Baseline Encoding and Transport of Pre-Congestion Information
draft-ietf-pcn-baseline-encoding-05

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Abstract

The objective of Pre-Congestion Notification (PCN) is to protect the quality of service (QoS) of inelastic flows within a Diffserv domain. The overall rate of the PCN-traffic is metered on every link in the PCN-domain, and PCN-packets are appropriately marked when certain configured rates are exceeded. The level of marking allows the boundary nodes to make decisions about whether to admit or block a new flow request, and (in abnormal circumstances) whether to terminate some of the existing flows, thereby protecting the QoS of previously admitted flows. This document specifies how such marks are to be encoded into the IP header by re-using the Explicit Congestion Notification (ECN) codepoints within this controlled domain. The baseline encoding described here provides for only two PCN encoding states, Not-marked and PCN-marked.

Table of Contents

1.  Introduction
2.  Requirements notation
3.  Terminology
4.  Encoding two PCN States in IP
    4.1.  Valid and Invalid Codepoint Transitions
    4.2.  Rationale for Encoding
    4.3.  PCN-Compatible Diffserv Codepoints
        4.3.1.  Co-existence of PCN and not-PCN traffic
5.  Rules for Experimental Encoding Schemes
6.  Backwards Compatibility
7.  IANA Considerations
8.  Security Considerations
9.  Conclusions
10.  Acknowledgements
11.  Comments Solicited
12.  References
    12.1.  Normative References
    12.2.  Informative References
Appendix A.  PCN Deployment Considerations (Informational)
    A.1.  Choice of Suitable DSCPs
    A.2.  Rationale for Using ECT(0) for Not-marked

1.  Introduction

The objective of Pre-Congestion Notification (PCN) [RFC5559] (Eardley, P., “Pre-Congestion Notification (PCN) Architecture,” June 2009.) is to protect the quality of service (QoS) of inelastic flows within a Diffserv domain, in a simple, scalable and robust fashion. The overall rate of the PCN-traffic is metered on every link in the PCN-domain, and PCN-packets are appropriately marked when certain configured rates are exceeded. These configured rates are below the rate of the link thus providing notification before any congestion occurs (hence “pre-congestion notification”). The level of marking allows the boundary nodes to make decisions about whether to admit or block a new flow request, and (in abnormal circumstances) whether to terminate some of the existing flows, thereby protecting the QoS of previously admitted flows.

This document specifies how these PCN marks are encoded into the IP header by re-using the bits of the Explicit Congestion Notification (ECN) field [RFC3168] (Ramakrishnan, K., Floyd, S., and D. Black, “The Addition of Explicit Congestion Notification (ECN) to IP,” September 2001.). It also describes how packets are identified as belonging to a PCN flow. Some deployment models require two PCN encoding states, others require more. The baseline encoding described here only provides for two PCN encoding states. However the encoding can be easily extended to provide more states. Rules for such extensions are given in Section 5 (Rules for Experimental Encoding Schemes).

Changes from previous drafts (to be removed by the RFC Editor):

From -04 to -05:

Clarified throughout that the PCN WG is not requesting a specific DSCP for PCN. Rather we are recommending a set of DSCPs that might be suitable. Appendix A.1 (Choice of Suitable DSCPs) has been re-written to reflect this. References to maintaining a list of PCN-compatible DSCPs have also been removed.

Last sentence of Section 6 (Backwards Compatibility) altered.

Several spelling corrections.

References updated throughout.

From -03 to -04:

Major WGLC comments addressed:

• Added Section 4.3.1 (Co-existence of PCN and not-PCN traffic) to clarify why we need the not-PCN codepoint.
• Stated that the PCN WG will maintain a list of PCN-compatible DSCPs. This should help avoid inter-operability issues.

Also addressed a number of WGLC nits.

From -02 to -03:

Extensive changes to address comments made by Gorry Fairhurst including:

• Abstract re-written.
• Clarified throughout that this re-uses the ECN bits in the IP header.
• Re-arranged order of terminology section for clarity.
• Table 2 replaced with new table and text.
• Security considerations re-written.
• Appendixes re-written to improve clarity.
• Numerous minor nits and language changes throughout.

Extensive other minor changes throughout.

From -01 to -02:

Removed Appendix A and replaced with reference to [I‑D.ietf‑tsvwg‑ecn‑tunnel] (Briscoe, B., “Tunnelling of Explicit Congestion Notification,” March 2010.)

Moved Appendix B into main body of text.

Changed Appendix C to give deployment advice.

Minor changes throughout including checking consistency of capitalisation of defined terms.

Clarified that LU was deliberately excluded from encoding.

From -00 to -01:

Added section on restrictions for extension encoding schemes.

Included table in Appendix showing encoding transitions at different PCN nodes.

Checked for consistency of terminology.

Minor language changes for clarity.

Changes from previous filename

Filename changed from draft-moncaster-pcn-baseline-encoding.

Terminology changed for clarity (PCN-compatible DSCP and PCN-enabled packet).

Minor changes throughout.

Modified meaning of ECT(1) state to EXP.

Moved text relevant to behaviour of nodes into appendix for later transfer to new document on edge behaviours.

From draft-moncaster -01 to -02:

Minor changes throughout including tightening up language to remain consistent with the PCN Architecture terminology.

From draft-moncaster -00 to -01:

Change of title from "Encoding and Transport of (Pre-)Congestion Information from within a Diffserv Domain to the Egress"

Extensive changes to Introduction and abstract.

Added a section on the implications of re-using a DSCP.

Added appendix listing possible operator scenarios for using this baseline encoding.

Minor changes throughout.

2.  Requirements notation

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] (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.).

3.  Terminology

The following terms are used in this document:

• PCN-compatible Diffserv codepoint - a Diffserv codepoint for which the ECN field is used to carry PCN markings rather than [RFC3168] (Ramakrishnan, K., Floyd, S., and D. Black, “The Addition of Explicit Congestion Notification (ECN) to IP,” September 2001.) markings.
• PCN-marked - codepoint indicating packets that have been marked at a PCN-interior-node using some PCN marking behaviour [I‑D.ietf‑pcn‑marking‑behaviour] (Eardley, P., “Metering and marking behaviour of PCN-nodes,” August 2009.). Abbreviated to PM.
• Not-marked - codepoint indicating packets that are PCN-capable, but are not PCN-marked. Abbreviated to NM.
• PCN-enabled codepoints - collective term for all NM and PM codepoints. By definition, packets carrying such codepoints are PCN-packets.
• not-PCN - packets that are not PCN-enabled.

In addition, the document uses the terminology defined in [RFC5559] (Eardley, P., “Pre-Congestion Notification (PCN) Architecture,” June 2009.).

4.  Encoding two PCN States in IP

The PCN encoding states are defined using a combination of the DSCP and ECN fields within the IP header. The baseline PCN encoding closely follows the semantics of ECN [RFC3168]. It allows the encoding of two PCN states: Not-marked and PCN-marked. It also allows for traffic that is not PCN-capable to be marked as such (not-PCN). Given the scarcity of codepoints within the IP header the baseline encoding leaves one codepoint free for experimental use. The following table defines how to encode these states in IP:

Where DSCP n is a PCN-compatible Diffserv codepoint (see Section 4.3 (PCN-Compatible Diffserv Codepoints)) and EXP means available for Experimental use. N.B. we deliberately reserve this codepoint for experimental use only (and not local use) to prevent future compatibility issues.

The following rules apply to all PCN traffic:

• PCN-traffic MUST be marked with a PCN-compatible Diffserv Codepoint. To conserve DSCPs, Diffserv Codepoints SHOULD be chosen that are already defined for use with admission controlled traffic. Appendix A.1 (Choice of Suitable DSCPs) gives guidance to implementiors on suitable DSCPs. Guidelines for mixing traffic-types within a PCN-domain are given in [I‑D.ietf‑pcn‑marking‑behaviour] (Eardley, P., “Metering and marking behaviour of PCN-nodes,” August 2009.).
• Any packet that is not-PCN but which shares the same Diffserv codepoint as PCN-enabled traffic MUST have the ECN field of its outermost IP header equal to 00.

4.1.  Valid and Invalid Codepoint Transitions

A PCN-ingress-node MUST set the Not-marked (10) codepoint on any arriving packet that belongs to a PCN-flow. It MUST set the not-PCN (00) codepoint on all other packets sharing a PCN-compatible Diffserv codepoint.

The only valid codepoint transitions within a PCN-interior-node are from NM to PM (which should occur if either meter indicates a need to PCN-mark a packet [I‑D.ietf‑pcn‑marking‑behaviour] (Eardley, P., “Metering and marking behaviour of PCN-nodes,” August 2009.)) and from EXP to PM (which MAY be allowed by some future experimental extensions). The following table gives the full set of valid and invalid codepoint transitions.

               +-------------------------------------------------+
               |                  Codepoint Out                  |
+--------------+-------------+-----------+-----------+-----------+
| Codepoint in | not-PCN(00) |   NM(10)  |  EXP(01)  |   PM(11)  |
+--------------+-------------+-----------+-----------+-----------+
|  not-PCN(00) |    Valid    | Not valid | Not valid | Not valid |
+--------------+-------------+-----------+-----------+-----------+
|       NM(10) |  Not valid  |   Valid   | Not valid |   Valid   |
+--------------+-------------+-----------+-----------+-----------+
|     EXP(01)* |  Not valid  | Not valid |   Valid   |   Valid*  |
+--------------+-------------+-----------+-----------+-----------+
|       PM(11) |  Not valid  | Not valid | Not valid |   Valid   |
+--------------+-------------+-----------+-----------+-----------+
* This SHOULD cause an alarm to be raised at a higher layer. The
    packet MUST be treated as if it carried the NM codepoint.

         Table 2: Valid and Invalid Codepoint Transitions for
                   PCN-packets at PCN-interior-nodes

A PCN-egress-node SHOULD set the not-PCN (00) codepoint on all packets it forwards out of the PCN-domain. The only exception to this is if the PCN-egress-node is certain that revealing other codepoints outside the PCN-domain won't contravene the guidance given in [RFC4774] (Floyd, S., “Specifying Alternate Semantics for the Explicit Congestion Notification (ECN) Field,” November 2006.).

4.2.  Rationale for Encoding

The exact choice of encoding was dictated by the constraints imposed by existing IETF RFCs, in particular [RFC3168] (Ramakrishnan, K., Floyd, S., and D. Black, “The Addition of Explicit Congestion Notification (ECN) to IP,” September 2001.), [RFC4301] (Kent, S. and K. Seo, “Security Architecture for the Internet Protocol,” December 2005.) and [RFC4774] (Floyd, S., “Specifying Alternate Semantics for the Explicit Congestion Notification (ECN) Field,” November 2006.). One of the tightest constraints was the need for any PCN encoding to survive being tunnelled through either an IP in IP tunnel or an IPsec Tunnel. [I‑D.ietf‑tsvwg‑ecn‑tunnel] (Briscoe, B., “Tunnelling of Explicit Congestion Notification,” March 2010.) explains this in more detail. The main effect of this constraint is that any PCN marking has to carry the 11 codepoint in the ECN field since this is the only codepoint that is guaranteed to be copied down into the inner header upon decapsulation. An additional constraint is the need to minimise the use of Diffserv codepoints because there is a limited supply of standards track codepoints remaining. Section 4.3 (PCN-Compatible Diffserv Codepoints) explains how we have minimised this still further by reusing pre-existing Diffserv codepoint(s) such that non-PCN traffic can still be distinguished from PCN traffic. There are a number of factors that were considered before choosing to set 10 as the NM state instead of 01. These included similarity to ECN, presence of tunnels within the domain, leakage into and out of PCN-domain and incremental deployment (see Appendix A.2 (Rationale for Using ECT(0) for Not-marked)).

The encoding scheme above seems to meet all these constraints and ends up looking very similar to ECN. This is perhaps not surprising given the similarity in architectural intent between PCN and ECN.

4.3.  PCN-Compatible Diffserv Codepoints

Equipment complying with the baseline PCN encoding MUST allow PCN to be enabled for certain Diffserv codepoints. This document defines the term "PCN-compatible Diffserv codepoint" for such a DSCP. To be clear, any packets with such a DSCP will be PCN enabled only if they are within a PCN-domain and have their ECN field set to indicate a codepoint other than not-PCN.

Enabling PCN marking behaviour for a specific DSCP disables any other marking behaviour (e.g. enabling PCN disables the default ECN marking behaviour introduced in [RFC3168] (Ramakrishnan, K., Floyd, S., and D. Black, “The Addition of Explicit Congestion Notification (ECN) to IP,” September 2001.)). All traffic metering and marking behaviours are discussed in [I‑D.ietf‑pcn‑marking‑behaviour] (Eardley, P., “Metering and marking behaviour of PCN-nodes,” August 2009.). This ensures compliance with the BCP guidance set out in [RFC4774] (Floyd, S., “Specifying Alternate Semantics for the Explicit Congestion Notification (ECN) Field,” November 2006.).

The PCN Working Group has chosen not to define a single DSCP for use with PCN for several reasons. Firstly the PCN mechanism is applicable to a variety of different traffic classes. Secondly standards track DSCPs are in increasingly short supply. Thirdly PCN should be seen as being essentially a marking behaviour similar to ECN but intended for inelastic traffic. More details are given in the informational appendix Appendix A.1 (Choice of Suitable DSCPs).

4.3.1.  Co-existence of PCN and not-PCN traffic

The scarcity of pool 1 DSCPs coupled with the fact that PCN is envisaged as a marking behaviour that could be applied to a number of different DSCPs makes it essential that we provide a not-PCN state. As stated above (and expanded in Appendix A.1 (Choice of Suitable DSCPs)) the aim is for PCN to re-use existing DSCPs. Because PCN re-defines the meaning of the ECN field for such DSCPs it is important to allow an operator to still use the DSCP for traffic that isn't PCN-enabled. This is achieved by providing a not-PCN state within the encoding scheme.

5.  Rules for Experimental Encoding Schemes

Any experimental encoding scheme MUST follow these rules to ensure backward compatibility with this baseline scheme:

• All Interior-nodes within a PCN-domain MUST interpret the 00 codepoint in the ECN field as not-PCN and MUST NOT change it to another value. Therefore an ingress node wishing to disable PCN marking for a packet within a PCN-compatible Diffserv Codepoint MUST set the ECN field to 00.
• The 11 codepoint in the ECN field MUST indicate PCN-marked (though this does not exclude the 01 Experimental codepoint from carrying the same meaning).
• Once set, the 11 codepoint in the ECN field MUST NOT be changed to any other codepoint.
• Any experimental scheme MUST include details of all valid and invalid codepoint transitions at any PCN nodes.
• Any experimental scheme MUST NOT update the meaning of the 00 and 11 codepoints defined above.

6.  Backwards Compatibility

BCP 124 [RFC4774] (Floyd, S., “Specifying Alternate Semantics for the Explicit Congestion Notification (ECN) Field,” November 2006.) gives guidelines for specifying alternative semantics for the ECN field. It sets out a number of factors to be taken into consideration. It also suggests various techniques to allow the co-existence of default ECN and alternative ECN semantics. The baseline encoding specified in this document defines PCN-compatible Diffserv codepoints as no longer supporting the default ECN semantics. As such this document is compatible with BCP 124. It should be noted that this baseline encoding effectively disables end-to-end ECN unless mechanisms are put in place to tunnel such traffic across the PCN-domain. Standard IP-in-IP or IPsec tunnels will always copy the CE codepoint from the outer header into the inner header in decapsulation (unless the inner packet is not-ECT). If an operator wishes to allow ECN to exist end-to-end they must ensure there are no tunnel end-points within the PCN-domain to prevent any risk of PCN-markings being exposed to endpoints.

7.  IANA Considerations

This document makes no direct request to IANA.

8.  Security Considerations

PCN-marking only carries a meaning within the confines of a PCN-domain. Packets wishing to be treated as belonging to a PCN-flow must carry a PCN-compatible DSCP and a PCN-Enabled ECN codepoint. This encoding document is intended to stand independently of the architecture used to determine how specific packets are authorised to be PCN-marked, which will be described in separate documents on PCN-boundary-node behaviour.

This document assumes the PCN-domain to be entirely under the control of a single operator, or a set of operators who trust each other. However future extensions to PCN might include inter-domain versions where trust cannot be assumed between domains. If such schemes are proposed they must ensure that they can operate securely despite the lack of trust. However such considerations are beyond the scope of this document.

9.  Conclusions

This document defines the baseline PCN encoding utilising a combination of a PCN-enabled DSCP and the ECN field in the IP header. This baseline encoding allows the existence of two PCN encoding states, not-Marked and PCN-marked. It also allows for the co-existence of competing traffic within the same DSCP so long as that traffic does not require ECN support within the PCN-domain. The encoding scheme is conformant with [RFC4774] (Floyd, S., “Specifying Alternate Semantics for the Explicit Congestion Notification (ECN) Field,” November 2006.). The Working Group has chosen not to define a single DSCP for use with PCN. The rationale for this decision along with advice relating to choice of suitable DSCPs can be found in Appendix A.1 (Choice of Suitable DSCPs).

10.  Acknowledgements

This document builds extensively on work done in the PCN working group by Kwok Ho Chan, Georgios Karagiannis, Philip Eardley, Anna Charny, Joe Babiarz and others. Thanks to Ruediger Geib and Gorry Fairhurst for providing detailed comments on this document.

11.  Comments Solicited

(To be removed by the RFC-Editor.) Comments and questions are encouraged and very welcome. They can be addressed to the IETF congestion and pre-congestion working group mailing list <pcn@ietf.org>, and/or to the authors.

12.  References

12.1. Normative References

12.2. Informative References

Appendix A.  PCN Deployment Considerations (Informational)

A.1.  Choice of Suitable DSCPs

The PCN Working Group chose not to define a single DSCP for use with PCN for several reasons. Firstly the PCN mechanism is applicable to a variety of different traffic classes. Secondly standards track DSCPs are in increasingly short supply. Thirdly PCN should be seen as being essentially a marking behaviour similar to ECN but intended for inelastic traffic. The choice of which DSCP is most suitable for a given PCN-domain is dependent on the nature of the traffic entering that domain and the link rates of all the links making up that domain. In PCN-domains with uniformly high link rates, the appropriate DSCPs would currently be those for the Real Time Traffic Class [RFC5127] (Chan, K., Babiarz, J., and F. Baker, “Aggregation of DiffServ Service Classes,” February 2008.). To be clear the PCN Working Group recommends using admission control for the following service classes:

• Telephony (EF)
• Real-time interactive (CS4)
• Broadcast Video (CS3)
• Multimedia Conferencing (AF4)

PCN marking is intended to provide a scalable admission control mechanism for traffic with a high degree of statistical multiplexing. PCN marking would therefore be appropriate to apply to traffic in the above classes, but only within a PCN region containing highly aggregated traffic. In such cases, the above service classes may well all be subject to a single forwarding treatment (treatment aggregate [RFC5127] (Chan, K., Babiarz, J., and F. Baker, “Aggregation of DiffServ Service Classes,” February 2008.)). However, this does not imply all such IP traffic would necessarily be identified by one DSCP - each service class might keep a distinct DSCP within the highly aggregated region [RFC5127] (Chan, K., Babiarz, J., and F. Baker, “Aggregation of DiffServ Service Classes,” February 2008.).

Additional service classes may be defined for which admission control is appropriate, whether through some future standards action or through local use by certain operators, e.g. the Multimedia Streaming service class (AF3). This document does not preclude the use of PCN in more cases than those listed above.

NOTE: The above discussion is informative not normative, as operators are ultimately free to decide whether to use admission control for certain service classes and whether to use PCN as their mechanism of choice.

A.2.  Rationale for Using ECT(0) for Not-marked

The choice of which ECT codepoint to use for the Not-marked state was based on the following considerations:

• [RFC3168] (Ramakrishnan, K., Floyd, S., and D. Black, “The Addition of Explicit Congestion Notification (ECN) to IP,” September 2001.) full functionality tunnel within the PCN-domain: Either ECT is safe.
• Leakage of traffic into PCN-domain: because of the lack of take-up of the ECN nonce [RFC3540] (Spring, N., Wetherall, D., and D. Ely, “Robust Explicit Congestion Notification (ECN) Signaling with Nonces,” June 2003.), leakage of ECT(1) is less likely to occur so might be considered safer.
• Leakage of traffic out of PCN-domain: Either ECT is equally unsafe (since this would incorrectly indicate the traffic was ECN-capable outside the controlled PCN-domain).
• Incremental deployment: Either codepoint is suitable providing that the codepoints are used consistently.
• Conceptual consistency with other schemes: ECT(0) is conceptually consistent with [RFC3168] (Ramakrishnan, K., Floyd, S., and D. Black, “The Addition of Explicit Congestion Notification (ECN) to IP,” September 2001.).

Overall this seemed to suggest ECT(0) was most appropriate to use.

Authors' Addresses