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Internet Engineering Task Force (IETF)
Request for Comments: 6040
Updates: 3168, 4301, 4774
Category: Standards Track
ISSN: 2070-1721
B. Briscoe
BT
November 2010

Tunnelling of Explicit Congestion Notification

Abstract

This document redefines how the explicit congestion notification (ECN) field of the IP header should be constructed on entry to and exit from any IP-in-IP tunnel. On encapsulation, it updates RFC 3168 to bring all IP-in-IP tunnels (v4 or v6) into line with RFC 4301 IPsec ECN processing. On decapsulation, it updates both RFC 3168 and RFC 4301 to add new behaviours for previously unused combinations of inner and outer headers. The new rules ensure the ECN field is correctly propagated across a tunnel whether it is used to signal one or two severity levels of congestion; whereas before, only one severity level was supported. Tunnel endpoints can be updated in any order without affecting pre-existing uses of the ECN field, thus ensuring backward compatibility. Nonetheless, operators wanting to support two severity levels (e.g., for pre-congestion notification -- PCN) can require compliance with this new specification. A thorough analysis of the reasoning for these changes and the implications is included. In the unlikely event that the new rules do not meet a specific need, RFC 4774 gives guidance on designing alternate ECN semantics, and this document extends that to include tunnelling issues.

Status of This Memo

This is an Internet Standards Track document.

This document is a product of the Internet Engineering Task Force (IETF). It represents the consensus of the IETF community. It has received public review and has been approved for publication by the Internet Engineering Steering Group (IESG). Further information on Internet Standards is available in Section 2 of RFC 5741.

Information about the current status of this document, any errata, and how to provide feedback on it may be obtained at http://www.rfc-editor.org/info/rfc6040.

Copyright Notice

Copyright © 2010 IETF Trust and the persons identified as the document authors. All rights reserved.

This document is subject to BCP 78 and the IETF Trust's Legal Provisions Relating to IETF Documents (http://trustee.ietf.org/license-info) in effect on the date of publication of this document. Please review these documents carefully, as they describe your rights and restrictions with respect to this document. Code Components extracted from this document must include Simplified BSD License text as described in Section 4.e of the Trust Legal Provisions and are provided without warranty as described in the Simplified BSD License.

Table of Contents

   1. Introduction ....................................................4
      1.1. Scope ......................................................5
   2. Terminology .....................................................6
   3. Summary of Pre-Existing RFCs ....................................7
      3.1. Encapsulation at Tunnel Ingress ............................7
      3.2. Decapsulation at Tunnel Egress .............................8
   4. New ECN Tunnelling Rules ........................................9
      4.1. Default Tunnel Ingress Behaviour ..........................10
      4.2. Default Tunnel Egress Behaviour ...........................10
      4.3. Encapsulation Modes .......................................12
      4.4. Single Mode of Decapsulation ..............................14
   5. Updates to Earlier RFCs ........................................15
      5.1. Changes to RFC 4301 ECN Processing ........................15
      5.2. Changes to RFC 3168 ECN Processing ........................16
      5.3. Motivation for Changes ....................................17
           5.3.1. Motivation for Changing Encapsulation ..............17
           5.3.2. Motivation for Changing Decapsulation ..............18
   6. Backward Compatibility .........................................21
      6.1. Non-Issues Updating Decapsulation .........................21
      6.2. Non-Update of RFC 4301 IPsec Encapsulation ................21
      6.3. Update to RFC 3168 Encapsulation ..........................22
   7. Design Principles for Alternate ECN Tunnelling Semantics .......22
   8. Security Considerations ........................................24
   9. Conclusions ....................................................26
   10. Acknowledgements ..............................................26
   11. References ....................................................27
      11.1. Normative References .....................................27
      11.2. Informative References ...................................27
   Appendix A.  Early ECN Tunnelling RFCs ............................29
   Appendix B.  Design Constraints ...................................29
     B.1.  Security Constraints ......................................29
     B.2.  Control Constraints .......................................31
     B.3.  Management Constraints ....................................32
   Appendix C.  Contribution to Congestion across a Tunnel ...........33
   Appendix D.  Compromise on Decap with ECT(1) Inner and ECT(0)
                Outer ................................................34
   Appendix E.  Open Issues ..........................................35

1. Introduction

Explicit congestion notification (ECN [RFC3168]) allows a forwarding element (e.g., a router) to notify the onset of congestion without having to drop packets. Instead, it can explicitly mark a proportion of packets in the two-bit ECN field in the IP header (Table 1 recaps the ECN codepoints).

The outer header of an IP packet can encapsulate one or more IP headers for tunnelling. A forwarding element using ECN to signify congestion will only mark the immediately visible outer IP header. When a tunnel decapsulator later removes this outer header, it follows rules to propagate congestion markings by combining the ECN fields of the inner and outer IP header into one outgoing IP header.

This document updates those rules for IPsec [RFC4301] and non-IPsec [RFC3168] tunnels to add new behaviours for previously unused combinations of inner and outer headers. It also updates the ingress behaviour of RFC 3168 tunnels to match that of RFC 4301 tunnels. Tunnel endpoints complying with the updated rules will be backward compatible when interworking with tunnel endpoints complying with RFC 4301, RFC 3168, or any earlier specification.

When ECN and its tunnelling was defined in RFC 3168, only the minimum necessary changes to the ECN field were propagated through tunnel endpoints -- just enough for the basic ECN mechanism to work. This was due to concerns that the ECN field might be toggled to communicate between a secure site and someone on the public Internet -- a covert channel. This was because a mutable field like ECN cannot be protected by IPsec's integrity mechanisms -- it has to be able to change as it traverses the Internet.

Nonetheless, the latest IPsec architecture [RFC4301] considered a bandwidth limit of two bits per packet on a covert channel to be a manageable risk. Therefore, for simplicity, an RFC 4301 ingress copied the whole ECN field to encapsulate a packet. RFC 4301 dispensed with the two modes of RFC 3168, one which partially copied the ECN field, and the other which blocked all propagation of ECN changes.

Unfortunately, this entirely reasonable sequence of standards actions resulted in a perverse outcome; non-IPsec tunnels (RFC 3168) blocked the two-bit covert channel, while IPsec tunnels (RFC 4301) did not -- at least not at the ingress. At the egress, both IPsec and non-IPsec tunnels still partially restricted propagation of the full ECN field.

The trigger for the changes in this document was the introduction of pre-congestion notification (PCN [RFC5670]) to the IETF Standards Track. PCN needs the ECN field to be copied at a tunnel ingress and it needs four states of congestion signalling to be propagated at the egress, but pre-existing tunnels only propagate three in the ECN field.

This document draws on currently unused (CU) combinations of inner and outer headers to add tunnelling of four-state congestion signalling to RFC 3168 and RFC 4301. Operators of tunnels who specifically want to support four states can require that all their tunnels comply with this specification. However, this is not a fork in the RFC series. It is an update that can be deployed first by those that need it, and subsequently by all tunnel endpoint implementations (RFC 4301, RFC 3168, RFC 2481, RFC 2401, RFC 2003), which can safely be updated to this new specification as part of general code maintenance. This will gradually add support for four congestion states to the Internet. Existing three state schemes will continue to work as before.

In fact, this document is the opposite of a fork. At the same time as supporting a fourth state, the opportunity has been taken to draw together divergent ECN tunnelling specifications into a single consistent behaviour, harmonising differences such as perverse covert channel treatment. Then, any tunnel can be deployed unilaterally, and it will support the full range of congestion control and management schemes without any modes or configuration. Further, any host or router can expect the ECN field to behave in the same way, whatever type of tunnel might intervene in the path.

1.1. Scope

This document only concerns wire protocol processing of the ECN field at tunnel endpoints and makes no changes or recommendations concerning algorithms for congestion marking or congestion response.

This document specifies common ECN field processing at encapsulation and decapsulation for any IP-in-IP tunnelling, whether IPsec or non- IPsec tunnels. It applies irrespective of whether IPv4 or IPv6 is used for either the inner or outer headers. It applies for packets with any destination address type, whether unicast or multicast. It applies as the default for all Diffserv per-hop behaviours (PHBs), unless stated otherwise in the specification of a PHB (but Section 4 strongly deprecates such exceptions). It is intended to be a good trade off between somewhat conflicting security, control, and management requirements.

[RFC2983] is a comprehensive primer on differentiated services and tunnels. Given ECN raises similar issues to differentiated services when interacting with tunnels, useful concepts introduced in RFC 2983 are used throughout, with brief recaps of the explanations where necessary.

2. Terminology

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 RFC 2119 [RFC2119].

Table 1 recaps the names of the ECN codepoints [RFC3168]. 

     +------------------+----------------+---------------------------+
     | Binary codepoint | Codepoint name | Meaning                   |
     +------------------+----------------+---------------------------+
     |        00        | Not-ECT        | Not ECN-capable transport |
     |        01        | ECT(1)         | ECN-capable transport     |
     |        10        | ECT(0)         | ECN-capable transport     |
     |        11        | CE             | Congestion experienced    |
     +------------------+----------------+---------------------------+

Table 1: Recap of Codepoints of the ECN Field [RFC3168]

in the IP Header

Further terminology used within this document: 

   Encapsulator:  The tunnel endpoint function that adds an outer IP
      header to tunnel a packet (also termed the 'ingress tunnel
      endpoint' or just the 'ingress' where the context is clear).
   
   Decapsulator:  The tunnel endpoint function that removes an outer IP
      header from a tunnelled packet (also termed the 'egress tunnel
      endpoint' or just the 'egress' where the context is clear).
   
   Incoming header:  The header of an arriving packet before
      encapsulation.
   
   Outer header:  The header added to encapsulate a tunnelled packet.
   
   Inner header:  The header encapsulated by the outer header.
   
   Outgoing header:  The header constructed by the decapsulator using
      logic that combines the fields in the outer and inner headers.
   
   Copying ECN:  On encapsulation, setting the ECN field of the new
      outer header to be a copy of the ECN field in the incoming header.
   
   Zeroing ECN:  On encapsulation, clearing the ECN field of the new
      outer header to Not-ECT ("00").
   
   Resetting ECN:  On encapsulation, setting the ECN field of the new
      outer header to be a copy of the ECN field in the incoming header
      except the outer ECN field is set to the ECT(0) codepoint if the
      incoming ECN field is CE.

3. Summary of Pre-Existing RFCs

This section is informative not normative, as it recaps pre-existing RFCs. Earlier relevant RFCs that were either Experimental or incomplete with respect to ECN tunnelling (RFC 2481, RFC 2401, and RFC 2003) are briefly outlined in Appendix A. The question of whether tunnel implementations used in the Internet comply with any of these RFCs is not discussed.

3.1. Encapsulation at Tunnel Ingress

At the encapsulator, the controversy has been over whether to propagate information about congestion experienced on the path so far into the outer header of the tunnel.

Specifically, RFC 3168 says that, if a tunnel fully supports ECN (termed a 'full-functionality' ECN tunnel in [RFC3168]), the encapsulator must not copy a CE marking from the incoming header into the outer header that it creates. Instead, the encapsulator must set the outer header to ECT(0) if the ECN field is marked CE in the arriving IP header. We term this 'resetting' a CE codepoint.

However, the new IPsec architecture in [RFC4301] reverses this rule, stating that the encapsulator must simply copy the ECN field from the incoming header to the outer header.

RFC 3168 also provided a Limited Functionality mode that turns off ECN processing over the scope of the tunnel by setting the outer header to Not-ECT ("00"). Then, such packets will be dropped to indicate congestion, rather than marked with ECN. This is necessary for the ingress to interwork with legacy decapsulators ([RFC2481], [RFC2401], and [RFC2003]) that do not propagate ECN markings added to the outer header. Otherwise, such legacy decapsulators would throw away congestion notifications before they reached the transport layer.

Neither Limited Functionality mode nor Full Functionality mode are used by an RFC 4301 IPsec encapsulator, which simply copies the incoming ECN field into the outer header. An earlier key-exchange phase ensures an RFC 4301 ingress will not have to interwork with a legacy egress that does not support ECN.

These pre-existing behaviours are summarised in Figure 1. 

    +-----------------+-----------------------------------------------+
    | Incoming Header |             Departing Outer Header            |
    | (also equal to  +---------------+---------------+---------------+
    | departing Inner |  RFC 3168 ECN |  RFC 3168 ECN | RFC 4301 IPsec|
    |     Header)     |    Limited    |     Full      |               |
    |                 | Functionality | Functionality |               |
    +-----------------+---------------+---------------+---------------+
    |    Not-ECT      |   Not-ECT     |   Not-ECT     |   Not-ECT     |
    |     ECT(0)      |   Not-ECT     |    ECT(0)     |    ECT(0)     |
    |     ECT(1)      |   Not-ECT     |    ECT(1)     |    ECT(1)     |
    |       CE        |   Not-ECT     |    ECT(0)     |      CE       |
    +-----------------+---------------+---------------+---------------+
    
    Figure 1: IP-in-IP Encapsulation: Recap of Pre-Existing Behaviours

3.2. Decapsulation at Tunnel Egress

RFC 3168 and RFC 4301 specify the decapsulation behaviour summarised in Figure 2. The ECN field in the outgoing header is set to the codepoint at the intersection of the appropriate arriving inner header (row) and arriving outer header (column). 

            +---------+------------------------------------------------+
            |Arriving |            Arriving Outer Header               |
            |   Inner +---------+------------+------------+------------+
            |  Header | Not-ECT | ECT(0)     | ECT(1)     |     CE     |
            +---------+---------+------------+------------+------------+
  RFC 3168->| Not-ECT | Not-ECT |Not-ECT     |Not-ECT     |  <drop>    |
  RFC 4301->| Not-ECT | Not-ECT |Not-ECT     |Not-ECT     |Not-ECT     |
            |  ECT(0) |  ECT(0) | ECT(0)     | ECT(0)     |     CE     |
            |  ECT(1) |  ECT(1) | ECT(1)     | ECT(1)     |     CE     |
            |    CE   |      CE |     CE     |     CE     |     CE     |
            +---------+---------+------------+------------+------------+

In pre-existing RFCs, the ECN field in the outgoing header was set to the codepoint at the intersection of the appropriate arriving inner header (row) and arriving outer header (column), or the packet was dropped where indicated.

Figure 2: IP in IP Decapsulation; Recap of Pre-Existing Behaviour

The behaviour in the table derives from the logic given in RFC 3168 and RFC 4301, briefly recapped as follows:

  • On decapsulation, if the inner ECN field is Not-ECT the outer is ignored. RFC 3168 (but not RFC 4301) also specified that the decapsulator must drop a packet with a Not-ECT inner and CE in the outer.
  • In all other cases, if the outer is CE, the outgoing ECN field is set to CE; otherwise, the outer is ignored and the inner is used for the outgoing ECN field.

Section 9.2.2 of RFC 3168 also made it an auditable event for an IPsec tunnel "if the ECN Field is changed inappropriately within an IPsec tunnel...". Inappropriate changes were not specifically enumerated. RFC 4301 did not mention inappropriate ECN changes.

4. New ECN Tunnelling Rules

The standards actions below in Section 4.1 (ingress encapsulation) and Section 4.2 (egress decapsulation) define new default ECN tunnel processing rules for any IP packet (v4 or v6) with any Diffserv codepoint.

If these defaults do not meet a particular requirement, an alternate ECN tunnelling scheme can be introduced as part of the definition of an alternate congestion marking scheme used by a specific Diffserv PHB (see [RFC4774] and Section 5 of [RFC3168]). When designing such alternate ECN tunnelling schemes, the principles in Section 7 should be followed. However, alternate ECN tunnelling schemes SHOULD be avoided whenever possible as the deployment burden of handling exceptional PHBs in implementations of all affected tunnels should not be underestimated. There is no requirement for a PHB definition to state anything about ECN tunnelling behaviour if the default behaviour in the present specification is sufficient.

4.1. Default Tunnel Ingress Behaviour

Two modes of encapsulation are defined here; a REQUIRED 'normal mode' and a 'compatibility mode', which is for backward compatibility with tunnel decapsulators that do not understand ECN. Note that these are modes of the ingress tunnel endpoint only, not the whole tunnel. Section 4.3 explains why two modes are necessary and specifies the circumstances in which it is sufficient to solely implement normal mode.

Whatever the mode, an encapsulator forwards the inner header without changing the ECN field.

In normal mode, an encapsulator compliant with this specification MUST construct the outer encapsulating IP header by copying the two-bit ECN field of the incoming IP header. In compatibility mode, it clears the ECN field in the outer header to the Not-ECT codepoint (the IPv4 header checksum also changes whenever the ECN field is changed). These rules are tabulated for convenience in Figure 3. 

            +-----------------+-------------------------------+
            | Incoming Header |    Departing Outer Header     |
            | (also equal to  +---------------+---------------+
            | departing Inner | Compatibility |    Normal     |
            |     Header)     |     Mode      |     Mode      |
            +-----------------+---------------+---------------+
            |    Not-ECT      |   Not-ECT     |   Not-ECT     |
            |     ECT(0)      |   Not-ECT     |    ECT(0)     |
            |     ECT(1)      |   Not-ECT     |    ECT(1)     |
            |       CE        |   Not-ECT     |      CE       |
            +-----------------+---------------+---------------+

Figure 3: New IP in IP Encapsulation Behaviours

4.2. Default Tunnel Egress Behaviour

To decapsulate the inner header at the tunnel egress, a compliant tunnel egress MUST set the outgoing ECN field to the codepoint at the intersection of the appropriate arriving inner header (row) and outer header (column) in Figure 4 (the IPv4 header checksum also changes whenever the ECN field is changed). There is no need for more than one mode of decapsulation, as these rules cater for all known requirements. 

            +---------+------------------------------------------------+
            |Arriving |            Arriving Outer Header               |
            |   Inner +---------+------------+------------+------------+
            |  Header | Not-ECT | ECT(0)     | ECT(1)     |     CE     |
            +---------+---------+------------+------------+------------+
            | Not-ECT | Not-ECT |Not-ECT(!!!)|Not-ECT(!!!)| <drop>(!!!)|
            |  ECT(0) |  ECT(0) | ECT(0)     | ECT(1)     |     CE     |
            |  ECT(1) |  ECT(1) | ECT(1) (!) | ECT(1)     |     CE     |
            |    CE   |      CE |     CE     |     CE(!!!)|     CE     |
            +---------+---------+------------+------------+------------+
   
    The ECN field in the outgoing header is set to the codepoint at the
      intersection of the appropriate arriving inner header (row) and
      arriving outer header (column), or the packet is dropped where
   indicated.  Currently unused combinations are indicated by '(!!!)' or
                                   '(!)'

Figure 4: New IP in IP Decapsulation Behaviour

This table for decapsulation behaviour is derived from the following logic:

  • If the inner ECN field is Not-ECT, the decapsulator MUST NOT propagate any other ECN codepoint onwards. This is because the inner Not-ECT marking is set by transports that rely on dropped packets as an indication of congestion and would not understand or respond to any other ECN codepoint [RFC4774]. Specifically:
  • If the inner ECN field is Not-ECT and the outer ECN field is CE, the decapsulator MUST drop the packet.
  • If the inner ECN field is Not-ECT and the outer ECN field is Not-ECT, ECT(0), or ECT(1), the decapsulator MUST forward the outgoing packet with the ECN field cleared to Not-ECT.
  • In all other cases where the inner supports ECN, the decapsulator MUST set the outgoing ECN field to the more severe marking of the outer and inner ECN fields, where the ranking of severity from highest to lowest is CE, ECT(1), ECT(0), Not-ECT. This in no way precludes cases where ECT(1) and ECT(0) have the same severity;
  • Certain combinations of inner and outer ECN fields cannot result from any transition in any current or previous ECN tunneling specification. These currently unused (CU) combinations are indicated in Figure 4 by '(!!!)' or '(!)', where '(!!!)' means the combination is CU and always potentially dangerous, while '(!)' means it is CU and possibly dangerous. In these cases, particularly the more dangerous ones, the decapsulator SHOULD log the event and MAY also raise an alarm.

Just because the highlighted combinations are currently unused, does not mean that all the other combinations are always valid. Some are only valid if they have arrived from a particular type of legacy ingress, and dangerous otherwise. Therefore, an implementation MAY allow an operator to configure logging and alarms for such additional header combinations known to be dangerous or CU for the particular configuration of tunnel endpoints deployed at run-time.

Alarms SHOULD be rate-limited so that the anomalous combinations will not amplify into a flood of alarm messages. It MUST be possible to suppress alarms or logging, e.g., if it becomes apparent that a combination that previously was not used has started to be used for legitimate purposes such as a new standards action.

The above logic allows for ECT(0) and ECT(1) to both represent the same severity of congestion marking (e.g., "not congestion marked"). But it also allows future schemes to be defined where ECT(1) is a more severe marking than ECT(0), in particular, enabling the simplest possible encoding for PCN [PCN3in1] (see Section 5.3.2). Treating ECT(1) as either the same as ECT(0) or as a higher severity level is explained in the discussion of the ECN nonce [RFC3540] in Section 8, which in turn refers to Appendix D.

4.3. Encapsulation Modes

Section 4.1 introduces two encapsulation modes: normal mode, and compatibility mode, defining their encapsulation behaviour (i.e., header copying or zeroing, respectively). Note that these are modes of the ingress tunnel endpoint only, not the tunnel as a whole.

To comply with this specification, a tunnel ingress MUST at least implement normal mode. Unless it will never be used with legacy tunnel egress nodes (RFC 2003, RFC 2401, or RFC 2481 or the limited functionality mode of RFC 3168), an ingress MUST also implement compatibility mode for backward compatibility with tunnel egresses that do not propagate explicit congestion notifications [RFC4774].

We can categorise the way that an ingress tunnel endpoint is paired with an egress as either static or dynamically discovered: 

   Static:  Tunnel endpoints paired together by prior configuration.

Some implementations of encapsulator might always be statically deployed, and constrained to never be paired with a legacy decapsulator (RFC 2003, RFC 2401 or RFC 2481 or the limited functionality mode of RFC 3168). In such a case, only normal mode needs to be implemented.

For instance, IPsec tunnel endpoints compatible with RFC 4301 invariably use Internet Key Exchange Protocol version 2 (IKEv2) [RFC5996] for key exchange, the original specification of which was introduced alongside RFC 4301. Therefore, both endpoints of an RFC 4301 tunnel can be sure that the other end is compatible with RFC 4301, because the tunnel is only formed after IKEv2 key management has completed, at which point both ends will be compliant with RFC 4301 by definition. Therefore an IPsec tunnel ingress does not need compatibility mode, as it will never interact with legacy ECN tunnels. To comply with the present specification, it only needs to implement the required normal mode, which is identical to the pre-existing RFC 4301 behaviour. 

   Dynamic Discovery:  Tunnel endpoints paired together by some form of
      tunnel endpoint discovery, typically finding an egress on the path
      taken by the first packet.

This specification does not require or recommend dynamic discovery and it does not define how dynamic negotiation might be done, but it recognises that proprietary tunnel endpoint discovery protocols exist. It therefore sets down some constraints on discovery protocols to ensure safe interworking.

If dynamic tunnel endpoint discovery might pair an ingress with a legacy egress (RFC 2003, RFC 2401, or RFC 2481 or the limited functionality mode of RFC 3168), the ingress MUST implement both normal and compatibility mode. If the tunnel discovery process is arranged to only ever find a tunnel egress that propagates ECN (RFC 3168 full functionality mode, RFC 4301, or this present specification), then a tunnel ingress can be compliant with the present specification without implementing compatibility mode.

While a compliant tunnel ingress is discovering an egress, it MUST send packets in compatibility mode in case the egress it discovers is a legacy egress. If, through the discovery protocol, the egress indicates that it is compliant with the present specification, with RFC 4301 or with RFC 3168 full functionality mode, the ingress can switch itself into normal mode. If the egress denies compliance with any of these or returns an error that implies it does not understand a request to work to any of these ECN specifications, the tunnel ingress MUST remain in compatibility mode.

If an ingress claims compliance with this specification, it MUST NOT permanently disable ECN processing across the tunnel (i.e., only using compatibility mode). It is true that such a tunnel ingress is at least safe with the ECN behaviour of any egress it may encounter, but it does not meet the central aim of this specification: introducing ECN support to tunnels.

Instead, if the ingress knows that the egress does support propagation of ECN (full functionality mode of RFC 3168 or RFC 4301 or the present specification), it SHOULD use normal mode, in order to support ECN where possible. Note that this section started by saying an ingress "MUST implement" normal mode, while it has just said an ingress "SHOULD use" normal mode. This distinction is deliberate, to allow the mode to be turned off in exceptional circumstances but to ensure all implementations make normal mode available. 

   Implementation note:  If a compliant node is the ingress for multiple
      tunnels, a mode setting will need to be stored for each tunnel
      ingress.  However, if a node is the egress for multiple tunnels,
      none of the tunnels will need to store a mode setting, because a
      compliant egress only needs one mode.

4.4. Single Mode of Decapsulation

A compliant decapsulator only needs one mode of operation. However, if a compliant egress is implemented to be dynamically discoverable, it may need to respond to discovery requests from various types of legacy tunnel ingress. This specification does not define how dynamic negotiation might be done by (proprietary) discovery protocols, but it sets down some constraints to ensure safe interworking.

Through the discovery protocol, a tunnel ingress compliant with the present specification might ask if the egress is compliant with the present specification, with RFC 4301 or with RFC 3168 full functionality mode. Or an RFC 3168 tunnel ingress might try to negotiate to use limited functionality or full functionality mode [RFC3168]. In all these cases, a decapsulating tunnel egress compliant with this specification MUST agree to any of these requests, since it will behave identically in all these cases.

If no ECN-related mode is requested, a compliant tunnel egress MUST continue without raising any error or warning, because its egress behaviour is compatible with all the legacy ingress behaviours that do not negotiate capabilities.

A compliant tunnel egress SHOULD raise a warning alarm about any requests to enter modes it does not recognise but, for 'forward compatibility' with standards actions possibly defined after it was implemented, it SHOULD continue operating.

5. Updates to Earlier RFCs

5.1. Changes to RFC 4301 ECN Processing

   Ingress:  An RFC 4301 IPsec encapsulator is not changed at all by the
      present specification.  It uses the normal mode of the present
      specification, which defines packet encapsulation identically to
      RFC 4301.
   
   Egress:  An RFC 4301 egress will need to be updated to the new
      decapsulation behaviour in Figure 4, in order to comply with the
      present specification.  However, the changes are backward
      compatible; combinations of inner and outer that result from any
      protocol defined in the RFC series so far are unaffected.  Only
      combinations that have never been used have been changed,
      effectively adding new behaviours to RFC 4301 decapsulation
      without altering existing behaviours.  The following specific
      updates to Section 5.1.2 of RFC 4301 have been made:
  • The outer, not the inner, is propagated when the outer is ECT(1) and the inner is ECT(0);
  • A packet with Not-ECT in the inner and an outer of CE is dropped rather than forwarded as Not-ECT;
  • Certain combinations of inner and outer ECN field have been identified as currently unused. These can trigger logging and/or raise alarms.
   Modes:  RFC 4301 tunnel endpoints do not need modes and are not
      updated by the modes in the present specification.  Effectively,
      an RFC 4301 IPsec ingress solely uses the REQUIRED normal mode of
      encapsulation, which is unchanged from RFC 4301 encapsulation.  It
      will never need the OPTIONAL compatibility mode as explained in
      Section 4.3.

5.2. Changes to RFC 3168 ECN Processing

   Ingress:  On encapsulation, the new rule in Figure 3 that a normal
      mode tunnel ingress copies any ECN field into the outer header
      updates the full functionality behaviour of an RFC 3168 ingress
      (Section 9.1.1 of [RFC3168]).  Nonetheless, the new compatibility
      mode encapsulates packets identically to the limited functionality
      mode of an RFC 3168 ingress.
   
   Egress:  An RFC 3168 egress will need to be updated to the new
      decapsulation behaviour in Figure 4, in order to comply with the
      present specification.  However, the changes are backward
      compatible; combinations of inner and outer that result from any
      protocol defined in the RFC series so far are unaffected.  Only
      combinations that have never been used have been changed,
      effectively adding new behaviours to RFC 3168 decapsulation
      without altering existing behaviours.  The following specific
      updates to Section 9.1.1 of RFC 3168 have been made:
  • The outer, not the inner, is propagated when the outer is ECT(1) and the inner is ECT(0);
  • Certain combinations of inner and outer ECN field have been identified as currently unused. These can trigger logging and/or raise alarms.
   Modes:  An RFC 3168 ingress will need to be updated if it is to
      comply with the present specification, whether or not it
      implemented the optional full functionality mode of Section 9.1.1
      of RFC 3168.

Section 9.1 of RFC 3168 defined a (required) limited functionality mode and an (optional) full functionality mode for a tunnel. In RFC 3168, modes applied to both ends of the tunnel, while in the present specification, modes are only used at the ingress -- a single egress behaviour covers all cases.

The normal mode of encapsulation is an update to the encapsulation behaviour of the full functionality mode of an RFC 3168 ingress. The compatibility mode of encapsulation is identical to the encapsulation behaviour of the limited functionality mode of an RFC 3168 ingress, except it is not always obligatory.

The constraints on how tunnel discovery protocols set modes in Sections 4.3 and 4.4 are an update to RFC 3168, but they are unlikely to require code changes as they document existing safe practice.

5.3. Motivation for Changes

An overriding goal is to ensure the same ECN signals can mean the same thing whatever tunnels happen to encapsulate an IP packet flow. This removes gratuitous inconsistency, which otherwise constrains the available design space and makes it harder to design networks and new protocols that work predictably.

5.3.1. Motivation for Changing Encapsulation

The normal mode in Section 4 updates RFC 3168 to make all IP-in-IP encapsulation of the ECN field consistent -- consistent with the way both RFC 4301 IPsec [RFC4301] and IP-in-MPLS or MPLS-in-MPLS encapsulation [RFC5129] construct the ECN field.

Compatibility mode has also been defined so that an ingress compliant with a version of IPsec prior to RFC 4301 can still switch to using drop across a tunnel for backward compatibility with legacy decapsulators that do not propagate ECN.

The trigger that motivated this update to RFC 3168 encapsulation was a Standards-Track proposal for pre-congestion notification (PCN [RFC5670]). PCN excess-traffic-marking only works correctly if the ECN field is copied on encapsulation (as in RFC 4301 and RFC 5129); it does not work if ECN is reset (as in RFC 3168). This is because PCN excess-traffic-marking depends on the outer header revealing any congestion experienced so far on the whole path, not just since the last tunnel ingress.

PCN allows a network operator to add flow admission and termination for inelastic traffic at the edges of a Diffserv domain, but without any per-flow mechanisms in the interior and without the generous provisioning typical of Diffserv, aiming to significantly reduce costs. The PCN architecture [RFC5559] states that RFC 3168 IP-in-IP tunnelling of the ECN field cannot be used for any tunnel ingress in a PCN domain. Prior to the present specification, this left a stark choice between not being able to use PCN for inelastic traffic control or not being able to use the many tunnels already deployed for Mobile IP, VPNs, and so forth.

The present specification provides a clean solution to this problem, so that network operators who want to use both PCN and tunnels can specify that every tunnel ingress in a PCN region must comply with this latest specification.

Rather than allow tunnel specifications to fragment further into one for PCN, one for IPsec, and one for other tunnels, the opportunity has been taken to consolidate the diverging specifications back into a single tunnelling behaviour. Resetting ECN was originally motivated by a covert channel concern that has been deliberately set aside in RFC 4301 IPsec. Therefore, the reset behaviour of RFC 3168 is an anomaly that we do not need to keep. Copying ECN on encapsulation is simpler than resetting. So, as more tunnel endpoints comply with this single consistent specification, encapsulation will be simpler as well as more predictable.

Appendix B assesses whether copying rather than resetting CE on ingress will cause any unintended side effects, from the three perspectives of security, control, and management. In summary, this analysis finds that:

  • From the control perspective, either copying or resetting works for existing arrangements, but copying has more potential for simplifying control and resetting breaks at least one proposal that is already on the Standards Track.
  • From the management and monitoring perspective, copying is preferable.
  • From the traffic security perspective (enforcing congestion control, mitigating denial of service, etc.), copying is preferable.
  • From the information security perspective, resetting is preferable, but the IETF Security Area now considers copying acceptable given the bandwidth of a two-bit covert channel can be managed.

Therefore, there are two points against resetting CE on ingress while copying CE causes no significant harm.

5.3.2. Motivation for Changing Decapsulation

The specification for decapsulation in Section 4 fixes three problems with the pre-existing behaviours found in both RFC 3168 and RFC 4301:

  1. The pre-existing rules prevented the introduction of alternate ECN semantics to signal more than one severity level of congestion [RFC4774], [RFC5559]. The four states of the two-bit ECN field provide room for signalling two severity levels in addition to not-congested and not-ECN-capable states. But, the pre-existing rules assumed that two of the states (ECT(0) and ECT(1)) are always equivalent. This unnecessarily restricts the use of one of four codepoints (half a bit) in the IP (v4 and v6) header. The new rules are designed to work in either case; whether ECT(1) is more severe than or equivalent to ECT(0).

As explained in Appendix B.1, the original reason for not forwarding the outer ECT codepoints was to limit the covert channel across a decapsulator to 1 bit per packet. However, now that the IETF Security Area has deemed that a two-bit covert channel through an encapsulator is a manageable risk, the same should be true for a decapsulator.

As well as being useful for general future-proofing, this problem is immediately pressing for standardisation of pre-congestion notification (PCN), which uses two severity levels of congestion. If a congested queue used ECT(1) in the outer header to signal more severe congestion than ECT(0), the pre-existing decapsulation rules would have thrown away this congestion signal, preventing tunnelled traffic from ever knowing that it should reduce its load.

Before the present specification was written, the PCN working group had to consider a number of wasteful or convoluted work- rounds to this problem. Without wishing to disparage the ingenuity of these work-rounds, none were chosen for the Standards Track because they were either somewhat wasteful, imprecise, or complicated. Instead, a baseline PCN encoding was specified [RFC5696] that supported only one severity level of congestion but allowed space for these work-rounds as experimental extensions.

By far the simplest approach is that taken by the current specification: just to remove the covert channel blockages from tunnelling behaviour -- now deemed unnecessary anyway. Then, network operators that want to support two congestion severity levels for PCN can specify that every tunnel egress in a PCN region must comply with this latest specification. Having taken this step, the simplest possible encoding for PCN with two severity levels of congestion [PCN3in1] can be used.

Not only does this make two congestion severity levels available for PCN, but also for other potential uses of the extra ECN codepoint (e.g., [VCP]). 

   2.  Cases are documented where a middlebox (e.g., a firewall) drops
       packets with header values that were currently unused (CU) when
       the box was deployed, often on the grounds that anything
       unexpected might be an attack.  This tends to bar future use of
       CU values.  The new decapsulation rules specify optional logging
       and/or alarms for specific combinations of inner and outer
       headers that are currently unused.  The aim is to give
       implementers a recourse other than drop if they are concerned
       about the security of CU values.  It recognises legitimate

security concerns about CU values, but still eases their future use. If the alarms are interpreted as an attack (e.g., by a management system) the offending packets can be dropped. However, alarms can be turned off if these combinations come into regular use (e.g., through a future standards action).

  1. While reviewing currently unused combinations of inner and outer headers, the opportunity was taken to define a single consistent behaviour for the three cases with a Not-ECT inner header but a different outer. RFC 3168 and RFC 4301 had diverged in this respect and even their common behaviours had never been justified.

None of these combinations should result from Internet protocols in the RFC series, but future standards actions might put any or all of them to good use. Therefore, it was decided that a decapsulator must forward a Not-ECT inner header unchanged when the arriving outer header is ECT(0) or ECT(1). For safety, it must drop a combination of Not-ECT inner and CE outer headers. Then, if some unfortunate misconfiguration resulted in a congested router marking CE on a packet that was originally Not-ECT, drop would be the only appropriate signal for the egress to propagate -- the only signal a non-ECN-capable transport (Not-ECT) would understand.

It may seem contradictory that the same argument has not been applied to the ECT(1) codepoint, given it is being proposed as an intermediate level of congestion in a scheme progressing through the IETF [PCN3in1]. Instead, a decapsulator must forward a Not-ECT inner unchanged when its outer is ECT(1). The rationale for not dropping this CU combination is to ensure it will be usable if needed in the future. If any misconfiguration led to ECT(1) congestion signals with a Not-ECT inner, it would not be disastrous for the tunnel egress to suppress them, because the congestion should then escalate to CE marking, which the egress would drop, thus at least preventing congestion collapse.

Problems 2 and 3 alone would not warrant a change to decapsulation, but it was decided they are worth fixing and making consistent at the same time as decapsulation code is changed to fix problem 1 (two congestion severity levels).

6. Backward Compatibility

A tunnel endpoint compliant with the present specification is backward compatible when paired with any tunnel endpoint compliant with any previous tunnelling RFC, whether RFC 4301, RFC 3168 (see Section 3), or the earlier RFCs summarised in Appendix A (RFC 2481, RFC 2401, and RFC 2003). Each case is enumerated below.

6.1. Non-Issues Updating Decapsulation

At the egress, this specification only augments the per-packet calculation of the ECN field (RFC 3168 and RFC 4301) for combinations of inner and outer headers that have so far not been used in any IETF protocols.

Therefore, all other things being equal, if an RFC 4301 IPsec egress is updated to comply with the new rules, it will still interwork with any ingress compliant with RFC 4301 and the packet outputs will be identical to those it would have output before (fully backward compatible).

And, all other things being equal, if an RFC 3168 egress is updated to comply with the same new rules, it will still interwork with any ingress complying with any previous specification (both modes of RFC 3168, both modes of RFC 2481, RFC 2401, and RFC 2003) and the packet outputs will be identical to those it would have output before (fully backward compatible).

A compliant tunnel egress merely needs to implement the one behaviour in Section 4 with no additional mode or option configuration at the ingress or egress nor any additional negotiation with the ingress. The new decapsulation rules have been defined in such a way that congestion control will still work safely if any of the earlier versions of ECN processing are used unilaterally at the encapsulating ingress of the tunnel (any of RFC 2003, RFC 2401, either mode of RFC 2481, either mode of RFC 3168, RFC 4301, and this present specification).

6.2. Non-Update of RFC 4301 IPsec Encapsulation

An RFC 4301 IPsec ingress can comply with this new specification without any update and it has no need for any new modes, options, or configuration. So, all other things being equal, it will continue to interwork identically with any egress it worked with before (fully backward compatible).

6.3. Update to RFC 3168 Encapsulation

The encapsulation behaviour of the new normal mode copies the ECN field, whereas an RFC 3168 ingress in full functionality mode reset it. However, all other things being equal, if an RFC 3168 ingress is updated to the present specification, the outgoing packets from any tunnel egress will still be unchanged. This is because all variants of tunnelling at either end (RFC 4301, both modes of RFC 3168, both modes of RFC 2481, RFC 2401, RFC 2003, and the present specification) have always propagated an incoming CE marking through the inner header and onward into the outgoing header; whether the outer header is reset or copied. Therefore, if the tunnel is considered a black box, the packets output from any egress will be identical with or without an update to the ingress. Nonetheless, if packets are observed within the black box (between the tunnel endpoints), CE markings copied by the updated ingress will be visible within the black box, whereas they would not have been before. Therefore, the update to encapsulation can be termed 'black-box backward compatible' (i.e., identical unless you look inside the tunnel).

This specification introduces no new backward compatibility issues when a compliant ingress talks with a legacy egress, but it has to provide similar safeguards to those already defined in RFC 3168. RFC 3168 laid down rules to ensure that an RFC 3168 ingress turns off ECN (limited functionality mode) if it is paired with a legacy egress (RFC 2481, RFC 2401, or RFC 2003), which would not propagate ECN correctly. The present specification carries forward those rules (Section 4.3). It uses compatibility mode whenever RFC 3168 would have used limited functionality mode, and their per-packet behaviours are identical. Therefore, all other things being equal, an ingress using the new rules will interwork with any legacy tunnel egress in exactly the same way as an RFC 3168 ingress (still black-box backward compatible).

7. Design Principles for Alternate ECN Tunnelling Semantics

This section is informative, not normative.

Section 5 of RFC 3168 permits the Diffserv codepoint (DSCP)[RFC2474] to 'switch in' alternative behaviours for marking the ECN field, just as it switches in different per-hop behaviours (PHBs) for scheduling. [RFC4774] gives best current practice for designing such alternative ECN semantics and very briefly mentions in Section 5.4 that tunnelling needs to be considered. The guidance below complements and extends RFC 4774, giving additional guidance on designing any alternate ECN semantics that would also require alternate tunnelling semantics.

The overriding guidance is: "Avoid designing alternate ECN tunnelling semantics, if at all possible". If a scheme requires tunnels to implement special processing of the ECN field for certain DSCPs, it will be hard to guarantee that every implementer of every tunnel will have added the required exception or that operators will have ubiquitously deployed the required updates. It is unlikely a single authority is even aware of all the tunnels in a network, which may include tunnels set up by applications between endpoints, or dynamically created in the network. Therefore, it is highly likely that some tunnels within a network or on hosts connected to it will not implement the required special case.

That said, if a non-default scheme for tunnelling the ECN field is really required, the following guidelines might prove useful in its design:

On encapsulation in any alternate scheme:

  1. The ECN field of the outer header ought to be cleared to Not- ECT ("00") unless it is guaranteed that the corresponding tunnel egress will correctly propagate congestion markings introduced across the tunnel in the outer header.
  1. If it has established that ECN will be correctly propagated, an encapsulator also ought to copy incoming congestion notification into the outer header. The general principle here is that the outer header should reflect congestion accumulated along the whole upstream path, not just since the tunnel ingress (Appendix B.3 on management and monitoring explains).

In some circumstances (e.g., PCN [RFC5559] and perhaps some pseudowires [RFC5659]), the whole path is divided into segments, each with its own congestion notification and feedback loop. In these cases, the function that regulates load at the start of each segment will need to reset congestion notification for its segment. Often, the point where congestion notification is reset will also be located at the start of a tunnel. However, the resetting function can be thought of as being applied to packets after the encapsulation function -- two logically separate functions even though they might run on the same physical box. Then, the code module doing encapsulation can keep to the copying rule and the load regulator module can reset congestion, without any code in either module being conditional on whether the other is there.

On decapsulation in any alternate scheme:

  1. If the arriving inner header is Not-ECT, the transport will not understand other ECN codepoints. If the outer header carries an explicit congestion marking, the alternate scheme would be expected to drop the packet -- the only indication of congestion the transport will understand. If the alternate scheme recommends forwarding rather than dropping such a packet, it will need to clearly justify this decision. If the inner is Not-ECT and the outer carries any other ECN codepoint that does not indicate congestion, the alternate scheme can forward the packet, but probably only as Not-ECT.
  1. If the arriving inner header is one other than Not-ECT, the ECN field that the alternate decapsulation scheme forwards ought to reflect the more severe congestion marking of the arriving inner and outer headers.
  1. Any alternate scheme will need to define a behaviour for all combinations of inner and outer headers, even those that would not be expected to result from standards known at the time and even those that would not be expected from the tunnel ingress paired with the egress at run-time. Consideration should be given to logging such unexpected combinations and raising an alarm, particularly if there is a danger that the invalid combination implies congestion signals are not being propagated correctly. The presence of currently unused combinations may represent an attack, but the new scheme should try to define a way to forward such packets, at least if a safe outgoing codepoint can be defined.

Raising an alarm allows a management system to decide whether the anomaly is indeed an attack, in which case it can decide to drop such packets. This is a preferable approach to hard- coded discard of packets that seem anomalous today, but may be needed tomorrow in future standards actions.

8. Security Considerations

Appendix B.1 discusses the security constraints imposed on ECN tunnel processing. The new rules for ECN tunnel processing (Section 4) trade-off between information security (covert channels) and traffic security (congestion monitoring and control). Ensuring congestion markings are not lost is itself an aspect of security, because if we allowed congestion notification to be lost, any attempt to enforce a response to congestion would be much harder.

Security issues in unlikely, but possible, scenarios:

Tunnels intersecting Diffserv regions with alternate ECN semantics:

If alternate congestion notification semantics are defined for a certain Diffserv PHB, the scope of the alternate semantics might typically be bounded by the limits of a Diffserv region or regions, as envisaged in [RFC4774] (e.g., the pre-congestion notification architecture [RFC5559]). The inner headers in tunnels crossing the boundary of such a Diffserv region but ending within the region can potentially leak the external congestion notification semantics into the region, or leak the internal semantics out of the region. [RFC2983] discusses the need for Diffserv traffic conditioning to be applied at these tunnel endpoints as if they are at the edge of the Diffserv region. Similar concerns apply to any processing or propagation of the ECN field at the endpoints of tunnels with one end inside and the other outside the domain. [RFC5559] gives specific advice on this for the PCN case, but other definitions of alternate semantics will need to discuss the specific security implications in each case. 

   ECN nonce tunnel coverage:  The new decapsulation rules improve the
   
      coverage of the ECN nonce [RFC3540] relative to the previous rules
      in RFC 3168 and RFC 4301.  However, nonce coverage is still not
      perfect, as this would have led to a safety problem in another
      case.  Both are corner-cases, so discussion of the compromise
      between them is deferred to Appendix D.
   
   Covert channel not turned off:  A legacy (RFC 3168) tunnel ingress
      could ask an RFC 3168 egress to turn off ECN processing as well as
      itself turning off ECN.  An egress compliant with the present
      specification will agree to such a request from a legacy ingress,
      but it relies on the ingress always sending Not-ECT in the outer
      header.  If the egress receives other ECN codepoints in the outer
      it will process them as normal, so it will actually still copy
      congestion markings from the outer to the outgoing header.
      Referring, for example, to Figure 5 (Appendix B.1), although the
      tunnel ingress 'I' will set all ECN fields in outer headers to
      Not-ECT, 'M' could still toggle CE or ECT(1) on and off to
      communicate covertly with 'B', because we have specified that 'E'
      only has one mode regardless of what mode it says it has
      negotiated.  We could have specified that 'E' should have a
      limited functionality mode and check for such behaviour.  However,
      we decided not to add the extra complexity of two modes on a
      compliant tunnel egress merely to cater for an historic security
      concern that is now considered manageable.

9. Conclusions

This document allows tunnels to propagate an extra level of congestion severity. It uses previously unused combinations of inner and outer headers to augment the rules for calculating the ECN field when decapsulating IP packets at the egress of IPsec (RFC 4301) and non-IPsec (RFC 3168) tunnels.

This document also updates the ingress tunnelling encapsulation of RFC 3168 ECN to bring all IP-in-IP tunnels into line with the new behaviour in the IPsec architecture of RFC 4301, which copies rather than resets the ECN field when creating outer headers.

The need for both these updated behaviours was triggered by the introduction of pre-congestion notification (PCN) onto the IETF Standards Track. Operators wanting to support PCN or other alternate ECN schemes that use an extra severity level can require that their tunnels comply with the present specification. This is not a fork in the RFC series, it is an update that can be deployed first by those that need it, and subsequently by all tunnel endpoint implementations during general code maintenance. It is backward compatible with all previous tunnelling behaviours, so existing single severity level schemes will continue to work as before, but support for two severity levels will gradually be added to the Internet.

The new rules propagate changes to the ECN field across tunnel endpoints that previously blocked them to restrict the bandwidth of a potential covert channel. Limiting the channel's bandwidth to two bits per packet is now considered sufficient.

At the same time as removing these legacy constraints, the opportunity has been taken to draw together diverging tunnel specifications into a single consistent behaviour. Then, any tunnel can be deployed unilaterally, and it will support the full range of congestion control and management schemes without any modes or configuration. Further, any host or router can expect the ECN field to behave in the same way, whatever type of tunnel might intervene in the path. This new certainty could enable new uses of the ECN field that would otherwise be confounded by ambiguity.

10. Acknowledgements

Thanks to David Black for his insightful reviews and patient explanations of better ways to think about function placement and alarms. Thanks to David and to Anil Agarwal for pointing out cases where it is safe to forward CU combinations of headers. Also, thanks to Arnaud Jacquet for the idea for Appendix C. Thanks to Gorry Fairhurst, Teco Boot, Michael Menth, Bruce Davie, Toby Moncaster,

Sally Floyd, Alfred Hoenes, Gabriele Corliano, Ingemar Johansson, Philip Eardley, and David Harrington for their thoughts and careful review comments, and to Stephen Hanna, Ben Campbell, and members of the IESG for respectively conducting the Security Directorate, General Area, and IESG reviews.

Bob Briscoe is partly funded by Trilogy, a research project (ICT- 216372) supported by the European Community under its Seventh Framework Programme.

11. References

11.1. Normative References

   [RFC2003]  Perkins, C., "IP Encapsulation within IP", RFC 2003,
              October 1996.
   
   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.
   
   [RFC3168]  Ramakrishnan, K., Floyd, S., and D. Black, "The Addition
              of Explicit Congestion Notification (ECN) to IP",
              RFC 3168, September 2001.
   
   [RFC4301]  Kent, S. and K. Seo, "Security Architecture for the
              Internet Protocol", RFC 4301, December 2005.

11.2. Informative References

   [PCN3in1]  Briscoe, B., Moncaster, T., and M. Menth, "Encoding 3 PCN-
              States in the IP header using a single DSCP", Work
              in Progress, July 2010.
   
   [RFC2401]  Kent, S. and R. Atkinson, "Security Architecture for the
              Internet Protocol", RFC 2401, November 1998.
   
   [RFC2474]  Nichols, K., Blake, S., Baker, F., and D. Black,
              "Definition of the Differentiated Services Field (DS
              Field) in the IPv4 and IPv6 Headers", RFC 2474,
              December 1998.
   
   [RFC2481]  Ramakrishnan, K. and S. Floyd, "A Proposal to add Explicit
              Congestion Notification (ECN) to IP", RFC 2481,
              January 1999.
   
   [RFC2983]  Black, D., "Differentiated Services and Tunnels",
              RFC 2983, October 2000.
   
   [RFC3540]  Spring, N., Wetherall, D., and D. Ely, "Robust Explicit
              Congestion Notification (ECN) Signaling with Nonces",
              RFC 3540, June 2003.
   
   [RFC4774]  Floyd, S., "Specifying Alternate Semantics for the
              Explicit Congestion Notification (ECN) Field", BCP 124,
              RFC 4774, November 2006.
   
   [RFC5129]  Davie, B., Briscoe, B., and J. Tay, "Explicit Congestion
              Marking in MPLS", RFC 5129, January 2008.
   
   [RFC5559]  Eardley, P., "Pre-Congestion Notification (PCN)
              Architecture", RFC 5559, June 2009.
   
   [RFC5659]  Bocci, M. and S. Bryant, "An Architecture for Multi-
              Segment Pseudowire Emulation Edge-to-Edge", RFC 5659,
              October 2009.
   
   [RFC5670]  Eardley, P., "Metering and Marking Behaviour of PCN-
              Nodes", RFC 5670, November 2009.
   
   [RFC5696]  Moncaster, T., Briscoe, B., and M. Menth, "Baseline
              Encoding and Transport of Pre-Congestion Information",
              RFC 5696, November 2009.
   
   [RFC5996]  Kaufman, C., Hoffman, P., Nir, Y., and P. Eronen,
              "Internet Key Exchange Protocol Version 2 (IKEv2)",
              RFC 5996, September 2010.
   
   [VCP]      Xia, Y., Subramanian, L., Stoica, I., and S. Kalyanaraman,
              "One more bit is enough", Proc. SIGCOMM'05, ACM
              CCR 35(4)37--48, 2005,
              <http://doi.acm.org/10.1145/1080091.1080098>.

Appendix A. Early ECN Tunnelling RFCs

IP-in-IP tunnelling was originally defined in [RFC2003]. On encapsulation, the incoming header was copied to the outer and on decapsulation, the outer was simply discarded. Initially, IPsec tunnelling [RFC2401] followed the same behaviour.

When ECN was introduced experimentally in [RFC2481], legacy (RFC 2003 or RFC 2401) tunnels would have discarded any congestion markings added to the outer header, so RFC 2481 introduced rules for calculating the outgoing header from a combination of the inner and outer on decapsulation. RFC 2481 also introduced a second mode for IPsec tunnels, which turned off ECN processing (Not-ECT) in the outer header on encapsulation because an RFC 2401 decapsulator would discard the outer on decapsulation. For RFC 2401 IPsec, this had the side effect of completely blocking the covert channel.

In RFC 2481, the ECN field was defined as two separate bits. But when ECN moved from Experimental to Standards Track [RFC3168], the ECN field was redefined as four codepoints. This required a different calculation of the ECN field from that used in RFC 2481 on decapsulation. RFC 3168 also had two modes; a 'full functionality mode' that restricted the covert channel as much as possible but still allowed ECN to be used with IPsec, and another that completely turned off ECN processing across the tunnel. This 'limited functionality mode' both offered a way for operators to completely block the covert channel and allowed an RFC 3168 ingress to interwork with a legacy tunnel egress (RFC 2481, RFC 2401, or RFC 2003).

The present specification includes a similar compatibility mode to interwork safely with tunnels compliant with any of these three earlier RFCs. However, unlike RFC 3168, it is only a mode of the ingress, as decapsulation behaviour is the same in either case.

Appendix B. Design Constraints

Tunnel processing of a congestion notification field has to meet congestion control and management needs without creating new information security vulnerabilities (if information security is required). This appendix documents the analysis of the trade-offs between these factors that led to the new encapsulation rules in Section 4.1.

B.1. Security Constraints

Information security can be assured by using various end-to-end security solutions (including IPsec in transport mode [RFC4301]), but a commonly used scenario involves the need to communicate between two physically protected domains across the public Internet. In this case, there are certain management advantages to using IPsec in tunnel mode solely across the publicly accessible part of the path. The path followed by a packet then crosses security 'domains'; the ones protected by physical or other means before and after the tunnel and the one protected by an IPsec tunnel across the otherwise unprotected domain. The scenario in Figure 5 will be used where endpoints 'A' and 'B' communicate through a tunnel. The tunnel ingress 'I' and egress 'E' are within physically protected edge domains, while the tunnel spans an unprotected internetwork where there may be 'men in the middle', M. 

                physically       unprotected     physically
            <-protected domain-><--domain--><-protected domain->
            +------------------+            +------------------+
            |                  |      M     |                  |
            |    A-------->I=========>==========>E-------->B   |
            |                  |            |                  |
            +------------------+            +------------------+
                           <----IPsec secured---->
                                   tunnel

Figure 5: IPsec Tunnel Scenario

IPsec encryption is typically used to prevent 'M' seeing messages from 'A' to 'B'. IPsec authentication is used to prevent 'M' masquerading as the sender of messages from 'A' to 'B' or altering their contents. 'I' can use IPsec tunnel mode to allow 'A' to communicate with 'B', but impose encryption to prevent 'A' leaking information to 'M'. Or 'E' can insist that 'I' uses tunnel mode authentication to prevent 'M' communicating information to 'B'.

Mutable IP header fields such as the ECN field (as well as the Time to Live (TTL) / Hop Limit and DS fields) cannot be included in the cryptographic calculations of IPsec. Therefore, if 'I' copies these mutable fields into the outer header that is exposed across the tunnel it will have allowed a covert channel from 'A' to 'M' that bypasses its encryption of the inner header. And if 'E' copies these fields from the outer header to the outgoing, even if it validates authentication from 'I', it will have allowed a covert channel from 'M' to 'B'.

ECN at the IP layer is designed to carry information about congestion from a congested resource towards downstream nodes. Typically, a downstream transport might feed the information back somehow to the point upstream of the congestion that can regulate the load on the congested resource, but other actions are possible [RFC3168], Section 6. In terms of the above unicast scenario, ECN effectively intends to create an information channel (for congestion signalling) from 'M' to 'B' (for 'B' to feed back to 'A'). Therefore, the goals of IPsec and ECN are mutually incompatible, requiring some compromise.

With respect to using the DS or ECN fields as covert channels, Section 5.1.2 of RFC 4301 says, "controls are provided to manage the bandwidth of this channel". Using the ECN processing rules of RFC 4301, the channel bandwidth is two bits per datagram from 'A' to 'M' and one bit per datagram from 'M' to 'B' (because 'E' limits the combinations of the 2-bit ECN field that it will copy). In both cases, the covert channel bandwidth is further reduced by noise from any real congestion marking. RFC 4301 implies that these covert channels are sufficiently limited to be considered a manageable threat. However, with respect to the larger (six-bit) DS field, the same section of RFC 4301 says not copying is the default, but a configuration option can allow copying "to allow a local administrator to decide whether the covert channel provided by copying these bits outweighs the benefits of copying". Of course, an administrator who plans to copy the DS field has to take into account that it could be concatenated with the ECN field, creating a covert channel with eight bits per datagram.

For tunnelling the six-bit Diffserv field, two conceptual models have had to be defined so that administrators can trade off security against the needs of traffic conditioning [RFC2983]: 

   The uniform model:  where the Diffserv field is preserved end-to-end
   
      by copying into the outer header on encapsulation and copying from
      the outer header on decapsulation.
   
   The pipe model:  where the outer header is independent of that in the
      inner header so it hides the Diffserv field of the inner header
      from any interaction with nodes along the tunnel.

However, for ECN, the new IPsec security architecture in RFC 4301 only standardised one tunnelling model equivalent to the uniform model. It deemed that simplicity was more important than allowing administrators the option of a tiny increment in security, especially given not copying congestion indications could seriously harm everyone's network service.

B.2. Control Constraints

Congestion control requires that any congestion notification marked into packets by a resource will be able to traverse a feedback loop back to a function capable of controlling the load on that resource. To be precise, rather than calling this function the data source, it will be called the 'Load Regulator'. This allows for exceptional cases where load is not regulated by the data source, but usually the two terms will be synonymous. Note the term "a function _capable of_ controlling the load" deliberately includes a source application that doesn't actually control the load but ought to (e.g., an application without congestion control that uses UDP). 

                 A--->R--->I=========>M=========>E-------->B

Figure 6: Simple Tunnel Scenario

A similar tunnelling scenario to the IPsec one just described will now be considered, but without the different security domains, because the focus now shifts to whether the control loop and management monitoring work (Figure 6). If resources in the tunnel are to be able to explicitly notify congestion and the feedback path is from 'B' to 'A', it will certainly be necessary for 'E' to copy any CE marking from the outer header to the outgoing header for onward transmission to 'B'; otherwise, congestion notification from resources like 'M' cannot be fed back to the Load Regulator ('A'). But it does not seem necessary for 'I' to copy CE markings from the incoming to the outer header. For instance, if resource 'R' is congested, it can send congestion information to 'B' using the congestion field in the inner header without 'I' copying the congestion field into the outer header and 'E' copying it back to the outgoing header. 'E' can still write any additional congestion marking introduced across the tunnel into the congestion field of the outgoing header.

All this shows that 'E' can preserve the control loop irrespective of whether 'I' copies congestion notification into the outer header or resets it.

That is the situation for existing control arrangements but, because copying reveals more information, it would open up possibilities for better control system designs. For instance, resetting CE marking on encapsulation breaks the Standards-Track PCN congestion marking scheme [RFC5670]. It ends up removing excessive amounts of traffic unnecessarily (Section 5.3.1). Whereas copying CE markings at ingress leads to the correct control behaviour.

B.3. Management Constraints

As well as control, there are also management constraints. Specifically, a management system may monitor congestion markings in passing packets, perhaps at the border between networks as part of a service level agreement. For instance, monitors at the borders of autonomous systems may need to measure how much congestion has accumulated so far along the path, perhaps to determine between them how much of the congestion is contributed by each domain.

In this document, the baseline of congestion marking (or the Congestion Baseline) is defined as the source of the layer that created (or most recently reset) the congestion notification field. When monitoring congestion, it would be desirable if the Congestion Baseline did not depend on whether or not packets were tunnelled. Given some tunnels cross domain borders (e.g., consider 'M' in Figure 6 is monitoring a border), it would therefore be desirable for 'I' to copy congestion accumulated so far into the outer headers, so that it is exposed across the tunnel.

For management purposes, it might be useful for the tunnel egress to be able to monitor whether congestion occurred across a tunnel or upstream of it. Superficially, it appears that copying congestion markings at the ingress would make this difficult, whereas it was straightforward when an RFC 3168 ingress reset them. However, Appendix C gives a simple and precise method for a tunnel egress to infer the congestion level introduced across a tunnel. It works irrespective of whether the ingress copies or resets congestion markings.

Appendix C. Contribution to Congestion across a Tunnel

This specification mandates that a tunnel ingress determines the ECN field of each new outer tunnel header by copying the arriving header. Concern has been expressed that this will make it difficult for the tunnel egress to monitor congestion introduced only along a tunnel, which is easy if the outer ECN field is reset at a tunnel ingress (RFC 3168 full functionality mode). However, in fact copying CE marks at ingress will still make it easy for the egress to measure congestion introduced across a tunnel, as illustrated below.

Consider 100 packets measured at the egress. Say it measures that 30 are CE marked in the inner and outer headers and 12 have additional CE marks in the outer but not the inner. This means packets arriving at the ingress had already experienced 30% congestion. However, it does not mean there was 12% congestion across the tunnel. The correct calculation of congestion across the tunnel is p_t = 12/ (100-30) = 12/70 = 17%. This is easy for the egress to measure. It is simply the proportion of packets not marked in the inner header (70) that have a CE marking in the outer header (12). This technique works whether the ingress copies or resets CE markings, so it can be used by an egress that is not sure with which RFC the ingress complies.

Figure 7 illustrates this in a combinatorial probability diagram. The square represents 100 packets. The 30% division along the bottom represents marking before the ingress, and the p_t division up the side represents marking introduced across the tunnel. 

        ^ outer header marking
        |
   100% +-----+---------+       The large square
        |     |         |       represents 100 packets
        | 30  |         |
        |     |         |   p_t = 12/(100-30)
    p_t +     +---------+       = 12/70
        |     |   12    |       = 17%
      0 +-----+---------+--->
        0    30%       100%  inner header marking

Figure 7: Tunnel Marking of Packets Already Marked at Ingress

Appendix D. Compromise on Decap with ECT(1) Inner and ECT(0) Outer

A packet with an ECT(1) inner and an ECT(0) outer should never arise from any known IETF protocol. Without giving a reason, RFC 3168 and RFC 4301 both say the outer should be ignored when decapsulating such a packet. This appendix explains why it was decided not to change this advice.

In summary, ECT(0) always means 'not congested' and ECT(1) may imply the same [RFC3168] or it may imply a higher severity congestion signal [RFC4774], [PCN3in1], depending on the transport in use. Whether or not they mean the same, at the ingress the outer should have started the same as the inner, and only a broken or compromised router could have changed the outer to ECT(0).

The decapsulator can detect this anomaly. But the question is, should it correct the anomaly by ignoring the outer, or should it reveal the anomaly to the end-to-end transport by forwarding the outer?

On balance, it was decided that the decapsulator should correct the anomaly, but log the event and optionally raise an alarm. This is the safe action if ECT(1) is being used as a more severe marking than ECT(0), because it passes the more severe signal to the transport. However, it is not a good idea to hide anomalies, which is why an optional alarm is suggested. It should be noted that this anomaly may be the result of two changes to the outer: a broken or compromised router within the tunnel might be erasing congestion markings introduced earlier in the same tunnel by a congested router.

In this case, the anomaly would be losing congestion signals, which needs immediate attention.

The original reason for defining ECT(0) and ECT(1) as equivalent was so that the data source could use the ECN nonce [RFC3540] to detect if congestion signals were being erased. However, in this case, the decapsulator does not need a nonce to detect any anomalies introduced within the tunnel, because it has the inner as a record of the header at the ingress. Therefore, it was decided that the best compromise would be to give precedence to solving the safety issue over revealing the anomaly, because the anomaly could at least be detected and dealt with internally.

Superficially, the opposite case where the inner and outer carry different ECT values, but with an ECT(1) outer and ECT(0) inner, seems to require a similar compromise. However, because that case is reversed, no compromise is necessary; it is best to forward the outer whether the transport expects the ECT(1) to mean a higher severity than ECT(0) or the same severity. Forwarding the outer either preserves a higher value (if it is higher) or it reveals an anomaly to the transport (if the two ECT codepoints mean the same severity).

Appendix E. Open Issues

The new decapsulation behaviour defined in Section 4.2 adds support for propagation of two severity levels of congestion. However, transports have no way to discover whether there are any legacy tunnels on their path that will not propagate two severity levels. It would have been nice to add a feature for transports to check path support, but this remains an open issue that will have to be addressed in any future standards action to define an end-to-end scheme that requires two severity levels of congestion. PCN avoids this problem because it is only for a controlled region, so all legacy tunnels can be upgraded by the same operator that deploys PCN.

Author's Address

   Bob Briscoe
   BT
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   Martlesham Heath
   Ipswich  IP5 3RE
   UK
   
   Phone: +44 1473 645196
   EMail: bob.briscoe@bt.com
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