The second paragraph of section 5 has been added.

Acknowledgements

Useful comments were made by members of the IETF Diffserv working

group.

References

[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate

Requirement Levels", BCP 14, RFC2119, March 1997.

[RFC2474] Nichols, K., Blake, S., Baker, F. and D. Black,

"Definition of the Differentiated Services Field (DS

Field) in the IPv4 and IPv6 Headers", RFC2474, December

1998.

[RFC2475] Blake, S., Black, D., Carlson, M., Davies, E., Wang, Z.

and W. Weiss, "An Architecture for Differentiated

Services", RFC2475, December 1998.

[RFC2597] Heinanen, J., Baker, F., Weiss, W. and J. Wroclawski,

"Assured Forwarding PHB Group", RFC2597, June 1999.

[RFC2836] Brim, S., Carpenter, B. and F. Le Faucheur, "Per Hop

Behavior Identification Codes", RFC2836, May 2000.

[MPLS-DS] Le Faucheur, F., et al., "MPLS Support of Differentiated

Services", Work in Progress.

Authors' Addresses

David L. Black

EMC Corporation

42 South St.

Hopkinton, MA 01748

EMail: black_david@emc.com

Scott W. Brim

146 Honness Lane

Ithaca, NY 14850

USA

EMail: sbrim@cisco.com

Brian E. Carpenter

IBM

c/o iCAIR

Suite 150

1890 Maple Avenue

Evanston, IL 60201

USA

EMail:

brian@icair.org

Francois Le Faucheur

Cisco Systems

Petra B - Les Lucioles

291, rue Albert Caquot

06560 Valbonne

France

EMail: flefauch@cisco.com

Intellectual Property

The IETF takes no position regarding the validity or scope of any

intellectual property or other rights that might be claimed to

pertain to the implementation or use of the technology described in

this document or the extent to which any license under such rights

might or might not be available; neither does it represent that it

has made any effort to identify any such rights. Information on the

IETF's procedures with respect to rights in standards-track and

standards-related documentation can be found in BCP-11. Copies of

claims of rights made available for publication and any assurances of

licenses to be made available, or the result of an attempt made to

oBTain a general license or permission for the use of such

proprietary rights by implementors or users of this specification can

be obtained from the IETF Secretariat.

The IETF invites any interested party to bring to its attention any

copyrights, patents or patent applications, or other proprietary

rights which may cover technology that may be required to practice

this standard. Please address the information to the IETF Executive

Director.

Full Copyright Statement

Copyright (C) The Internet Society (2001). All Rights Reserved.

This document and translations of it may be copied and furnished to

others, and derivative works that comment on or otherwise explain it

or assist in its implementation may be prepared, copied, published

and distributed, in whole or in part, without restriction of any

kind, provided that the above copyright notice and this paragraph are

included on all such copies and derivative works. However, this

document itself may not be modified in any way, such as by removing

the copyright notice or references to the Internet Society or other

Internet organizations, except as needed for the purpose of

developing Internet standards in which case the procedures for

copyrights defined in the Internet Standards process must be

followed, or as required to translate it into languages other than

English.

The limited permissions granted above are perpetual and will not be

revoked by the Internet Society or its successors or assigns.

This document and the information contained herein is provided on an

"AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING

TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING

BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION

HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF

MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

Funding for the RFCEditor function is currently provided by the

Internet Society.

 
 
 
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