Traffic Selector for Internet Key Exchange version 2 to add support Differentiated Services Field Codepoints (DSCP)

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Table of Contents

1. Introduction

[RFC4301] does not include Differentiated Services Field Codepoints (DSCP) as Traffic Selectors (TS). [RFC4301], Section 4.1 acknowledges that aggregating traffic with mutliple DSCP over the same SA may result in inappropriate discarding of lower priority packets due to the windowing mechanism used by this feature. However, to address such concern, [RFC4301], Section 4.1 recommends the sender implements a "classifier" mechanism which dispatches the traffic over multiple SAs.¶

Such "classifier" results in inbound and outbound traffic may take SA negotiated via different IKEv2 sessions and thus makes SA management more complex with an unnecessary SAs. This causes both a resource issue - especially with hardware implementations that are designed with a limited number of SAs - as well operational and management issues.
Typically, if the DSCP values are negotiated the initiator and the responder can agree to send a set of DSCP value over one SA and another set of DSCP value over a second channel. If DSCP values are not agreed and between (for example) 2 SAs, it is unlikely the initiator and the responder miraculously select the same subset of DSCP values over the same SAs. Instead each peer is likely that inbound and outbound traffic take different SA and as such does not solve the issue of discarding lower priority packets associated to different class of traffic sharing a given SA. This makes traffic management at least much harder as if not impossible. Increasing the number of SAs as to lower the traffic rate over each of these SA might reduce the probability of packet being dropped, but is not deterministic and as such cannot be considered as a solution especially when considering hardware with a hard limitation on the number of SAs.¶

This document specifies a new Traffic Selector Type TS_DSCP for IKEv2 that can be used to negotiate DSCP as additional selectors for the Security Policy Database (SPD) to further restrict the type of traffic allowed to be sent and received over the IPsec SA.¶

This document follows the clarification between Traffic Selector and Traffic Selector payload (TS) described in [I-D.ietf-ipsecme-labeled-ipsec], Section 1.2 and uses TS only to designate the TSi/TSr payload. This document uses TS_DSCP to designates the TS_TYPE value of the Traffic Selector payload with a specific TS_TYPE set to TS_DSCP.¶

2. TS_DSCP Traffic Selector Type

This document defines a new TS_TYPE, TS_DSCP that contains a list of opaque DSCP value.¶

3. Traffic Selector negotiation

TS_DSCP MUST MUST be used along with an IP address selector type such as TS_IPV4_ADDR_RANGE and/or TS_IPV6_ADDR_RANGE. If this condition is not met an TS_UNACCEPTABLE Error Notify message MUST be returned.¶

If the TS contains a TS_DSCP along with another TS_TYPE, the responder MUST create each TS response for the Traffic Selector of TS_TYPE TS_IPV4_ADDR_RANGE or TS_IPV6_ADDR_RANGE, using its normal rules specified for each of those TS_TYPE. The responder includes the acceptable DSCP values. These values will apply to all Traffic Selectors mentioned in the resulting TS. If this is not possible, it MUST return a TS_UNACCEPTABLE Error Notify payload.¶

The responder MAY respond select a subset of the DSCP values sent by the initiator and send a TS_DSCP payload or omit that TS_DSCP payload. If the responder provides the TS_DSCP, the initiator creates the SA with the specified subset of DSCP values.¶

If the subset of DSCP values do no match those of the initiator, this may indicate the responder's policy might be stricter regarding DSCP values. The initiator created the Child SA with the subset of DSCP values. The initiator SHOULD (upon local configuration) try to negotiate a separate SA associated to the missing DSCP values. The other selectors of different TS_TYPE SHOULD take the same values as the initial ones.¶

If the TS_DSCP is omitted the initiator (upon local configuration) MAY accept the response in which case DSCP is not considered as a traffic selector, that is to say all DSCP values are considered matching the policy. On the other hand, the initiator (upon local configuration) MAY also reject the offer and send a Delete Notify Payload.¶

If the responder returns a TS_UNACCEPTABLE Error Notify Payload, this might result in the responder not supporting TS_DSCP - though discarding the TS_DSCP would have been more appropriated. The initiator (upon local configuration) MAY restart the IKE negotiation with the same TSi/Tsr but removing the TS_DSCP.¶

4. IANA Considerations

IANA is requested to allocate two values in the "IKEv2 Traffic Selector Types" registry (available at https://www.iana.org/assignments/ikev2-parameters/ikev2-parameters.xhtml#ikev2-parameters-16) with the following definition:¶

+=======+======================+
| Value | TS Type  | REFERENCE |
+=======+ =====================+
| TBD1  | TS_DSCP  | This-RFC  |
+-------+----------------------+

5. Security Considerations

A packet that matches an SPD entry for all components except the DSCP values would be treated as "not matching". If no other SPD entries match, the traffic might end up being transmitted in the clear.¶

It is not different from ensuring that IP traffic is not sent in clear text and it is presumed that the IPsec subsystem itself would install a REJECT/DISCARD rule in the SPD to prevent that traffic from being transmitted without IPsec protection.¶

To avoid such situation, it is RECOMMENDED to introduce a SP to does not consider the DSCP values after those SP specifying the DSCP. This is very similar to placing a default SP that protects all traffic by default.¶

Upon receiving a TS_UNACCEPTABLE Error Notify or an incorrect response, the initiator MAY retry the IKEv2 negotiation without specifying the DSCP values. The initiator MAY handle the DSCP value on its own for outbound traffic, but MUST be prepared to receive any DSCP values from the responder.¶

6. Acknowledgements

Appendix A. Illustrative Example

The example shows a negotiation where each TSi / TSr are negotiating different values of DSCP. The purpose of this example is to show this is theoretically feasible, but we currently expect that TS_DSCP_LIST1 and TS_DSCP_LIST2 have the same values.¶

``` Initiator Responder ------------------------------------------------------------------- HDR, SK {N(REKEY_SA), SA, Ni, [KEi,] TSi, TSr} --> with: TSi = ( TS_IPV6_ADDR_RANGE, TS_DSCP_LIST1 ) TSr = ( TS_IPV6_ADDR_RANGE, TS_DSCP_LIST2 )¶

                            <--  HDR, SK {SA, Nr, [KEr,]
                                     TSi, TSr}
with:
  TSi = ( TS_IPV6_ADDR_RANGE, TS_DSCP_LIST1 )
  TSr = ( TS_IPV6_ADDR_RANGE, TS_DSCP_LIST2 ) ```

Authors' Addresses