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Versions: 00 01 02 03 RFC 3168

Internet Engineering Task Force                       K. K. Ramakrishnan
INTERNET DRAFT                                        TeraOptic Networks
draft-ietf-tsvwg-ecn-01.txt                                  Sally Floyd
                                                                   ACIRI
                                                                D. Black
                                                                     EMC
                                                           January, 2001
                                                     Expires: July, 2001


      The Addition of Explicit Congestion Notification (ECN) to IP



                          Status of this Memo


   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-
   Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet- Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

Abstract

   This document specifies the incorporation of ECN (Explicit Congestion
   Notification) to TCP and IP, including ECN's use of two bits in the
   IP header.  We begin by describing TCP's use of packet drops as an
   indication of congestion.  Next we explain that with the addition of
   active queue management (e.g., RED) to the Internet infrastructure,
   where routers detect congestion before the queue overflows, routers
   are no longer limited to packet drops as an indication of congestion.
   Routers can instead set the Congestion Experienced (CE) bit in the IP
   header of packets from ECN-capable transports.  We describe when the
   CE bit is to be set in routers, and describe modifications needed to



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   TCP to make it ECN-capable.  Modifications to other transport
   protocols (e.g., unreliable unicast or multicast, reliable multicast,
   other reliable unicast transport protocols) could be considered as
   those protocols are developed and advance through the standards
   process.

   We also describe in this document the issues involving the use of ECN
   within IP tunnels, and within IPsec tunnels in particular.

   One of the guiding principles for this document is that all the
   mechanisms specified here are incrementally deployable.

Table of Contents






































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     1.  Introduction
     2.  Conventions and Acronyms
     3.  Assumptions and General Principles
     4.  Active Queue Management (AQM)
     5.  Explicit Congestion Notification in IP
     5.1.  ECN as an Indication of Persistent Congestion
     5.2.  Dropped or Corrupted Packets
     6.  Support from the Transport Protocol
     6.1.  TCP
     6.1.1.  TCP Initialization
     6.1.1.1.  Robust TCP Initialization with an Echoed Reserve Field
     6.1.2.  The TCP Sender
     6.1.3.  The TCP Receiver
     6.1.4.  Congestion on the ACK-path
     6.1.5.  Retransmitted TCP packets
     6.1.6.  TCP Window Probes.
     7.  Non-compliance by the End Nodes
     8.  Non-compliance in the Network
     8.1.  Complications Introduced by Split Paths
     9.  Encapsulated Packets
     9.1.  IP packets encapsulated in IP
     9.1.1.  The Limited-functionality and Full-functionality Options
     9.1.2.  Changes to the ECN Field within an IP Tunnel.
     9.2.  IPsec Tunnels
     9.2.1.  Negotiation between Tunnel Endpoints
     9.2.1.1.  ECN Tunnel Security Association Database Field
     9.2.1.2.  ECN Tunnel Security Association Attribute
     9.2.1.3.  Changes to IPsec Tunnel Header Processing
     9.2.2.  Changes to the ECN Field within an IPsec Tunnel.
     9.2.3.  Comments for IPsec Support
     9.3.  IP packets encapsulated in non-IP packet headers.
     10.  Issues Raised by Monitoring and Policing Devices
     11.  Evaluations of ECN
     12.  Summary of changes required in IP and TCP
     13.  Conclusions
     14.  Acknowledgements
     15.  References
     16.  Security Considerations
     17.  IPv4 Header Checksum Recalculation
     18.  Possible Changes to the ECN Field in the Network
     18.1.  Possible Changes to the IP Header
     18.1.1.  Erasing the Congestion Indication
     18.1.2.  Falsely Reporting Congestion
     18.1.3.  Disabling ECN-Capability
     18.1.4.  Falsely Indicating ECN-Capability
     18.1.5.  Changes with No Functional Effect
     18.2.  Information carried in the Transport Header
     18.3.  Split Paths



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     19.  Implications of Subverting End-to-End Congestion Control
     19.1.  Implications for the Network and for Competing Flows
     19.2.  Implications for the Subverted Flow
     19.3.  Non-ECN-Based Methods of Subverting End-to-end Congestion Control
     20.  The Motivation for the ECT bit.
     21.  Why use Two Bits in the IP Header?
     22.  Historical Definitions for the IPv4 TOS Octet
     23.  IANA Considerations


RFC EDITOR - REMOVE THE FOLLOWING PARAGRAPH ON PUBLICATION - To compare
this with draft-ietf-tsvwg-ecn-00, compare the following:
"http://www.aciri.org/floyd/papers/draft-ietf-tsvwg-ecn-00.troff"
"http://www.aciri.org/floyd/papers/draft-ietf-tsvwg-ecn-01.troff"
Changes from draft-ietf-tsvwg-ecn-00:
* Deleted Section 6.1.1.2. on "Robust TCP Initialization with no
response to the SYN", and modified the paragraph in the Conclusions
referring to this.
* Added Section 23 on IANA Considerations.
* Added two paragraphs to Section 18.2 on denial-of-service attacks.
* Added some text about the ECN nonce being a research issue.
* Moved two paragraphs about setting the CWR bit from Section 6.1.3 to
  Section 6.1.2.
* Various small changes:
  Adding several small clarifying sentences in Section 12, 22.
  Small clarification to text in Section 19.2.
  Deleted a few unnecessary sentences in Section 9.
  Updated some references to Section X.
  Added more references to RFC 2780.
  Deleted references to internet-drafts.
  Clarified terminology for "non-ECN-setup SYN packet", including the
following:  "Receivers MUST correctly handle all forms of the non-ECN-
setup SYN and SYN-ACK packets."

1.  Introduction

   TCP's congestion control and avoidance algorithms are based on the
   notion that the network is a black-box [Jacobson88, Jacobson90].  The
   network's state of congestion or otherwise is determined by end-sys-
   tems probing for the network state, by gradually increasing the load
   on the network (by increasing the window of packets that are out-
   standing in the network) until the network becomes congested and a
   packet is lost.  Treating the network as a "black-box" and treating
   loss as an indication of congestion in the network is appropriate for
   pure best-effort data carried by TCP, with little or no sensitivity
   to delay or loss of individual packets.  In addition, TCP's conges-
   tion management algorithms have techniques built-in (such as Fast
   Retransmit and Fast Recovery) to minimize the impact of losses, from



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   a throughput perspective.  However, these mechanisms are not intended
   to help applications that are in fact sensitive to the delay or loss
   of one or more individual packets.  Interactive traffic such as tel-
   net, web-browsing, and transfer of audio and video data can be sensi-
   tive to packet losses (especially when using an unreliable data
   delivery transport such as UDP) or to the increased latency of the
   packet caused by the need to retransmit the packet after a loss (with
   the reliable data delivery semantics provided by TCP).

   Since TCP determines the appropriate congestion window to use by
   gradually increasing the window size until it experiences a dropped
   packet, this causes the queues at the bottleneck router to build up.
   With most packet drop policies at the router that are not sensitive
   to the load placed by each individual flow (e.g., tail-drop on queue
   overflow), this means that some of the packets of latency-sensitive
   flows may be dropped. In addition, such drop policies lead to syn-
   chronization of loss across multiple flows.

   Active queue management mechanisms detect congestion before the queue
   overflows, and provide an indication of this congestion to the end
   nodes.  Thus, active queue management can reduce unnecessary queueing
   delay for all traffic sharing that queue.  The advantages of active
   queue management are discussed in RFC 2309 [RFC2309].  Active queue
   management avoids some of the bad properties of dropping on queue
   overflow, including the undesirable synchronization of loss across
   multiple flows.  More importantly, active queue management means that
   transport protocols with mechanisms for congestion control (e.g.,
   TCP) do not have to rely on buffer overflow as the only indication of
   congestion.

   Active queue management mechanisms may use one of several methods for
   indicating congestion to end-nodes. One is to use packet drops, as is
   currently done. However, active queue management allows the router to
   separate policies of queueing or dropping packets from the policies
   for indicating congestion. Thus, active queue management allows
   routers to use the Congestion Experienced (CE) bit in a packet header
   as an indication of congestion, instead of relying solely on packet
   drops. This has the potential of reducing the impact of loss on
   latency-sensitive flows.

   This document is intended to obsolete RFC 2481, "A Proposal to add
   Explicit Congestion Notification (ECN) to IP", which defined ECN as
   an Experimental Protocol for the Internet Community.

   RFC EDITOR - REMOVE THE FOLLOWING PARAGRAPH ON PUBLICATION - This
   document obsoletes three subsequent internet-drafts on ECN, "IPsec
   Interactions with ECN", "ECN Interactions with IP Tunnels", and "TCP
   with ECN: The Treatment of Retransmitted Data Packets".  This



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   document is intended largely to merge the earlier documents all into
   a single document, for greater clarity, in preparation to becoming a
   Proposed Standard.

2.  Conventions and Acronyms

   The keywords MUST, MUST NOT, REQUIRED, SHALL, SHALL NOT, SHOULD,
   SHOULD NOT, RECOMMENDED, MAY, and OPTIONAL, when they appear in this
   document, are to be interpreted as described in [B97].

3.  Assumptions and General Principles

   In this section, we describe some of the important design principles
   and assumptions that guided the design choices in this proposal.

   * Because ECN is likely to be adopted gradually, accommodating migra-
   tion is essential. Some routers may still only drop packets to indi-
   cate congestion, and some end-systems may not be ECN-capable. The
   most viable strategy is one that accommodates incremental deployment
   without having to resort to "islands" of ECN-capable and non-ECN-
   capable environments.
   * New mechanisms for congestion control and avoidance need to co-
   exist and cooperate with existing mechanisms for congestion control.
   In particular, new mechanisms have to co-exist with TCP's current
   methods of adapting to congestion and with routers' current practice
   of dropping packets in periods of congestion.
   * Congestion may persist over different time-scales. The time scales
   that we are concerned with are congestion events that may last longer
   than a round-trip time.
   * The number of packets in an individual flow (e.g., TCP connection
   or an exchange using UDP) may range from a small number of packets to
   quite a large number. We are interested in managing the congestion
   caused by flows that send enough packets so that they are still
   active when network feedback reaches them.
   * Asymmetric routing is likely to be a normal occurrence in the
   Internet. The path (sequence of links and routers) followed by data
   packets may be different from the path followed by the acknowledgment
   packets in the reverse direction.
   * Many routers process the "regular" headers in IP packets more effi-
   ciently than they process the header information in IP options.  This
   suggests keeping congestion experienced information in the regular
   headers of an IP packet.
   * It must be recognized that not all end-systems will cooperate in
   mechanisms for congestion control. However, new mechanisms shouldn't
   make it easier for TCP applications to disable TCP congestion con-
   trol.  The benefit of lying about participating in new mechanisms
   such as ECN-capability should be small.




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4.  Active Queue Management (AQM)

   Random Early Detection (RED) is one mechanism for Active Queue Man-
   agement (AQM) that has been proposed to detect incipient congestion
   [FJ93], and is currently being deployed in the Internet [RFC2309].
   AQM is meant to be a general mechanism using one of several alterna-
   tives for congestion indication, but in the absence of ECN, AQM is
   restricted to using packet drops as a mechanism for congestion indi-
   cation.  AQM drops packets based on the average queue length exceed-
   ing a threshold, rather than only when the queue overflows.  However,
   because AQM may drop packets before the queue actually overflows, AQM
   is not always forced by memory limitations to discard the packet.

   AQM can set a Congestion Experienced (CE) bit in the packet header
   instead of dropping the packet, when such a bit is provided in the IP
   header and understood by the transport protocol.  The use of the CE
   bit with ECN allows the receiver(s) to receive the packet, avoiding
   the potential for excessive delays due to retransmissions after
   packet losses.  We use the term 'CE packet' to denote a packet that
   has the CE bit set.

5.  Explicit Congestion Notification in IP

   This document specifies that the Internet provide a congestion indi-
   cation for incipient congestion (as in RED and earlier work [RJ90])
   where the notification can sometimes be through marking packets
   rather than dropping them.  This uses an ECN field in the IP header
   with two bits.  The ECN-Capable Transport (ECT) bit is set by the
   data sender to indicate that the end-points of the transport protocol
   are ECN-capable.  The CE bit is set by the router to indicate conges-
   tion to the end nodes.  Routers that have a packet arriving at a full
   queue drop the packet, just as they do in the absence of ECN.

   Bits 6 and 7 in the IPv4 TOS octet are designated as the ECN field.
   Bit 6 is designated as the ECT bit, and bit 7 is designated as the CE
   bit.  The IPv4 TOS octet corresponds to the Traffic Class octet in
   IPv6.  The definitions for the IPv4 TOS octet [RFC791] and the IPv6
   Traffic Class octet have been superseded by the six-bit DS (Differen-
   tiated Services) Field [RFC2474, RFC2780].  Bits 6 and 7 are listed
   in [RFC2474] as Currently Unused, and are specified in RFC 2780 as
   approved for experimental use for ECN.  Section 19 gives a brief his-
   tory of the TOS octet.









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            0     1     2     3     4     5     6     7
         +-----+-----+-----+-----+-----+-----+-----+-----+
         |             DS FIELD              | ECN FIELD |
         |                                   |           |
         |               DSCP                | ECT | CE  |
         +-----+-----+-----+-----+-----+-----+-----+-----+

           DSCP: differentiated services codepoint
           ECN:  Explicit Congestion Notification

          Figure 1: The Differentiated Services and ECN Fields in IP.

   Because of the unstable history of the TOS octet, the use of the ECN
   field as specified in this document cannot be guaranteed to be back-
   wards compatible with all past uses of these two bits.  The potential
   dangers of this lack of backwards compatibility are discussed in Sec-
   tion 19.

   Upon the receipt by an ECN-Capable transport of a single CE packet,
   the congestion control algorithms followed at the end-systems MUST be
   essentially the same as the congestion control response to a *single*
   dropped packet.  For example, for ECN-Capable TCP the source TCP is
   required to halve its congestion window for any window of data con-
   taining either a packet drop or an ECN indication.

   One reason for requiring that the congestion-control response to the
   CE packet be essentially the same as the response to a dropped packet
   is to accommodate the incremental deployment of ECN in both end-sys-
   tems and in routers.  Some routers may drop ECN-Capable packets
   (e.g., using the same AQM policies for congestion detection) while
   other routers set the CE bit, for equivalent levels of congestion.
   Similarly, a router might drop a non-ECN-Capable packet but set the
   CE bit in an ECN-Capable packet, for equivalent levels of congestion.
   If there were different congestion control responses to a CE bit
   indication than to a packet drop, this could result in unfair treat-
   ment for different flows.

   An additional goal is that the end-systems should react to congestion
   at most once per window of data (i.e., at most once per round-trip
   time), to avoid reacting multiple times to multiple indications of
   congestion within a round-trip time.

   For a router, the CE bit of an ECN-Capable packet should only be set
   if the router would otherwise have dropped the packet as an indica-
   tion of congestion to the end nodes. When the router's buffer is not
   yet full and the router is prepared to drop a packet to inform end
   nodes of incipient congestion, the router should first check to see
   if the ECT bit is set in that packet's IP header.  If so, then



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   instead of dropping the packet, the router MAY instead set the CE bit
   in the IP header.

   An environment where all end nodes were ECN-Capable could allow new
   criteria to be developed for setting the CE bit, and new congestion
   control mechanisms for end-node reaction to CE packets.  However,
   this is a research issue, and as such is not addressed in this docu-
   ment.

   When a CE packet (i.e., a packet that has the CE bit set) is received
   by a router, the CE bit is left unchanged, and the packet is trans-
   mitted as usual. When severe congestion has occurred and the router's
   queue is full, then the router has no choice but to drop some packet
   when a new packet arrives.  We anticipate that such packet losses
   will become relatively infrequent when a majority of end-systems
   become ECN-Capable and participate in TCP or other compatible conges-
   tion control mechanisms. In an ECN-Capable environment that is ade-
   quately-provisioned network, packet losses should occur primarily
   during transients or in the presence of non-cooperating sources.

   We expect that routers will set the CE bit in response to incipient
   congestion as indicated by the average queue size, using the RED
   algorithms suggested in [FJ93, RFC2309].  To the best of our knowl-
   edge, this is the only proposal currently under discussion in the
   IETF for routers to drop packets proactively, before the buffer over-
   flows.  However, this document does not attempt to specify a particu-
   lar mechanism for active queue management, leaving that endeavor, if
   needed, to other areas of the IETF.  While ECN is inextricably tied
   up with the need to have a reasonable active queue management mecha-
   nism at the router, the reverse does not hold; active queue manage-
   ment mechanisms have been developed and deployed independent of ECN,
   using packet drops as indications of congestion in the absence of ECN
   in the IP architecture.

5.1.  ECN as an Indication of Persistent Congestion

   We emphasize that a *single* packet with the CE bit set in an IP
   packet causes the transport layer to respond, in terms of congestion
   control, as it would to a packet drop.  The instantaneous queue size
   is likely to see considerable variations even when the router does
   not experience persistent congestion.  As such, it is important that
   transient congestion at a router, reflected by the instantaneous
   queue size reaching a threshold much smaller than the capacity of the
   queue, not trigger a reaction at the transport layer.  Therefore, the
   CE bit should not be set by a router based on the instantaneous queue
   size.

   For example, since the ATM and Frame Relay mechanisms for congestion



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   indication have typically been defined without an associated notion
   of average queue size as the basis for determining that an intermedi-
   ate node is congested, we believe that they provide a very noisy sig-
   nal. The TCP-sender reaction specified in this document for ECN is
   NOT the appropriate reaction for such a noisy signal of congestion
   notification.  However, if the routers that interface to the ATM net-
   work have a way of maintaining the average queue at the interface,
   and use it to come to a reliable determination that the ATM subnet is
   congested, they may use the ECN notification that is defined here.

   We continue to encourage experiments in techniques at layer 2 (e.g.,
   in ATM switches or Frame Relay switches) to take advantage of ECN.
   For example, using a scheme such as RED (where packet marking is
   based on the average queue length exceeding a threshold), layer 2
   devices could provide a reasonably reliable indication of congestion.
   When all the layer 2 devices in a path set that layer's own Conges-
   tion Experienced bit (e.g., the EFCI bit for ATM, the FECN bit in
   Frame Relay) in this reliable manner, then the interface router to
   the layer 2 network could copy the state of that layer 2 Congestion
   Experienced bit into the CE bit in the IP header.  We recognize that
   this is not the current practice, nor is it in current standards.
   However, encouraging experimentation in this manner may provide the
   information needed to enable evolution of existing layer 2 mechanisms
   to provide a more reliable means of congestion indication, when they
   use a single bit for indicating congestion.

5.2.  Dropped or Corrupted Packets

   For the proposed use for ECN in this document (that is, for a trans-
   port protocol such as TCP for which a dropped data packet is an indi-
   cation of congestion), end nodes detect dropped data packets, and the
   congestion response of the end nodes to a dropped data packet is at
   least as strong as the congestion response to a received CE packet.
   To ensure the reliable delivery of the congestion indication of the
   CE bit, the ECT bit MUST NOT be set in a packet unless the loss of
   that packet in the network would be detected by the end nodes and
   interpreted as an indication of congestion.

   Transport protocols such as TCP do not necessarily detect all packet
   drops, such as the drop of a "pure" ACK packet; for example, TCP does
   not reduce the arrival rate of subsequent ACK packets in response to
   an earlier dropped ACK packet.  Any proposal for extending ECN-Capa-
   bility to such packets would have to address issues such as the case
   of an ACK packet that was marked with the CE bit but was later
   dropped in the network. We believe that this aspect is still the sub-
   ject of research, so this document specifies that at this time,
   "pure" ACK packets MUST NOT indicate ECN-Capability.




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   Similarly, if a CE packet is dropped later in the network due to cor-
   ruption (bit errors), the end nodes should still invoke congestion
   control, just as TCP would today in response to a dropped data
   packet. This issue of corrupted CE packets would have to be consid-
   ered in any proposal for the network to distinguish between packets
   dropped due to corruption, and packets dropped due to congestion or
   buffer overflow.  In particular, the ubiquitous deployment of ECN
   would not, in and of itself, be a sufficient development to allow
   end-nodes to interpret packet drops as indications of corruption
   rather than congestion.

6.  Support from the Transport Protocol

   ECN requires support from the transport protocol, in addition to the
   functionality given by the ECN field in the IP packet header. The
   transport protocol might require negotiation between the endpoints
   during setup to determine that all of the endpoints are ECN-capable,
   so that the sender can set the ECT bit in transmitted packets.  Sec-
   ond, the transport protocol must be capable of reacting appropriately
   to the receipt of CE packets.  This reaction could be in the form of
   the data receiver informing the data sender of the received CE packet
   (e.g., TCP), of the data receiver unsubscribing to a layered multi-
   cast group (e.g., RLM [MJV96]), or of some other action that ulti-
   mately reduces the arrival rate of that flow on that congested link.

   This document only addresses the addition of ECN Capability to TCP,
   leaving issues of ECN in other transport protocols to further
   research.  For TCP, ECN requires three new pieces of functionality:
   negotiation between the endpoints during connection setup to deter-
   mine if they are both ECN-capable; an ECN-Echo (ECE) flag in the TCP
   header so that the data receiver can inform the data sender when a CE
   packet has been received; and a Congestion Window Reduced (CWR) flag
   in the TCP header so that the data sender can inform the data
   receiver that the congestion window has been reduced. The support
   required from other transport protocols is likely to be different,
   particularly for unreliable or reliable multicast transport proto-
   cols, and will have to be determined as other transport protocols are
   brought to the IETF for standardization.

6.1.  TCP

   The following sections describe in detail the proposed use of ECN in
   TCP.  This proposal is described in essentially the same form in
   [Floyd94]. We assume that the source TCP uses the standard congestion
   control algorithms of Slow-start, Fast Retransmit and Fast Recovery
   [RFC 2001].

   This proposal specifies two new flags in the Reserved field of the



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   TCP header.  The TCP mechanism for negotiating ECN-Capability uses
   the ECN-Echo (ECE) flag in the TCP header.  Bit 9 in the Reserved
   field of the TCP header is designated as the ECN-Echo flag.  The
   location of the 6-bit Reserved field in the TCP header is shown in
   Figure 3 of RFC 793 [RFC793] (and is reproduced below for complete-
   ness).  This specification of the ECN Field leaves the Reserved field
   as a 4-bit field using bits 4-7.

   To enable the TCP receiver to determine when to stop setting the ECN-
   Echo flag, we introduce a second new flag in the TCP header, the CWR
   flag.  The CWR flag is assigned to Bit 8 in the Reserved field of the
   TCP header.

         0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
       |               |                       | U | A | P | R | S | F |
       | Header Length |        Reserved       | R | C | S | S | Y | I |
       |               |                       | G | K | H | T | N | N |
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

        Figure 2: The old definition of bytes 13 and 14 of the TCP
   header.

         0   1   2   3   4   5   6   7   8   9  10  11  12  13  14  15
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+
       |               |               | C | E | U | A | P | R | S | F |
       | Header Length |    Reserved   | W | C | R | C | S | S | Y | I |
       |               |               | R | E | G | K | H | T | N | N |
       +---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+---+

        Figure 3: The new definition of bytes 13 and 14 of the TCP
   Header.

   Thus, ECN uses the ECT and CE flags in the IP header (as shown in
   Figure 1) for signaling between routers and connection endpoints, and
   uses the ECN-Echo and CWR flags in the TCP header (as shown in Figure
   3) for TCP-endpoint to TCP-endpoint signaling.  For a TCP connection,
   a typical sequence of events in an ECN-based reaction to congestion
   is as follows:
      * The ECT bit is set in packets transmitted by the sender to indi-
      cate that ECN is supported by the transport entities for these
      packets.
      * An ECN-capable router detects impending congestion and detects
      that the ECT bit is set in the packet it is about to drop.
      Instead of dropping the packet, the router chooses to set the CE
      bit in the IP header and forwards the packet.
      * The receiver receives the packet with the CE bit set, and sets
      the ECN-Echo flag in its next TCP ACK sent to the sender.



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      * The sender receives the TCP ACK with ECN-Echo set, and reacts to
      the congestion as if a packet had been dropped.
      * The sender sets the CWR flag in the TCP header of the next
      packet sent to the receiver to acknowledge its receipt of and
      reaction to the ECN-Echo flag.

   The negotiation for using ECN by the TCP transport entities and the
   use of the ECN-Echo and CWR flags is described in more detail in the
   sections below.

6.1.1  TCP Initialization

   In the TCP connection setup phase, the source and destination TCPs
   exchange information about their willingness to use ECN.  Subsequent
   to the completion of this negotiation, the TCP sender sets the ECT
   bit in the IP header of data packets to indicate to the network that
   the transport is capable and willing to participate in ECN for this
   packet. This indicates to the routers that they may mark this packet
   with the CE bit, if they would like to use that as a method of con-
   gestion notification. If the TCP connection does not wish to use ECN
   notification for a particular packet, the sending TCP sets the ECT
   bit equal to 0 (i.e., not set), and the TCP receiver ignores the CE
   bit in the received packet.

   For this discussion, we designate the initiating host as Host A and
   the responding host as Host B.  We call a SYN packet with the ECE and
   CWR flags set an "ECN-setup SYN packet", and we call a SYN packet
   with at least one of the ECE and CWR flags not set a "non-ECN-setup
   SYN packet".  Similarly, we call a SYN-ACK packet with only the ECE
   flag set but the CWR flag not set an "ECN-setup SYN-ACK packet", and
   we call a SYN-ACK packet with any other configuration of the ECE and
   CWR flags a "non-ECN-setup SYN-ACK packet".

   Before a TCP connection can use ECN, Host A sends an ECN-setup SYN
   packet, and Host B sends an ECN-setup SYN-ACK packet.  For a SYN
   packet, the setting of both ECE and CWR in the ECN-setup SYN packet
   is defined as an indication that the sending TCP is ECN-Capable,
   rather than as an indication of congestion or of response to conges-
   tion. More precisely, an ECN-setup SYN packet indicates that the TCP
   implementation transmitting the SYN packet will participate in ECN as
   both a sender and receiver.  Specifically, as a receiver, it will
   respond to incoming data packets that have the CE bit set in the IP
   header by setting ECE in outgoing TCP Acknowledgement (ACK) packets.
   As a sender, it will respond to incoming packets that have ECE set by
   reducing the congestion window and setting CWR when appropriate.  An
   ECN-setup SYN packet does not commit the TCP sender to setting the
   ECT bit in any or all of the packets it may transmit.  However, the
   commitment to respond appropriately to incoming packets with the CE



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   bit set remains even if the TCP sender in a later transmission,
   within this TCP connection, sends a SYN packet without ECE and CWR
   set.

   When Host B sends an ECN-setup SYN-ACK packet, it sets the ECE flag
   but not the CWR flag.  An ECN-setup SYN-ACK packet is defined as an
   indication that the TCP transmitting the SYN-ACK packet is ECN-Capa-
   ble.  As with the SYN packet, an ECN-setup SYN-ACK packet does not
   commit the TCP host to setting the ECT bit in transmitted packets.

   The following rules apply to the sending of ECN-setup packets:

   * If a host has received an ECN-setup SYN packet, then it MAY send an
   ECN-setup SYN-ACK packet.  Otherwise, it MUST NOT send an ECN-setup
   SYN-ACK packet.
   * A host MUST NOT set ECT on data packets unless it has sent at least
   one ECN-setup SYN or ECN-setup SYN-ACK packet, and has received at
   least one ECN-setup SYN or ECN-setup SYN-ACK packet, and has sent no
   non-ECN-setup SYN or non-ECN-setup SYN-ACK packet.  If a host has
   received at least one non-ECN-setup SYN or non-ECN-setup SYN-ACK
   packet, then it SHOULD NOT set ECT on data packets.
   * If a host ever sets the ECT bit on a data packet, then that host
   MUST correctly set/clear the CWR TCP bit on all subsequent packets in
   the connection.
   * If a host has sent at least one ECN-setup SYN or ECN-setup SYN-ACK
   packet, and has received no non-ECN-setup SYN or non-ECN-setup SYN-
   ACK packet, then if that host receives TCP data packets with ECT and
   CE bits set in the IP header, then that host MUST process these pack-
   ets as specified for an ECN-capable connection.  * A host that is not
   willing to use ECN on a TCP connection SHOULD clear both the ECE and
   CWR flags in all non-ECN-setup SYN and/or SYN-ACK packets that it
   sends to indicate this unwillingness.  Receivers MUST correctly han-
   dle all forms of the non-ECN-setup SYN and SYN-ACK packets.

6.1.1.1.  Robust TCP Initialization with an Echoed Reserve Field

   There is the question of why we chose to have the TCP sending the SYN
   set two ECN-related flags in the Reserved field of the TCP header for
   the SYN packet, while the responding TCP sending the SYN-ACK sets
   only one ECN-related flag in the SYN-ACK packet.  This asymmetry is
   necessary for the robust negotiation of ECN-capability with some
   deployed TCP implementations.  There exists at least one faulty TCP
   implementation in which TCP receivers set the Reserved field of the
   TCP header in ACK packets (and hence the SYN-ACK) simply to reflect
   the Reserved field of the TCP header in the received data packet.
   Because the TCP SYN packet sets the ECN-Echo and CWR flags to indi-
   cate ECN-capability, while the SYN-ACK packet sets only the ECN-Echo
   flag, the sending TCP correctly interprets a receiver's reflection of



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   its own flags in the Reserved field as an indication that the
   receiver is not ECN-capable.  The sending TCP is not mislead by a
   faulty TCP implementation sending a SYN-ACK packet that simply
   reflects the Reserved field of the incoming SYN packet.

6.1.2.  The TCP Sender

   For a TCP connection using ECN, new data packets are transmitted with
   the ECT bit set in the IP header (set to a "1").  If the sender
   receives an ECN-Echo (ECE) ACK packet (that is, an ACK packet with
   the ECN-Echo flag set in the TCP header), then the sender knows that
   congestion was encountered in the network on the path from the sender
   to the receiver.  The indication of congestion should be treated just
   as a congestion loss in non-ECN-Capable TCP. That is, the TCP source
   halves the congestion window "cwnd" and reduces the slow start
   threshold "ssthresh".  The sending TCP SHOULD NOT increase the con-
   gestion window in response to the receipt of an ECN-Echo ACK packet.

   TCP should not react to congestion indications more than once every
   window of data (or more loosely, more than once every round-trip
   time). That is, the TCP sender's congestion window should be reduced
   only once in response to a series of dropped and/or CE packets from a
   single window of data.  In addition, the TCP source should not
   decrease the slow-start threshold, ssthresh, if it has been decreased
   within the last round trip time.  However, if any retransmitted pack-
   ets are dropped, then this is interpreted by the source TCP as a new
   instance of congestion.

   After the source TCP reduces its congestion window in response to a
   CE packet, incoming acknowledgements that continue to arrive can
   "clock out" outgoing packets as allowed by the reduced congestion
   window.  If the congestion window consists of only one MSS (maximum
   segment size), and the sending TCP receives an ECN-Echo ACK packet,
   then the sending TCP should in principle still reduce its congestion
   window in half. However, the value of the congestion window is
   bounded below by a value of one MSS.  If the sending TCP were to con-
   tinue to send, using a congestion window of 1 MSS, this results in
   the transmission of one packet per round-trip time.  It is necessary
   to still reduce the sending rate of the TCP sender even further, on
   receipt of an ECN-Echo packet when the congestion window is one.  We
   use the retransmit timer as a means of reducing the rate further in
   this circumstance.  Therefore, the sending TCP MUST reset the
   retransmit timer on receiving the ECN-Echo packet when the congestion
   window is one.  The sending TCP will then be able to send a new
   packet only when the retransmit timer expires.

   When an ECN-Capable TCP sender reduces its congestion window for any
   reason (because of a retransmit timeout, a Fast Retransmit, or in



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   response to an ECN Notification), the TCP sender sets the CWR flag in
   the TCP header of the first new data packet sent after the window
   reduction.  If that data packet is dropped in the network, then the
   sending TCP will have to reduce the congestion window again and
   retransmit the dropped packet.

   We ensure that the "Congestion Window Reduced" information is reli-
   ably delivered to the TCP receiver.  This comes about from the fact
   that if the new data packet carrying the CWR flag is dropped, then
   the TCP sender will have to again reduce its congestion window, and
   send another new data packet with the CWR flag set.  Thus, the CWR
   bit in the TCP header SHOULD NOT be set on retransmitted packets.
   When the TCP data sender is ready to set the CWR bit after reducing
   the congestion window, it SHOULD set the CWR bit only on the first
   new data packet that it transmits.

   [Floyd94] discusses TCP's response to ECN in more detail.  [Floyd98]
   discusses the validation test in the ns simulator, which illustrates
   a wide range of ECN scenarios. These scenarios include the following:
   an ECN followed by another ECN, a Fast Retransmit, or a Retransmit
   Timeout; a Retransmit Timeout or a Fast Retransmit followed by an
   ECN; and a congestion window of one packet followed by an ECN.

   TCP follows existing algorithms for sending data packets in response
   to incoming ACKs, multiple duplicate acknowledgements, or retransmit
   timeouts [RFC2581].  TCP also follows the normal procedures for
   increasing the congestion window when it receives ACK packets without
   the ECN-Echo bit set [RFC2581].

6.1.3.  The TCP Receiver

   When TCP receives a CE data packet at the destination end-system, the
   TCP data receiver sets the ECN-Echo flag in the TCP header of the
   subsequent ACK packet.  If there is any ACK withholding implemented,
   as in current "delayed-ACK" TCP implementations where the TCP
   receiver can send an ACK for two arriving data packets, then the ECN-
   Echo flag in the ACK packet will be set to the OR of the CE bits of
   all of the data packets being acknowledged.  That is, if any of the
   received data packets are CE packets, then the returning ACK has the
   ECN-Echo flag set.

   To provide robustness against the possibility of a dropped ACK packet
   carrying an ECN-Echo flag, the TCP receiver sets the ECN-Echo flag in
   a series of ACK packets sent subsequently.  The TCP receiver uses the
   CWR flag received from the TCP sender to determine when to stop set-
   ting the ECN-Echo flag.

   After a TCP receiver sends an ACK packet with the ECN-Echo bit set,



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   that TCP receiver continues to set the ECN-Echo flag in all the ACK
   packets it sends (whether they acknowledge CE data packets or non-CE
   data packets) until it receives a CWR packet (a packet with the CWR
   flag set).  After the receipt of the CWR packet, acknowledgements for
   subsequent non-CE data packets do not have the ECN-Echo flag set. If
   another CE packet is received by the data receiver, the receiver
   would once again send ACK packets with the ECN-Echo flag set.  While
   the receipt of a CWR packet does not guarantee that the data sender
   received the ECN-Echo message, this does suggest that the data sender
   reduced its congestion window at some point *after* it sent the data
   packet for which the CE bit was set.

   We have already specified that a TCP sender is not required to reduce
   its congestion window more than once per window of data.  Some care
   is required if the TCP sender is to avoid unnecessary reductions of
   the congestion window when a window of data includes both dropped
   packets and (marked) CE packets.  This is illustrated in [Floyd98].

6.1.4.  Congestion on the ACK-path

   For the current generation of TCP congestion control algorithms, pure
   acknowledgement packets (e.g., packets that do not contain any accom-
   panying data) should be sent with the ECT bit off. Current TCP
   receivers have no mechanisms for reducing traffic on the ACK-path in
   response to congestion notification.  Mechanisms for responding to
   congestion on the ACK-path are areas for current and future research.
   (One simple possibility would be for the sender to reduce its conges-
   tion window when it receives a pure ACK packet with the CE bit set).
   For current TCP implementations, a single dropped ACK generally has
   only a very small effect on the TCP's sending rate.

6.1.5.  Retransmitted TCP packets

   This document specifies that for ECN-capable TCP implementations, the
   ECT bit (ECN-Capable Transport) in the IP header MUST NOT be set on
   retransmitted data packets, and that the TCP data receiver SHOULD
   ignore the ECN field on arriving data packets that are outside of the
   receiver's current window.  This is for greater security against
   denial-of-service attacks, as well as for robustness of the ECN con-
   gestion indication with packets that are dropped later in the net-
   work.

   First, we note that if the TCP sender were to set the ECT bit on a
   retransmitted packet, then if an unnecessarily-retransmitted packet
   was later dropped in the network, the end nodes would never receive
   the indication of congestion from the router setting the CE bit.
   Thus, setting the ECT bit on retransmitted data packets is not con-
   sistent with the robust delivery of the congestion indication even



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   for packets that are later dropped in the network.

   In addition, an attacker capable of spoofing the IP source address of
   the TCP sender could send data packets with arbitrary sequence num-
   bers, with both the ECT and CE bits set in the IP header.  On receiv-
   ing this spoofed data packet, the TCP data receiver would determine
   that the data does not lie in the current receive window, and return
   a duplicate acknowledgement.  We define an out-of-window packet at
   the TCP data receiver as a data packet that lies outside the
   receiver's current window.  On receiving an out-of-window packet, the
   TCP data receiver has to decide whether or not to treat the CE bit in
   the packet header as a valid indication of congestion, and therefore
   whether to return ECN-Echo indications to the TCP data sender.  If
   the TCP data receiver ignored the CE bit in an out-of-window packet,
   then the TCP data sender would not receive this possibly-legitimate
   indication of congestion from the network, resulting in a violation
   of end-to-end congestion control.  On the other hand, if the TCP data
   receiver honors the CE indication in the out-of-window packet, and
   reports the indication of congestion to the TCP data sender, then the
   malicious node that created the spoofed, out-of-window packet has
   successfully "attacked" the TCP connection by forcing the data sender
   to unnecessarily reduce (halve) its congestion window.  To prevent
   such a denial-of-service attack, we specify that a legitimate TCP
   data sender MUST NOT set the ECT bit on retransmitted data packets,
   and that the TCP data receiver SHOULD ignore the CE bit on out-of-
   window packets.

   One drawback of not setting ECT on retransmitted packets denies ECN
   protection for retransmitted packets.  However, for an ECN-capable
   TCP connection in a fully-ECN-capable environment with mild conges-
   tion, packets should rarely be dropped due to congestion in the first
   place, and so instances of retransmitted packets should rarely arise.
   If packets are being retransmitted, then there are already packet
   losses (from corruption or from congestion) that ECN has been unable
   to prevent.

   We note that if the router sets the CE bit for an ECN-capable data
   packet within a TCP connection, then the TCP connection is guaranteed
   to receive that indication of congestion, or to receive some other
   indication of congestion within the same window of data, even if this
   packet is dropped or reordered in the network.  We consider two
   cases, when the packet is later retransmitted, and when the packet is
   not later retransmitted.

   In the first case, if the packet is either dropped or delayed, and at
   some point retransmitted by the data sender, then the retransmission
   is a result of a Fast Retransmit or a Retransmit Timeout for either
   that packet or for some prior packet in the same window of data.  In



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   this case, because the data sender already has retransmitted this
   packet, we know that the data sender has already responded to an
   indication of congestion for some packet within the same window of
   data as the original packet.  Thus, even if the first transmission of
   the packet is dropped in the network, or is delayed, if it had the CE
   bit set, and is later ignored by the data receiver as an out-of-win-
   dow packet, this is not a problem, because the sender has already
   responded to an indication of congestion for that window of data.

   In the second case, if the packet is never retransmitted by the data
   sender, then this data packet is the only copy of this data received
   by the data receiver, and therefore arrives at the data receiver as
   an in-window packet, regardless of how much the packet might be
   delayed or reordered.  In this case, if the CE bit is set on the
   packet within the network, this will be treated by the data receiver
   as a valid indication of congestion.

6.1.6.  TCP Window Probes.

   When the TCP data receiver advertises a zero window, the TCP data
   sender sends window probes to determine if the receiver's window has
   increased.  Window probe packets do not contain any user data except
   for the sequence number, which is a byte.  If a window probe packet
   is dropped in the network, this loss is not detected by the receiver.
   Therefore, the TCP data sender MUST NOT set either the ECT or CWR
   bits on window probe packets.

   However, because window probes use exact sequence numbers, they can-
   not be easily spoofed in denial-of-service attacks.  Therefore, if a
   window probe arrives with ECT and CE set, then the receiver SHOULD
   respond to the ECN indications.

7.  Non-compliance by the End Nodes

   This section discusses concerns about the vulnerability of ECN to
   non-compliant end-nodes (i.e., end nodes that set the ECT bit in
   transmitted packets but do not respond to received CE packets).  We
   argue that the addition of ECN to the IP architecture will not sig-
   nificantly increase the current vulnerability of the architecture to
   unresponsive flows.

   Even for non-ECN environments, there are serious concerns about the
   damage that can be done by non-compliant or unresponsive flows (that
   is, flows that do not respond to congestion control indications by
   reducing their arrival rate at the congested link).  For example, an
   end-node could "turn off congestion control" by not reducing its con-
   gestion window in response to packet drops. This is a concern for the
   current Internet.  It has been argued that routers will have to



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   deploy mechanisms to detect and differentially treat packets from
   non-compliant flows [RFC2309,FF99].  It has also been suggested that
   techniques such as end-to-end per-flow scheduling and isolation of
   one flow from another, differentiated services, or end-to-end reser-
   vations could remove some of the more damaging effects of unrespon-
   sive flows.

   It might seem that dropping packets in itself is an adequate deter-
   rent for non-compliance, and that the use of ECN removes this deter-
   rent.  We would argue in response that (1) ECN-capable routers pre-
   serve packet-dropping behavior in times of high congestion; and (2)
   even in times of high congestion, dropping packets in itself is not
   an adequate deterrent for non-compliance.

   First, ECN-Capable routers will only mark packets (as opposed to
   dropping them) when the packet marking rate is reasonably low. During
   periods where the average queue size exceeds an upper threshold, and
   therefore the potential packet marking rate would be high, our recom-
   mendation is that routers drop packets rather then set the CE bit in
   packet headers.

   During the periods of low or moderate packet marking rates when ECN
   would be deployed, there would be little deterrent effect on unre-
   sponsive flows of dropping rather than marking those packets. For
   example, delay-insensitive flows using reliable delivery might have
   an incentive to increase rather than to decrease their sending rate
   in the presence of dropped packets.  Similarly, delay-sensitive flows
   using unreliable delivery might increase their use of FEC in response
   to an increased packet drop rate, increasing rather than decreasing
   their sending rate.  For the same reasons, we do not believe that
   packet dropping itself is an effective deterrent for non-compliance
   even in an environment of high packet drop rates, when all flows are
   sharing the same packet drop rate.

   Several methods have been proposed to identify and restrict non-com-
   pliant or unresponsive flows. The addition of ECN to the network
   environment would not in any way increase the difficulty of designing
   and deploying such mechanisms. If anything, the addition of ECN to
   the architecture would make the job of identifying unresponsive flows
   slightly easier.  For example, in an ECN-Capable environment routers
   are not limited to information about packets that are dropped or have
   the CE bit set at that router itself; in such an environment, routers
   could also take note of arriving CE packets that indicate congestion
   encountered by that packet earlier in the path.







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8.  Non-compliance in the Network

   This section considers the issues when a router is operating, possi-
   bly maliciously, to modify either of the bits in the ECN field.  In
   this section we represent the ECN field in the IP header by the tuple
   (ECT bit, CE bit).

   By tampering with the bits in the ECN field, an adversary (or a bro-
   ken router) could do one or more of the following: falsely report
   congestion, disable ECN-Capability for an individual packet, erase
   the ECN congestion indication, or falsely indicate ECN-Capability.
   Section 18 systematically examines the various cases by which the ECN
   field could be modified.  The important criterion considered in
   determining the consequences of such modifications is whether it is
   likely to lead to poorer behavior in any dimension (throughput,
   delay, fairness or functionality) than if a router were to drop a
   packet.

   The first two possible changes, falsely reporting congestion or dis-
   abling ECN-Capability for an individual packet, are no worse than if
   the router were to simply drop the packet.  From a congestion control
   point of view, setting the CE bit in the absence of congestion by a
   non-compliant router would be no worse than a router dropping a
   packet unnecessarily. By "erasing" the ECT bit of a packet that is
   later dropped in the network, a router's actions could result in an
   unnecessary packet drop for that packet later in the network.

   However, as discussed in Section 18, a router that erases the ECN
   congestion indication or falsely indicates ECN-Capability could
   potentially do more damage to the flow that if it has simply dropped
   the packet.  A rogue or broken router that "erased" the CE bit in
   arriving CE packets would prevent that indication of congestion from
   reaching downstream receivers.  This could result in the failure of
   congestion control for that flow and a resulting increase in conges-
   tion in the network, ultimately resulting in subsequent packets
   dropped for this flow as the average queue size increased at the con-
   gested gateway.

   Section 19 considers the potential repercussions of subverting end-
   to-end congestion control by either falsely indicating ECN-Capabil-
   ity, or by erasing the congestion indication in ECN (the CE-bit).  We
   observe in Section 19 that the consequence of subverting ECN-based
   congestion control may lead to potential unfairness, but this is
   likely to be no worse than the subversion of either ECN-based or
   packet-based congestion control by the end nodes.






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8.1.  Complications Introduced by Split Paths

   If a router or other network element has access to all of the packets
   of a flow, then that router could do no more damage to a flow by
   altering the ECN field than it could by simply dropping all of the
   packets from that flow.  However, in some cases, a malicious or bro-
   ken router might have access to only a subset of the packets from a
   flow.  The question is as follows:  can this router, by altering the
   ECN field in this subset of the packets, do more damage to that flow
   than if it has simply dropped that set of the packets?

   This is also discussed in detail in Section 18, which conclude as
   follows:  It is true that the adversary that has access only to a
   subset of packets in an aggregate might, by subverting ECN-based con-
   gestion control, be able to deny the benefits of ECN to the other
   packets in the aggregate.  While this is undesirable, this is not a
   sufficient concern to result in disabling ECN.

9.  Encapsulated Packets

9.1.  IP packets encapsulated in IP

   The encapsulation of IP packet headers in tunnels is used in many
   places, including IPsec and IP in IP [RFC2003].  This section consid-
   ers issues related to interactions between ECN and IP tunnels, and
   specifies two alternative solutions.  This discussion is complemented
   by RFC 2983's discussion of interactions between Differentiated Ser-
   vices and IP tunnels of various forms [RFC 2983], as Differentiated
   Services uses the remaining six bits of the IP header octet that is
   used by ECN (see Figure 1 in Section 5).


   Some IP tunnel modes are based on adding a new "outer" IP header that
   encapsulates the original, or "inner" IP header and its associated
   packet.  In many cases, the new "outer" IP header may be added and
   removed at intermediate points along a connection, enabling the net-
   work to establish a tunnel without requiring endpoint participation.
   We denote tunnels that specify that the outer header be discarded at
   tunnel egress as "simple tunnels".

   ECN uses the ECT and CE flags in the IP header for signaling between
   routers and connection endpoints.  ECN interacts with IP tunnels
   based on the treatment of these flags in the IP header.  In simple IP
   tunnels the octet containing these flags is copied or mapped from the
   inner IP header to the outer IP header at IP tunnel ingress, and the
   outer header's copy of this field is discarded at IP tunnel egress.
   If the outer header were to be simply discarded without taking care
   to deal with the ECN related flags, and an ECN-capable router were to



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   set the CE (Congestion Experienced) bit within a packet in a simple
   IP tunnel, this indication would be discarded at tunnel egress, los-
   ing the indication of congestion.

   Thus, the use of ECN over simple IP tunnels would result in routers
   attempting to use the outer IP header to signal congestion to end-
   points, but those congestion warnings never arriving because the
   outer header is discarded at the tunnel egress point.  This problem
   was encountered with ECN and IPsec in tunnel mode, and RFC 2481 rec-
   ommended that ECN not be used with the older simple IPsec tunnels in
   order to avoid this behavior and its consequences.  When ECN becomes
   widely deployed, then simple tunnels likely to carry ECN-capable
   traffic will have to be changed.

   From a security point of view, the use of ECN in the outer header of
   an IP tunnel might raise security concerns because an adversary could
   tamper with the ECN information that propagates beyond the tunnel
   endpoint.  Based on an analysis in Sections 18 and 19 of these con-
   cerns and the resultant risks, our overall approach is to make sup-
   port for ECN an option for IP tunnels, so that an IP tunnel can be
   specified or configured either to use ECN or not to use ECN in the
   outer header of the tunnel.  Thus, in environments or tunneling pro-
   tocols where the risks of using ECN are judged to outweigh its bene-
   fits, the tunnel can simply not use ECN in the outer header.  Then
   the only indication of congestion experienced at routers within the
   tunnel would be through packet loss.

   The result is that there are two viable options for the behavior of
   ECN-capable connections over an IP tunnel, especially IPsec tunnels:
      * A limited-functionality option in which ECN is preserved in the
      inner header, but disabled in the outer header.  The only mecha-
      nism available for signaling congestion occurring within the tun-
      nel in this case is dropped packets.
      * A full-functionality option that supports ECN in both the inner
      and outer headers, and propagates congestion warnings from nodes
      within the tunnel to endpoints.

   Support for these options requires varying amounts of changes to IP
   header processing at tunnel ingress and egress.  A small subset of
   these changes sufficient to support only the limited-functionality
   option would be sufficient to eliminate any incompatibility between
   ECN and IP tunnels.

   One goal of this document is to give guidance about the tradeoffs
   between the limited-functionality and full-functionality options.  A
   full discussion of the potential effects of an adversary's modifica-
   tions of the CE and ECT bits is given in Sections 18 and 19.




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9.1.1.  The Limited-functionality and Full-functionality Options

   The limited-functionality option for ECN encapsulation in IP tunnels
   is for the ECT bit in the outside (encapsulating) header to be off
   (i.e., set to 0), regardless of the value of the ECT bit in the
   inside (encapsulated) header.  With this option, the ECN field in the
   inner header is not altered upon de-capsulation.  The disadvantage of
   this approach is that the flow does not have ECN support for that
   part of the path that is using IP tunneling, even if the encapsulated
   packet (from the original TCP sender) is ECN-Capable.  That is, if
   the encapsulated packet arrives at a congested router that is ECN-
   capable, and the router can decide to drop or mark the packet as an
   indication of congestion to the end nodes, the router will not be
   permitted to set the CE bit in the packet header, but instead will
   have to drop the packet.

   The full-functionality option for ECN encapsulation is to copy the
   ECT bit of the inside header to the outside header on encapsulation,
   and to OR the CE bit from the outer header with the CE bit of the
   inside header on decapsulation.  That is, for full ECN support the
   encapsulation and decapsulation processing involves the following:
   At tunnel ingress, the full-functionality option copies the value of
   ECT (bit 6) in the inner header to the outer header.  CE (bit 7) is
   set to 0 in the outer header.  Upon decapsulation at the tunnel
   egress, the full-functionality option sets CE to 1 in the inner
   header if the value of ECT (bit 6) in the inner header is 1, and the
   value of CE (bit 7) in the outer header is 1.  Otherwise, no change
   is made to this field of the inner header.

   With the full-functionality option, a flow can take advantage of ECN
   in those parts of the path that might use IP tunneling.  The disad-
   vantage of the full-functionality option from a security perspective
   is that the IP tunnel cannot protect the flow from certain modifica-
   tions to the ECN bits in the IP header within the tunnel.  The poten-
   tial dangers from modifications to the ECN bits in the IP header are
   described in detail in Sections 18 and 19.

      (1) An IP tunnel MUST modify the handling of the DS field octet at
      IP tunnel endpoints by implementing either the limited-functional-
      ity or the full-functionality option.
      (2) Optionally, an IP tunnel MAY enable the endpoints of an IP
      tunnel to negotiate the choice between the limited-functionality
      and the full-functionality option for ECN in the tunnel.

   The minimum required to make ECN usable with IP tunnels is the lim-
   ited-functionality option, which prevents ECN from being enabled in
   the outer header of an IPsec tunnel.  Full support for ECN requires
   the use of the full-functionality option.  If there are no optional



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   mechanisms for the tunnel endpoints to negotiate a choice between the
   limited-functionality or full-functionality option, there can be a
   pre-existing agreement between the tunnel endpoints about whether to
   support the limited-functionality or the full-functionality ECN
   option.

   In addition, it is RECOMMENDED that packets with ECT and CE both set
   to 1 in the outer header be dropped if they arrive at the tunnel
   egress point for a tunnel that uses the limited-functionality option,
   or for a tunnel that uses the full-functionality option but for which
   the ECT bit in the inner header is set to zero.  This is motivated by
   backwards compatibility and to ensure that no unauthorized modifica-
   tions of the ECN field take place, and is discussed further in the
   next Section (9.1.2).

9.1.2.  Changes to the ECN Field within an IP Tunnel.

   The presence of a copy of the ECN field in the inner header of an IP
   tunnel mode packet provides an opportunity for detection of unautho-
   rized modifications to the ECT bit in the outer header.  Comparison
   of the ECT bits in the inner and outer headers falls into two cate-
   gories for implementations that conform to this document:
      * If the IP tunnel uses the full-functionality option, then the
      values of the ECT bits in the inner and outer headers should be
      identical.
      * If the tunnel uses the limited-functionality option, then the
      ECT bit in the outer header should be 0.

   Receipt of a packet not satisfying the appropriate condition could be
   a cause of concern.

   Consider the case of an IP tunnel where the tunnel ingress point has
   not been updated to this document's requirements, while the tunnel
   egress point has been updated to support ECN.  In this case, the IP
   tunnel is not explicitly configured to support the full-functionality
   ECN option. However, the tunnel ingress point is behaving identically
   to a tunnel ingress point that supports the full-functionality
   option.  If packets from an ECN-capable connection use this tunnel,
   ECT will be set to 1 in the outer header at the tunnel ingress point.
   Congestion within the tunnel may then result in ECN-capable routers
   setting CE in the outer header.  Because the tunnel has not been
   explicitly configured to support the full-functionality option, the
   tunnel egress point expects the ECT bit in the outer header to be 0.
   When an ECN-capable tunnel egress point receives a packet with the
   ECT bit in the outer header set to 1, in a tunnel that has not been
   configured to support the full-functionality option, that packet
   should be processed, according to whether CE bit was set, as follows.
   It is RECOMMENDED that such packets, with the ECT bit in the outer



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   header set to 1 on a tunnel that has not been configured to support
   the full-functionality option, be dropped at the egress point if CE
   is set to 1 in the outer header but 0 in the inner header, and for-
   warded otherwise.

   An IP tunnel cannot provide protection against erasure of congestion
   indications based on resetting the value of the CE bit in packets for
   which ECT is set in the outer header.  The erasure of congestion
   indications may impact the network and other flows in ways that would
   not be possible in the absence of ECN.  It is important to note that
   erasure of congestion indications can only be performed to congestion
   indications placed by nodes within the tunnel; the copy of the CE bit
   in the inner header preserves congestion notifications from nodes
   upstream of the tunnel ingress.  If erasure of congestion notifica-
   tions is judged to be a security risk that exceeds the congestion
   management benefits of ECN, then tunnels could be specified or con-
   figured to use the limited-functionality option.

9.2.  IPsec Tunnels

   IPsec supports secure communication over potentially insecure network
   components such as intermediate routers.  IPsec protocols support two
   operating modes, transport mode and tunnel mode, that span a wide
   range of security requirements and operating environments.  Transport
   mode security protocol header(s) are inserted between the IP (IPv4 or
   IPv6) header and higher layer protocol headers (e.g., TCP), and hence
   transport mode can only be used for end-to-end security on a connec-
   tion.  IPsec tunnel mode is based on adding a new "outer" IP header
   that encapsulates the original, or "inner" IP header and its associ-
   ated packet.  Tunnel mode security headers are inserted between these
   two IP headers.  In contrast to transport mode, the new "outer" IP
   header and tunnel mode security headers can be added and removed at
   intermediate points along a connection, enabling security gateways to
   secure vulnerable portions of a connection without requiring endpoint
   participation in the security protocols.  An important aspect of tun-
   nel mode security is that in the original specification, the outer
   header is discarded at tunnel egress, ensuring that security threats
   based on modifying the IP header do not propagate beyond that tunnel
   endpoint.  Further discussion of IPsec can be found in [RFC2401].

   The IPsec protocol as originally defined in [ESP, AH] required that
   the inner header's ECN field not be changed by IPsec decapsulation
   processing at a tunnel egress node; this would have ruled out the
   possibility of full-functionality mode for ECN.  At the same time,
   this would ensure that an adversary's modifications to the ECN field
   cannot be used to launch theft- or denial-of-service attacks across
   an IPsec tunnel endpoint, as any such modifications will be discarded
   at the tunnel endpoint.



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   In principle, permitting the use of ECN functionality in the outer
   header of an IPsec tunnel raises security concerns because an adver-
   sary could tamper with the information that propagates beyond the
   tunnel endpoint.  Based on an analysis (included in Sections 18 and
   19) of these concerns and the associated risks, our overall approach
   has been to provide configuration support for IPsec changes to remove
   the conflict with ECN.

   In particular, in tunnel mode the IPsec tunnel MUST support either
   the limited-functionality or the full-functionality mode outlined in
   Section 9.1.1.

   This makes permission to use ECN functionality in the outer header of
   an IPsec tunnel a configurable part of the corresponding IPsec Secu-
   rity Association (SA), so that it can be disabled in situations where
   the risks are judged to outweigh the benefits.  The result is that an
   IPsec security administrator is presented with two alternatives for
   the behavior of ECN-capable connections within an IPsec tunnel, the
   limited-functionality alternative and full-functionality alternative
   described earlier.  All IPsec implementations MUST implement either
   the limited-functionality or the full-functionality alternative in
   order to eliminate incompatibility between ECN and IPsec tunnels, but
   implementers MAY choose to implement either alternative.

   In addition, this document specifies how the endpoints of an IPsec
   tunnel could negotiate enabling ECN functionality in the outer head-
   ers of that tunnel based on security policy.  The ability to negoti-
   ate ECN usage between tunnel endpoints would enable a security admin-
   istrator to disable ECN in situations where she believes the risks
   (e.g., of lost congestion notifications) outweigh the benefits of
   ECN.

   The IPsec protocol, as defined in [ESP, AH], does not include the IP
   header's ECN field in any of its cryptographic calculations (in the
   case of tunnel mode, the outer IP header's ECN field is not
   included).  Hence modification of the ECN field by a network node has
   no effect on IPsec's end-to-end security, because it cannot cause any
   IPsec integrity check to fail.  As a consequence, IPsec does not pro-
   vide any defense against an adversary's modification of the ECN field
   (i.e., a man-in-the-middle attack), as the adversary's modification
   will also have no effect on IPsec's end-to-end security.  In some
   environments, the ability to modify the ECN field without affecting
   IPsec integrity checks may constitute a covert channel; if it is nec-
   essary to eliminate such a channel or reduce its bandwidth, then the
   IPsec tunnel should be run in limited-functionality mode.






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9.2.1.  Negotiation between Tunnel Endpoints

   This section describes the detailed changes to enable usage of ECN
   over IPsec tunnels, including the negotiation of ECN support between
   tunnel endpoints.  This is supported by three changes to IPsec:
      * An optional Security Association Database (SAD) field indicating
      whether tunnel encapsulation and decapsulation processing allows
      or forbids ECN usage in the outer IP header.
      * An optional Security Association Attribute that enables negotia-
      tion of this SAD field between the two endpoints of an SA that
      supports tunnel mode.
      * Changes to tunnel mode encapsulation and decapsulation process-
      ing to allow or forbid ECN usage in the outer IP header based on
      the value of the SAD field.  When ECN usage is allowed in the
      outer IP header, ECT is set in the outer header for ECN-capable
      connections and congestion notifications (indicated by the CE bit)
      from such connections are propagated to the inner header at tunnel
      egress.

   If negotiation of ECN usage is implemented, then the SAD field SHOULD
   also be implemented.  On the other hand, negotiation of ECN usage is
   OPTIONAL in all cases, even for implementations that support the SAD
   field.  The encapsulation and decapsulation processing changes are
   REQUIRED, but MAY be implemented without the other two changes by
   assuming that ECN usage is always forbidden.  The full-functionality
   alternative for ECN usage over IPsec tunnels consists of the SAD
   field and the full version of encapsulation and decapsulation pro-
   cessing changes, with or without the OPTIONAL negotiation support.
   The limited-functionality alternative consists of a subset of the
   encapsulation and decapsulation changes that always forbids ECN
   usage.

   These changes are covered further in the following three subsections.

9.2.1.1.  ECN Tunnel Security Association Database Field

   Full ECN functionality adds a new field to the SAD (see [RFC2401]):

      ECN Tunnel: allowed or forbidden.

      Indicates whether ECN-capable connections using this SA in tunnel
      mode are permitted to receive ECN congestion notifications for
      congestion occurring within the tunnel.  The allowed value enables
      ECN congestion notifications.  The forbidden value disables such
      notifications, causing all congestion to be indicated via dropped
      packets.

      [OPTIONAL.  The value of this field SHOULD be assumed to be



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      "forbidden" in implementations that do not support it.]

   If this attribute is implemented, then the SA specification in a
   Security Policy Database (SPD) entry MUST support a corresponding
   attribute, and this SPD attribute MUST be covered by the SPD adminis-
   trative interface (currently described in Section 4.4.1 of
   [RFC2401]).

9.2.1.2.  ECN Tunnel Security Association Attribute

   A new IPsec Security Association Attribute is defined to enable the
   support for ECN congestion notifications based on the outer IP header
   to be negotiated for IPsec tunnels (see [RFC2407]).  This attribute
   is OPTIONAL, although implementations that support it SHOULD also
   support the SAD field defined in Section 9.2.1.1.

   Attribute Type

           class               value           type
     -------------------------------------------------
     ECN Tunnel                 10             Basic

   The IPsec SA Attribute value 10 has been allocated by IANA to indi-
   cate that the ECN Tunnel SA Attribute is being negotiated; the type
   of this attribute is Basic (see Section 4.5 of [RFC2407]).  The Class
   Values are used to conduct the negotiation.  See [RFC2407, RFC2408,
   RFC2409] for further information including encoding formats and
   requirements for negotiating this SA attribute.

   Class Values

     ECN Tunnel

       Specifies whether ECN functionality is allowed to
       be used with Tunnel Encapsulation Mode.
       This affects tunnel encapsulation and decapsulation processing -
       see Section 9.2.1.3.

       RESERVED          0
       Allowed           1
       Forbidden         2

       Values 3-61439 are reserved to IANA.  Values 61440-65535 are for
       private use.

       If unspecified, the default shall be assumed to be Forbidden.

   ECN Tunnel is a new SA attribute, and hence initiators that use it



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   can expect to encounter responders that do not understand it, and
   therefore reject proposals containing it.  For backwards compatibil-
   ity with such implementations initiators SHOULD always also include a
   proposal without the ECN Tunnel attribute to enable such a responder
   to select a transform or proposal that does not contain the ECN Tun-
   nel attribute.  RFC 2407 currently requires responders to reject all
   proposals if any proposal contains an unknown attribute; this
   requirement is expected to be changed to require a responder not to
   select proposals or transforms containing unknown attributes.

9.2.1.3.  Changes to IPsec Tunnel Header Processing

   For full ECN support, the encapsulation and decapsulation processing
   for the IPv4 TOS field and the IPv6 Traffic Class field are changed
   from that specified in [RFC2401] to the following:

                           <-- How Outer Hdr Relates to Inner Hdr -->
                           Outer Hdr at                 Inner Hdr at
      IPv4                 Encapsulator                 Decapsulator
        Header fields:     --------------------         ------------
          DS Field         copied from inner hdr (5)    no change
          ECN Field        constructed (7)              constructed (8)

      IPv6
        Header fields:
          DS Field         copied from inner hdr (6)    no change
          ECN Field        constructed (7)              constructed (8)

      (5)(6) If the packet will immediately enter a domain for which the
      DSCP value in the outer header is not appropriate, that value MUST
      be mapped to an appropriate value for the domain [RFC 2474].  Also
      see [RFC 2475] for further information.

      (7) If the value of the ECN Tunnel field in the SAD entry for this
      SA is "allowed" and the value of ECT (bit 0) is 1 in the inner
      header, set ECT to 1 in the outer header, else set ECT to 0 in the
      outer header.  Set CE (bit 1) to 0 in the outer header.

      (8) If the value of the ECN tunnel field in the SAD entry for this
      SA is "allowed" and the value of ECT (bit 0) in the inner header
      is 1, then set the CE bit (bit 1) in the inner header to the logi-
      cal OR of the CE bit in the inner header with the CE bit in the
      outer header, else make no change to the ECN field.

      (5) and (6) are identical to match usage in [RFC2401], although
      they are different in [RFC2401].

   The above description applies to implementations that support the ECN



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   Tunnel field in the SAD; such implementations MUST implement this
   processing instead of the processing of the IPv4 TOS octet and IPv6
   Traffic Class octet defined in [RFC2401].  This constitutes the full-
   functionality alternative for ECN usage with IPsec tunnels.

   An implementation that does not support the ECN Tunnel field in the
   SAD MUST implement this processing by assuming that the value of the
   ECN Tunnel field of the SAD is "forbidden" for every SA.  In this
   case, the processing of the ECN field reduces to:

      (7) Set the ECN field (ECT and CE bits) to zero in the outer
      header.
      (8) Make no change to the ECN field in the inner header.

   This constitutes the limited functionality alternative for ECN usage
   with IPsec tunnels.

   For backwards compatibility, packets with ECT and CE both set to 1 in
   the outer header SHOULD be dropped if they arrive on an SA that is
   using the limited-functionality option, or that is using the full-
   functionality option (i.e., and has set the ECT flag in the outer
   header to 1) for a packet with the ECT flag set to 0 in the inner
   header.

9.2.2.  Changes to the ECN Field within an IPsec Tunnel.

   If the ECN Field is changed inappropriately within an IPsec tunnel,
   and this change is detected at the tunnel egress, then the receipt of
   a packet not satisfying the appropriate condition for its SA is an
   auditable event.  An implementation MAY create audit records with
   per-SA counts of incorrect packets over some time period rather than
   creating an audit record for each erroneous packet.  Any such audit
   record SHOULD contain the headers from at least one erroneous packet,
   but need not contain the headers from every packet represented by the
   entry.

9.2.3.  Comments for IPsec Support

   Substantial comments were received on two areas of this document dur-
   ing review by the IPsec working group.  This section describes these
   comments and explains why the proposed changes were not incorporated.

   The first comment indicated that per-node configuration is easier to
   implement than per-SA configuration.  After serious thought and
   despite some initial encouragement of per-node configuration, it no
   longer seems to be a good idea. The concern is that as ECN-awareness
   is progressively deployed in IPsec, many ECN-aware IPsec implementa-
   tions will find themselves communicating with a mixture of ECN-aware



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   and ECN-unaware IPsec tunnel endpoints.  In such an environment with
   per-node configuration, the only reasonable thing to do is forbid ECN
   usage for all IPsec tunnels, which is not the desired outcome.

   In the second area, several reviewers noted that SA negotiation is
   complex, and adding to it is non-trivial.  One reviewer suggested
   using ICMP after tunnel setup as a possible alternative.  The addi-
   tion to SA negotiation in this document is OPTIONAL and will remain
   so; implementers are free to ignore it.  The authors believe that the
   assurance it provides can be useful in a number of situations.  In
   practice, if this is not implemented, it can be deleted at a subse-
   quent stage in the standards process.  Extending ICMP to negotiate
   ECN after tunnel setup is more complex than extending SA attribute
   negotiation.  Some tunnels do not permit traffic to be addressed to
   the tunnel egress endpoint, hence the ICMP packet would have to be
   addressed to somewhere else, scanned for by the egress endpoint, and
   discarded there or at its actual destination.  In addition, ICMP
   delivery is unreliable, and hence there is a possibility of an ICMP
   packet being dropped, entailing the invention of yet another
   ack/retransmit mechanism.  It seems better simply to specify an
   OPTIONAL extension to the existing SA negotiation mechanism.

9.3.  IP packets encapsulated in non-IP packet headers.

   A different set of issues are raised, relative to ECN, when IP pack-
   ets are encapsulated in tunnels with non-IP packet headers.  This
   occurs with MPLS [MPLS], GRE [GRE], L2TP [L2TP], and PPTP [PPTP].
   For these protocols, there is no conflict with ECN; it is just that
   ECN cannot be used within the tunnel unless an ECN codepoint can be
   specified for the header of the encapsulating protocol.  Earlier work
   considered a preliminary proposal for incorporating ECN into MPLS,
   and proposals for incorporating ECN into GRE, L2TP, or PPTP will be
   considered as the need arises.

10.  Issues Raised by Monitoring and Policing Devices

   One possibility is that monitoring and policing devices (or more
   informally, "penalty boxes") will be installed in the network to mon-
   itor whether best-effort flows are appropriately responding to con-
   gestion, and to preferentially drop packets from flows determined not
   to be using adequate end-to-end congestion control procedures.

   We recommend that any "penalty box" that detects a flow or an aggre-
   gate of flows that is not responding to end-to-end congestion control
   first change from marking to dropping packets from that flow, before
   taking any additional action to restrict the bandwidth available to
   that flow.  Thus, initially, the router may drop packets in which the
   router would otherwise would have set the CE bit.  This could include



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   dropping those arriving packets for that flow that are ECN-Capable
   and that already have the CE bit set.  In this way, any congestion
   indications seen by that router for that flow will be guaranteed to
   also be seen by the end nodes, even in the presence of malicious or
   broken routers elsewhere in the path.  If we assume that the first
   action taken at any "penalty box" for an ECN-capable flow will be to
   drop packets instead of marking them, then there is no way that an
   adversary that subverts ECN-based end-to-end congestion control can
   cause a flow to be characterized as being non-cooperative and placed
   into a more severe action within the "penalty box".

   The monitoring and policing devices that are actually deployed could
   fall short of the `ideal' monitoring device described above, in that
   the monitoring is applied not to a single flow, but to an aggregate
   of flows (e.g., those sharing a single IPsec tunnel).  In this case,
   the switch from marking to dropping would apply to all of the flows
   in that aggregate, denying the benefits of ECN to the other flows in
   the aggregate also.  At the highest level of aggregation, another
   form of the disabling of ECN happens even in the absence of monitor-
   ing and policing devices, when ECN-Capable RED queues switch from
   marking to dropping packets as an indication of congestion when the
   average queue size has exceeded some threshold.

   If there were serious operational problems with routers inappropri-
   ately erasing the CE bit in packet headers, this could be addressed
   to some extent by including a one-bit ECN nonce in packet headers.
   Routers would erase the nonce when they set the CE bit [SCWA99].
   Routers that erased the CE bit would face additional difficulty in
   reconstructing the original nonce, and thus repeated erasure of the
   CE bit would be more likely to be detected by the end-nodes.  (This
   could in fact be done without adding any extra bits for ECN in the IP
   header, by using the ECN codepoints (ECT=1, CE=0) and (ECT=0, CE=1)
   as the two values for the nonce, and by defining the codepoint
   (ECT=0, CE=1) to mean exactly the same as the codepoint (ECT=1,
   CE=0).)  However, at this point the potential danger of misbehaving
   routers does not seem of sufficient concern to warrant this addi-
   tional complication of adding an ECN nonce to protect against the
   erasure of the CE bit.  Additional research is also needed to better
   understand the value of such a nonce and appropriate means of gener-
   ating sequences of nonce values that an adversary will find suffi-
   ciently difficult to reconstruct.

   An ECN nonce would also address the problem of misbehaving transport
   receivers lying to the transport sender about whether or not the CE
   bit was set in a packet.  However, another possibility is for the
   data sender to test for a misbehaving receiver directly, by occasion-
   ally sending a data packet with ECT and CE set, to see if the
   receiver reports receiving the CE bit.  Of course, if these packets



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   encountered congestion in the network, the router would make no
   change in the packets, because the CE bit would already be set.
   Thus, for packets sent with the ECT and CE bits set, the TCP end-
   nodes could not determine if some router intended to set the CE bit
   in these packets.  For this reason, sending packets with the ECT and
   CE bits would have to be done very sparingly.  In addition, the TCP
   sender would have to remember which packets were sent with the ECT
   and CE bits set, so that it doesn't react to them as if there was
   congestion in the network.  We believe that further research is
   needed on possible transport-based mechanisms for verifying that the
   transport receiver does not lie to the transport sender about the
   receipt of congestion indications.

11.  Evaluations of ECN

   This section discusses some of the related work evaluating the use of
   ECN.  The ECN Web Page [ECN] has pointers to other papers, as well as
   to implementations of ECN.

   [Floyd94] considers the advantages and drawbacks of adding ECN to the
   TCP/IP architecture.  As shown in the simulation-based comparisons,
   one advantage of ECN is to avoid unnecessary packet drops for short
   or delay-sensitive TCP connections.  A second advantage of ECN is in
   avoiding some unnecessary retransmit timeouts in TCP.  This paper
   discusses in detail the integration of ECN into TCP's congestion con-
   trol mechanisms.  The possible disadvantages of ECN discussed in the
   paper are that a non-compliant TCP connection could falsely advertise
   itself as ECN-capable, and that a TCP ACK packet carrying an ECN-Echo
   message could itself be dropped in the network.  The first of these
   two issues is discussed in the appendix of this document, and the
   second is addressed by the addition of the CWR flag in the TCP
   header.

   Experimental evaluations of ECN include [RFC2884,K98].  The conclu-
   sions of [K98] and [RFC2884] are that ECN TCP gets moderately better
   throughput than non-ECN TCP; that ECN TCP flows are fair towards non-
   ECN TCP flows; and that ECN TCP is robust with two-way traffic (with
   congestion in both directions) and with multiple congested gateways.
   Experiments with many short web transfers show that, while most of
   the short connections have similar transfer times with or without
   ECN, a small percentage of the short connections have very long
   transfer times for the non-ECN experiments as compared to the ECN
   experiments.

12.  Summary of changes required in IP and TCP

   This document specified two bits in the IP header, the ECN-Capable
   Transport (ECT) bit and the Congestion Experienced (CE) bit, to be



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   used for ECN.  The ECT bit set to "0" indicates that the transport
   protocol will ignore the CE bit.  This is the default value for the
   ECT bit.  The ECT bit set to "1" indicates that the transport proto-
   col is willing and able to participate in ECN.

   The default value for the CE bit is "0".  The router sets the CE bit
   to "1" to indicate congestion to the end nodes.  The CE bit in a
   packet header MUST NOT be reset by a router from "1" to "0".

   When viewed in terms of code points, this document has defined three
   code points for the ECN field, for "not ECT" (ECT=0, CE=0), "ECT but
   not CE" (ECT=1, CE=0), and "ECT and CE" (ECT=1, CE=1).  The code
   point of (ECT=0, CE=1) is not defined in this document.  One possi-
   bility would be for this code point to be used, some time in the
   future, for some other function for non-ECN-capable packets.  A sec-
   ond possibility would be for this code point to be used as an ECN
   nonce, as described earlier in the document.  A third possibility
   would be for the code point (ECT=0, CE=1) to be used to indicate that
   the packet is ECN-capable for an alternate semantics for the Conges-
   tion Experienced indication.  However, at this time the code point
   (ECT=0, CE=1) remains undefined.

   TCP requires three changes for ECN, a setup phase and two new flags
   in the TCP header. The ECN-Echo flag is used by the data receiver to
   inform the data sender of a received CE packet.  The Congestion Win-
   dow Reduced (CWR) flag is used by the data sender to inform the data
   receiver that the congestion window has been reduced.

   When ECN (Explicit Congestion Notification [RFC2481]) is used, it is
   required that congestion indications generated within an IP tunnel
   not be lost at the tunnel egress.  We specified a minor modification
   to the IP protocol's handling of the ECN field during encapsulation
   and de-capsulation to allow flows that will undergo IP tunneling to
   use ECN.

   Two options for ECN in tunnels were specified:
   1) A limited-functionality option that does not use ECN inside the IP
   tunnel, by turning the ECT bit in the outer header off, and not
   altering the inner header at the time of decapsulation.
   2) The full-functionality option, which copies the ECT bit of the
   inner header to the encapsulating header. At decapsulation, if the
   ECT bit is set in the inner header, the CE bit on the outer header is
   ORed with the CE bit of the inner header to update the CE bit of the
   packet.

   All IP tunnels MUST implement one of the two alternative approaches
   described above.  For IPsec tunnels, this document also defines an
   optional IPsec Security Association (SA) attribute that enables



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   negotiation of ECN usage within IPsec tunnels and an optional field
   in the Security Association Database to indicate whether ECN is per-
   mitted in tunnel mode on a SA.  The required changes to IPsec tunnels
   for ECN usage modify RFC 2401 [RFC2401], which defines the IPsec
   architecture and specifies some aspects of its implementation.  The
   new IPsec SA attribute is in addition to those already defined in
   Section 4.5 of [RFC2407].

   This document is intended to obsolete RFC 2481, "A Proposal to add
   Explicit Congestion Notification (ECN) to IP", which defined ECN as
   an Experimental Protocol for the Internet Community.  The rest of
   this section describes the relationship between this document and its
   predecessor.

   RFC 2481 included a brief discussion of the use of ECN with encapsu-
   lated packets, and noted that for the IPsec specifications at the
   time (January 1999), flows could not safely use ECN if they were to
   traverse IPsec tunnels.  RFC 2481 also described the changes that
   could be made to IPsec tunnel specifications to made them compatible
   with ECN.

   This document also incorporates work that was done after RFC 2481,
   First was to describe the changes to IPsec tunnels in detail, and
   extensively discuss the security implications of ECN (now included as
   Sections 18 and 19 of this document).  Second was to extend the dis-
   cussion of IPsec tunnels to include all IP tunnels.  Because older IP
   tunnels are not compatible with a flow's use of ECN, the deployment
   of ECN in the Internet will create strong pressure for older IP tun-
   nels to be updated to an ECN-compatible version, using either the
   limited-functionality or the full-functionality option.

   This document does not address the issue of including ECN in non-IP
   tunnels such as MPLS, GRE, L2TP, or PPTP.  An earlier preliminary
   document about adding ECN support to MPLS was not advanced.

   A third new piece of work after RFC2481 was to describe the ECN pro-
   cedure with retransmitted data packets, that the ECT bit should not
   be set on retransmitted data packets.  The motivation for this addi-
   tional specification is to eliminate a possible avenue for denial-of-
   service attacks on an existing TCP connection.  Some prior deploy-
   ments of ECN-capable TCP might not conform to the (new) requirement
   not to set the ECT bit on retransmitted packets; we do not believe
   this will cause significant problems in practice.

   This document also expands slightly on the specification of the use
   of SYN packets for the negotiation of ECN.  While some prior deploy-
   ments of ECN-capable TCP might not conform to the requirements speci-
   fied in this document, we do not believe that this will lead to any



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   performance or compatibility problems for TCP connections with a com-
   bination of TCP implementations at the endpoints.

13.  Conclusions

   Given the current effort to implement AQM, we believe this is the
   right time to deploy congestion avoidance mechanisms that do not
   depend on packet drops alone.  With the increased deployment of
   applications and transports sensitive to the delay and loss of a sin-
   gle packet (e.g., realtime traffic, short web transfers), depending
   on packet loss as a normal congestion notification mechanism appears
   to be insufficient (or at the very least, non-optimal).

   We examined the consequence of modifications of the ECN field within
   the network, analyzing all the opportunities for an adversary to
   change the ECN field.  In many cases, the change to the ECN field is
   no worse than dropping a packet. However, we noted that some changes
   have the more serious consequence of subverting end-to-end congestion
   control.  However, we point out that even then the potential damage
   is limited, and is similar to the threat posed by end-systems inten-
   tionally failing to cooperate with end-to-end congestion control.

14.  Acknowledgements

   Many people have made contributions to this work and this document,
   including many that we have not managed to directly acknowledge in
   this document.  In addition, we would like to thank Kenjiro Cho for
   the proposal for the TCP mechanism for negotiating ECN-Capability,
   Kevin Fall for the proposal of the CWR bit, Steve Blake for material
   on IPv4 Header Checksum Recalculation, Jamal Hadi-Salim for discus-
   sions of ECN issues, and Steve Bellovin, Jim Bound, Brian Carpenter,
   Paul Ferguson, Stephen Kent, Greg Minshall, and Vern Paxson for dis-
   cussions of security issues.  We also thank the Internet End-to-End
   Research Group for ongoing discussions of these issues.

   Email discussions with a number of people, including Alexey
   Kuznetsov, Jamal Hadi-Salim, and Venkat Venkatsubra, have addressed
   the issues raised by non-conformant equipment in the Internet that
   does not respond to TCP SYN packets with the ECE and CWR flags set.
   We thank Mark Handley, Jitentra Padhye, and others for discussions on
   the TCP initialization procedures.

   The discussion of ECN and IP tunnel considerations draws heavily on
   related discussions and documents from the Differentiated Services
   Working Group.  We thank Tabassum Bint Haque from Dhaka, Bangladesh,
   for feedback on IP tunnels.  We thank Derrell Piper and Kero Tivinen
   for proposing modifications to RFC 2407 that improve the usability of
   negotiating the ECN Tunnel SA attribute.



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15.  References

   [AH] Kent, S. and R. Atkinson, "IP Authentication Header", RFC 2402,
   November 1998.

   [B97] Bradner, S., "Key words for use in RFCs to Indicate Requirement
   Levels", BCP 14, RFC 2119, March 1997.

   [ECN] "The ECN Web Page", URL "http://www.aciri.org/floyd/ecn.html".
   Reference for informational purposes only.

   [ESP] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload",
   RFC 2406, November 1998.

   [FJ93] Floyd, S., and Jacobson, V., "Random Early Detection gateways
   for Congestion Avoidance", IEEE/ACM Transactions on Networking, V.1
   N.4, August 1993, p.  397-413.

   [Floyd94] Floyd, S., "TCP and Explicit Congestion Notification", ACM
   Computer Communication Review, V. 24 N. 5, October 1994, p. 10-23.

   [Floyd98] Floyd, S., "The ECN Validation Test in the NS Simulator",
   URL "http://www-mash.cs.berkeley.edu/ns/", test tcl/test/test-all-
   ecn.  Reference for informational purposes only.

   [FF99] Floyd, S., and Fall, K., "Promoting the Use of End-to-End Con-
   gestion Control in the Internet", IEEE/ACM Transactions on Network-
   ing, August 1999.

   [FRED] Lin, D., and Morris, R., "Dynamics of Random Early Detection",
   SIGCOMM '97, September 1997.

   [GRE] S. Hanks, T. Li, D. Farinacci, and P. Traina, Generic Routing
   Encapsulation (GRE), RFC 1701, October 1994.

   [Jacobson88] V. Jacobson, "Congestion Avoidance and Control", Proc.
   ACM SIGCOMM '88, pp. 314-329.

   [Jacobson90] V. Jacobson, "Modified TCP Congestion Avoidance Algo-
   rithm", Message to end2end-interest mailing list, April 1990. URL
   "ftp://ftp.ee.lbl.gov/email/vanj.90apr30.txt".

   [K98] Krishnan, H., "Analyzing Explicit Congestion Notification (ECN)
   benefits for TCP", Master's thesis, UCLA, 1998, URL
   "http://www.cs.ucla.edu/~hari/software/ecn/ ecn_report.ps.gz".

   [L2TP] W. Townsley, A. Valencia, A. Rubens, G. Pall, G. Zorn, and B.
   Palter Layer Two Tunneling Protocol "L2TP", RFC 2661, August 1999.



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   [MJV96] S. McCanne, V. Jacobson, and M. Vetterli, "Receiver- driven
   Layered Multicast", SIGCOMM '96, August 1996, pp.  117-130.

   [MPLS] D. Awduche, J. Malcolm, J. Agogbua, M. O'Dell, J. McManus,
   Requirements for Traffic Engineering Over MPLS, RFC 2702, September
   1999.

   [PPTP] Hamzeh, K., Pall, G., Verthein, W., Taarud, J., Little, W.
   and G. Zorn, "Point-to-Point Tunneling Protocol (PPTP)", RFC 2637,
   July 1999.

   [RFC791] Postel, J., "Internet Protocol", STD 5, RFC 791, September
   1981.

   [RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
   September 1981.

   [RFC1141] Mallory, T. and A. Kullberg, "Incremental Updating of the
   Internet Checksum", RFC 1141, January 1990.

   [RFC1349] Almquist, P., "Type of Service in the Internet Protocol
   Suite", RFC 1349, July 1992.

   [RFC1455] Eastlake, D., "Physical Link Security Type of Service", RFC
   1455, May 1993.

   [RFC1701] Hanks, S., Li, T., Farinacci, D., and P. Traina, Generic
   Routing Encapsulation (GRE), RFC 1701, October 1994.

   [RFC1702] Hanks, S., Li, T., Farinacci, D., and P. Traina, Generic
   Routing Encapsulation over IPv4 networks, RFC 1702, October 1994.

   [RFC2003]  Perkins, C., IP Encapsulation within IP, RFC 2003, October
   1996.

   [RFC 2119] S. Bradner, Key words for use in RFCs to Indicate Require-
   ment Levels, RFC 2119, March 1997.

   [RFC2309] Braden, B., et al., "Recommendations on Queue Management
   and Congestion Avoidance in the Internet", RFC 2309, April 1998.

   [RFC2401] S. Kent and R. Atkinson, Security Architecture for the
   Internet Protocol, RFC 2401, November 1998.

   [RFC2407] D. Piper, The Internet IP Security Domain of Interpretation
   for ISAKMP, RFC 2407, November 1998.

   [RFC2408] D. Maughan, M. Schertler, M. Schneider, and J. Turner,



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   Internet Security Association and Key Management Protocol (ISAKMP),
   RFC 2409, November 1998.

   [RFC2409] D. Harkins and D. Carrel, The Internet Key Exchange (IKE),
   RFC 2409, 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.

   [RFC2475] S. Blake, D. Black, M. Carlson, E. Davies, Z. Wang, and W.
   Weiss, An Architecture for Differentiated Services, RFC 2475, Decem-
   ber 1998.

   [RFC2481] K. Ramakrishnan and S. Floyd, A Proposal to add Explicit
   Congestion Notification (ECN) to IP, RFC 2481, January 1999.

   [RFC2581] M. Allman, V. Paxson, W. Stevens, "TCP Congestion Control",
   RFC 2581, April 1999.

   [RFC2884] Jamal Hadi Salim and Uvaiz Ahmed, "Performance Evaluation
   of Explicit Congestion Notification (ECN) in IP Networks", RFC 2884,
   July 2000.

   [RFC2983] D. Black, "Differentiated Services and Tunnels", RFC2983,
   October 2000.

   [RFC2780] S. Bradner and V. Paxson, "IANA Allocation Guidelines For
   Values In the Internet Protocol and Related Headers", RFC 2780, March
   2000.

   [RJ90] K. K. Ramakrishnan and Raj Jain, "A Binary Feedback Scheme for
   Congestion Avoidance in Computer Networks", ACM Transactions on Com-
   puter Systems, Vol.8, No.2, pp.  158-181, May 1990.

   [SCWA99] Stefan Savage, Neal Cardwell, David Wetherall, and Tom
   Anderson, TCP Congestion Control with a Misbehaving Receiver, ACM
   Computer Communications Review, October 1999.

16.  Security Considerations

   Security considerations have been discussed in Sections 7, 8, 18, and
   19.

17.  IPv4 Header Checksum Recalculation

   IPv4 header checksum recalculation is an issue with some high-end
   router architectures using an output-buffered switch, since most if



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   not all of the header manipulation is performed on the input side of
   the switch, while the ECN decision would need to be made local to the
   output buffer. This is not an issue for IPv6, since there is no IPv6
   header checksum. The IPv4 TOS octet is the last byte of a 16-bit
   half-word.

   RFC 1141 [RFC1141] discusses the incremental updating of the IPv4
   checksum after the TTL field is decremented.  The incremental updat-
   ing of the IPv4 checksum after the CE bit was set would work as fol-
   lows: Let HC be the original header checksum, and let HC' be the new
   header checksum after the CE bit has been set.  Then for header
   checksums calculated with one's complement subtraction, HC' would be
   recalculated as follows:


        HC' = { HC - 1     HC > 1
              { 0x0000     HC = 1

For header checksums calculated on two's complement machines, HC' would
be recalculated as follows after the CE bit was set:


        HC' = { HC - 1     HC > 0
              { 0xFFFE     HC = 0


18.  Possible Changes to the ECN Field in the Network

   This section discusses in detail possible changes to the ECN field in
   the network, such as falsely reporting congestion, disabling ECN-
   Capability for an individual packet, erasing the ECN congestion indi-
   cation, or falsely indicating ECN-Capability.  We represent the ECN
   bits in the IP header by the tuple (ECT bit, CE bit).

18.1.  Possible Changes to the IP Header

18.1.1.  Erasing the Congestion Indication

   First, we consider the changes that a router could make that would
   result in effectively erasing the congestion indication after it had
   been set by a router upstream.  The convention followed is:
   (ECT, CE) of received packet -> (ECT, CE) of packet transmitted.

   (1, 1) -> (1, 0): erase only the CE bit that was set.
   (1, 1) -> (0, 0): erase both the ECT bit and the CE bit.
   (1, 1) -> (0, 1): erase the ECT bit

   The first change turns off the CE bit after it has been set by some



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   upstream router along the path.  The consequence for the upstream
   router is that there is a potential for congestion to build for a
   time, because the congestion indication does not reach the source.
   However, the packet would be received and acknowledged.

   The potential effect of erasing the congestion indication is complex,
   and is discussed in depth in Section 19 below.  Note that the effect
   of erasing the congestion indication is different from dropping a
   packet in the network.  When a data packet is dropped, the drop is
   detected by the TCP sender, and interpreted as an indication of con-
   gestion.  Similarly, if a sufficient number of consecutive acknowl-
   edgement packets are dropped, causing the cumulative acknowledgement
   field not to be advanced at the sender, the sender is limited by the
   congestion window from sending additional packets, and ultimately the
   retransmit timer expires.

   In contrast, a systematic erasure of the CE bit by a downstream
   router can have the effect of causing a queue buildup at an upstream
   router, including the possible loss of packets due to buffer over-
   flow.  There is a potential of unfairness in that another flow that
   goes through the congested router could react to the CE bit set while
   the flow that has the CE bit erased could see better performance.
   The limitations on this potential unfairness are discussed in more
   detail in Section 19 below.

   The second change is to turn off both the ECT and the CE bits, thus
   erasing the congestion indication and disabling ECN-Capability at the
   same time.  The third change turns off only the ECT bit, disabling
   ECN-Capability.

   Within an IP tunnel using the full-functionality option, the third
   change would not erase the congestion indication, but would only dis-
   able ECN-Capability for that packet within the rest of the tunnel.
   However, when performed outside of an IP tunnel, the third change
   would also effectively erase the congestion indication, because an
   ECN field of (0, 1) is undefined.

   The `erasure' of the congestion indication is only effective if the
   packet does not end up being marked or dropped again by a downstream
   router.  With the first change, the packet remains ECN-Capable, and
   could be either marked or dropped by a downstream router as an indi-
   cation of congestion.  With the second and third changes, the packet
   is no longer ECN-capable, and can therefore be dropped but not marked
   by a downstream router as an indication of congestion.







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18.1.2.  Falsely Reporting Congestion

   (1, 0) -> (1, 1)

   This change is to set the CE bit when the ECT bit was already set,
   even though there was no congestion.  This change does not affect the
   treatment of that packet along the rest of the path.  In particular,
   a router does not examine the CE bit in deciding whether to drop or
   mark an arriving packet.

   However, this could result in the application unnecessarily invoking
   end-to-end congestion control, and reducing its arrival rate.  By
   itself, this is no worse (for the application or for the network)
   than if the tampering router had actually dropped the packet.

18.1.3.  Disabling ECN-Capability

   (1, 0) -> (0, *)

   This change is to turn off the ECT bit of a packet that does not have
   the CE bit set.  (Section 18.1.1 discussed the case of turning off
   the ECT bit of a packet that does have the CE bit set.)  This means
   that if the packet later encounters congestion (e.g., by arriving to
   a RED queue with a moderate average queue size), it will be dropped
   instead of being marked.  By itself, this is no worse (for the appli-
   cation) than if the tampering router had actually dropped the packet.
   The saving grace in this particular case is that there is no con-
   gested router upstream expecting a reaction from setting the CE bit.

18.1.4.  Falsely Indicating ECN-Capability
   This change would incorrectly label a packet as ECN-Capable. The
   packet may have been sent either by an ECN-Capable transport or a
   transport that is not ECN-Capable.

   (0, *) -> (1, 0);
   (0, *) -> (1, 1);

   If the packet later encounters moderate congestion at an ECN-Capable
   router, the router could set the CE bit instead of dropping the
   packet.  If the transport protocol in fact is not ECN-Capable, then
   the transport will never receive this indication of congestion, and
   will not reduce its sending rate in response.  The potential conse-
   quences of falsely indicating ECN-capability are discussed further in
   Section 19 below.

   If the packet never later encounters congestion at an ECN-Capable
   router, then the first of these two changes would have no effect.
   The second change, however, would have the effect of giving false



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   reports of congestion to a monitoring device along the path.  If the
   transport protocol is ECN-Capable, then the second of these two
   changes (when, for example, (0,0) was changed to (1,1)) could also
   have an effect at the transport level, by combining falsely indicat-
   ing ECN-Capability with falsely reporting congestion.  For an ECN-
   capable transport, this would cause the transport to unnecessarily
   react to congestion.  In this particular case, the router that is
   incorrectly changing the ECN field could have dropped the packet.
   Thus for this case of an ECN-capable transport, the consequence of
   this change to the ECN field is no worse than dropping the packet.

18.1.5.  Changes with No Functional Effect

   (0, *) -> (0, *)

   The CE bit is ignored in a packet that does not have the ECT bit set.
   Thus, this change would have no effect, in terms of ECN.

18.2.  Information carried in the Transport Header

   For TCP, an ECN-capable TCP receiver informs its TCP peer that it is
   ECN-capable at the TCP level, conveying this information in the TCP
   header at the time the connection is setup.  This document does not
   consider potential dangers introduced by changes in the transport
   header within the network.  In the case of IPsec tunnels, the IPsec
   tunnel protects the transport header.

   Another issue concerns TCP packets with a spoofed IP source address
   carrying invalid ECN information in the transport header.  For com-
   pleteness, we examine here some possible ways that a node spoofing
   the IP source address of another node could use the two ECN flags in
   the TCP header to launch a denial-of-service attack. However, these
   attacks would require an ability for the attacker to use valid TCP
   sequence numbers, and any attacker with this ability and with the
   ability to spoof IP source addresses could damage the TCP connection
   without using the ECN flags.  Therefore, ECN does not add any new
   vulnerabilities in this respect.

   An acknowledgement packet with a spoofed IP source address of the TCP
   data receiver could include the ECE bit set.  If accepted by the TCP
   data sender as a valid packet, this spoofed acknowledgement packet
   could result in the TCP data sender unnecessarily halving its conges-
   tion window.  However, to be accepted by the data sender, such a
   spoofed acknowledgement packet would have to have the correct 32-bit
   sequence number as well as a valid acknowledgement number.  An
   attacker that could successfully send such a spoofed acknowledgement
   packet could also send a spoofed RST packet, or do other equally dam-
   aging operations to the TCP connection.



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   Packets with a spoofed IP source address of the TCP data sender could
   include the CWR bit set.  Again, to be accepted, such a packet would
   have to have a valid sequence number.  In addition, such a spoofed
   packet would have a limited performance impact.  Spoofing a data
   packet with the CWR bit set could result in the TCP data receiver
   sending fewer ECE packets than it would otherwise, if the data
   receiver was sending ECE packets when it received the spoofed CWR
   packet.

18.3.  Split Paths

   In some cases, a malicious or broken router might have access to only
   a subset of the packets from a flow.  The question is as follows:
   can this router, by altering the ECN field in this subset of the
   packets, do more damage to that flow than if it had simply dropped
   that set of packets?

   We will classify the packets in the flow as A packets and B packets,
   and assume that the adversary only has access to A packets.  Assume
   that the adversary is subverting end-to-end congestion control along
   the path traveled by A packets only, by either falsely indicating
   ECN-Capability upstream of the point where congestion occurs, or
   erasing the congestion indication downstream.  Consider also that
   there exists a monitoring device that sees both the A and B packets,
   and will "punish" both the A and B packets if the total flow is
   determined not to be properly responding to indications of conges-
   tion.  Another key characteristic that we believe is likely to be
   true is that the monitoring device, before `punishing' the A&B flow,
   will first drop packets instead of setting the CE bit, and will drop
   arriving packets of that flow that already have the ECT and CE bits
   set.  If the end nodes are in fact using end-to-end congestion con-
   trol, they will see all of the indications of congestion seen by the
   monitoring device, and will begin to respond to these indications of
   congestion. Thus, the monitoring device is successful in providing
   the indications to the flow at an early stage.

   It is true that the adversary that has access only to the A packets
   might, by subverting ECN-based congestion control, be able to deny
   the benefits of ECN to the other packets in the A&B aggregate.  While
   this is unfortunate, this is not a reason to disable ECN within an
   IPsec tunnel.

   A variant of falsely reporting congestion occurs when there are two
   adversaries along a path, where the first adversary falsely reports
   congestion, and the second adversary `erases' those reports. (Unlike
   packet drops, ECN congestion reports can be `reversed' later in the
   network by a malicious or broken router.)  While this would be trans-
   parent to the end node, it is possible that a monitoring device



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   between the first and second adversaries would see the false indica-
   tions of congestion.  Keep in mind our recommendation in this docu-
   ment, that before `punishing' a flow for not responding appropriately
   to congestion, the router will first switch to dropping rather than
   marking as an indication of congestion, for that flow.  When this
   includes dropping arriving packets from that flow that have the CE
   bit set, this ensures that these indications of congestion are being
   seen by the end nodes.  Thus, there is no additional harm that we are
   able to postulate as a result of multiple conflicting adversaries.

19.  Implications of Subverting End-to-End Congestion Control

   This section focuses on the potential repercussions of subverting
   end-to-end congestion control by either falsely indicating ECN-Capa-
   bility, or by erasing the congestion indication in ECN (the CE-bit).
   Subverting end-to-end congestion control by either of these two meth-
   ods can have consequences both for the application and for the net-
   work.  We discuss these separately below.

   The first method to subvert end-to-end congestion control, that of
   falsely indicating ECN-Capability, effectively subverts end-to-end
   congestion control only if the packet later encounters congestion
   that results in the setting of the CE bit.  In this case, the trans-
   port protocol (which may not be ECN-capable) does not receive the
   indication of congestion from these downstream congested routers.

   The second method to subvert end-to-end congestion control, `erasing'
   the (set) CE bit in a packet, effectively subverts end-to-end conges-
   tion control only when the CE bit in the packet was set earlier by a
   congested router.  In this case, the transport protocol does not
   receive the indication of congestion from the upstream congested
   routers.

   Either of these two methods of subverting end-to-end congestion con-
   trol can potentially introduce more damage to the network (and possi-
   bly to the flow itself) than if the adversary had simply dropped
   packets from that flow.  However, as we discuss later in this section
   and in Section 7, this potential damage is limited.

19.1.  Implications for the Network and for Competing Flows

   The CE bit of the ECN field is only used by routers as an indication
   of congestion during periods of *moderate* congestion.  ECN-capable
   routers should drop rather than mark packets during heavy congestion
   even if the router's queue is not yet full.  For example, for routers
   using active queue management based on RED, the router should drop
   rather than mark packets that arrive while the average queue sizes
   exceed the RED queue's maximum threshold.



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   One consequence for the network of subverting end-to-end congestion
   control is that flows that do not receive the congestion indications
   from the network might increase their sending rate until they drive
   the network into heavier congestion.  Then, the congested router
   could begin to drop rather than mark arriving packets.  For flows
   that are not isolated by some form of per-flow scheduling or other
   per-flow mechanisms, but are instead aggregated with other flows in a
   single queue in an undifferentiated fashion, this packet-dropping at
   the congested router would apply to all flows that share that queue.
   Thus, the consequences would be to increase the level of congestion
   in the network.

   In some cases, the increase in the level of congestion will lead to a
   substantial buffer buildup at the congested queue that will be suffi-
   cient to drive the congested queue from the packet-marking to the
   packet-dropping regime.  This transition could occur either because
   of buffer overflow, or because of the active queue management policy
   described above that drops packets when the average queue is above
   RED's maximum threshold.  At this point, all flows, including the
   subverted flow, will begin to see packet drops instead of packet
   marks, and a malicious or broken router will no longer be able to
   `erase' these indications of congestion in the network.  If the end
   nodes are deploying appropriate end-to-end congestion control, then
   the subverted flow will reduce its arrival rate in response to con-
   gestion.  When the level of congestion is sufficiently reduced, the
   congested queue can return from the packet-dropping regime to the
   packet-marking regime.  The steady-state pattern could be one of the
   congested queue oscillating between these two regimes.

   In other cases, the consequences of subverting end-to-end congestion
   control will not be severe enough to drive the congested link into
   sufficiently-heavy congestion that packets are dropped instead of
   being marked.  In this case, the implications for competing flows in
   the network will be a slightly-increased rate of packet marking or
   dropping, and a corresponding decrease in the bandwidth available to
   those flows.  This can be a stable state if the arrival rate of the
   subverted flow is sufficiently small, relative to the link bandwidth,
   that the average queue size at the congested router remains under
   control.  In particular, the subverted flow could have a limited
   bandwidth demand on the link at this router, while still getting more
   than its "fair" share of the link.  This limited demand could be due
   to a limited demand from the data source; a limitation from the TCP
   advertised window; a lower-bandwidth access pipe; or other factors.
   Thus the subversion of ECN-based congestion control can still lead to
   unfairness, which we believe is appropriate to note here.

   The threat to the network posed by the subversion of ECN-based con-
   gestion control in the network is essentially the same as the threat



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   posed by an end-system that intentionally fails to cooperate with
   end-to-end congestion control.  The deployment of mechanisms in
   routers to address this threat is an open research question, and is
   discussed further in Section 10.

   Let us take the example described in Section 18.1.1, where the CE bit
   that was set in a packet is erased: {(1, 1) -> (1, 0)}.  The conse-
   quence for the congested upstream router that set the CE bit is that
   this congestion indication does not reach the end nodes for that
   flow. The source (even one which is completely cooperative and not
   malicious) is thus allowed to continue to increase its sending rate
   (if it is a TCP flow, by increasing its congestion window).  The flow
   potentially achieves better throughput than the other flows that also
   share the congested router, especially if there are no policing mech-
   anisms or per-flow queueing mechanisms at that router.  Consider the
   behavior of the other flows, especially if they are cooperative: that
   is, the flows that do not experience subverted end-to-end congestion
   control.  They are likely to reduce their load (e.g., by reducing
   their window size) on the congested router, thus benefiting our sub-
   verted flow. This results in unfairness.  As we discussed above, this
   unfairness could either be transient (because the congested queue is
   driven into the packet-marking regime), oscillatory (because the con-
   gested queue oscillates between the packet marking and the packet
   dropping regime), or more moderate but a persistent stable state
   (because the congested queue is never driven to the packet dropping
   regime).

   The results would be similar if the subverted flow was intentionally
   avoiding end-to-end congestion control.  One difference is that a
   flow that is intentionally avoiding end-to-end congestion control at
   the end nodes can avoid end-to-end congestion control even when the
   congested queue is in packet-dropping mode, by refusing to reduce its
   sending rate in response to packet drops in the network.  Thus the
   problems for the network from the subversion of ECN-based congestion
   control are less severe than the problems caused by the intentional
   avoidance of end-to-end congestion control in the end nodes.  It is
   also the case that it is considerably more difficult to control the
   behavior of the end nodes than it is to control the behavior of the
   infrastructure itself.  This is not to say that the problems for the
   network posed by the network's subversion of ECN-based congestion
   control are small; just that they are dwarfed by the problems for the
   network posed by the subversion of either ECN-based or other cur-
   rently known packet-based congestion control mechanisms by the end
   nodes.







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19.2.  Implications for the Subverted Flow

   When a source indicates that it is ECN-capable, there is an expecta-
   tion that the routers in the network that are capable of participat-
   ing in ECN will use the CE bit for indication of congestion. There is
   the potential benefit of using ECN in reducing the amount of packet
   loss (in addition to the reduced queueing delays because of active
   queue management policies).  When the packet flows through a tunnel
   where the nodes that the tunneled packets traverse are untrusted in
   some way, the expectation is that IPsec will protect the flow from
   subversion that results in undesirable consequences.

   In many cases, a subverted flow will benefit from the subversion of
   end-to-end congestion control for that flow in the network, by
   receiving more bandwidth than it would have otherwise, relative to
   competing non-subverted flows.  If the congested queue reaches the
   packet-dropping stage, then the subversion of end-to-end congestion
   control might or might not be of overall benefit to the subverted
   flow, depending on that flow's relative tradeoffs between throughput,
   loss, and delay.

   One form of subverting end-to-end congestion control is to falsely
   indicate ECN-capability by setting the ECT bit.  This has the conse-
   quence of downstream congested routers setting the CE bit in vain.
   However, as described in Section 9.1.2, if the ECT bit is changed in
   an IP tunnel, this can be detected at the egress point of the tunnel,
   as long as the inner header was not changed within the tunnel.

   The second form of subverting end-to-end congestion control is to
   erase the congestion indication, either by erasing the CE bit
   directly, or by erasing the ECT bit when the CE bit is already set.
   In this case, it is the upstream congested routers that set the CE
   bit in vain.

   If the ECT bit is erased within an IP tunnel, then this can be
   detected at the egress point of the tunnel, as long as the inner
   header was not changed within the tunnel.  If the CE bit is set
   upstream of the IP tunnel, then any erasure of the outer header's CE
   bit within the tunnel will have no effect because the inner header
   preserves the set value of the CE bit.  However, if the CE bit is set
   within the tunnel, and erased either within or downstream of the tun-
   nel, this is not necessarily detected at the egress point of the tun-
   nel.

   With this subversion of end-to-end congestion control, an end-system
   transport does not respond to the congestion indication.  Along with
   the increased unfairness for the non-subverted flows described in the
   previous section, the congested router's queue could continue to



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   build, resulting in packet loss at the congested router - which is a
   means for indicating congestion to the transport in any case.  In the
   interim, the flow might experience higher queueing delays, possibly
   along with an increased bandwidth relative to other non-subverted
   flows.  But transports do not inherently make assumptions of consis-
   tently experiencing carefully managed queueing in the path.  We
   believe that these forms of subverting end-to-end congestion control
   are no worse for the subverted flow than if the adversary had simply
   dropped the packets of that flow itself.

19.3.  Non-ECN-Based Methods of Subverting End-to-end Congestion Control

   We have shown that, in many cases, a malicious or broken router that
   is able to change the bits in the ECN field can do no more damage
   than if it had simply dropped the packet in question.  However, this
   is not true in all cases, in particular in the cases where the broken
   router subverted end-to-end congestion control by either falsely
   indicating ECN-Capability or by erasing the ECN congestion indication
   (in the CE-bit).  While there are many ways that a router can harm a
   flow by dropping packets, a router cannot subvert end-to-end conges-
   tion control by dropping packets.  As an example, a router cannot
   subvert TCP congestion control by dropping data packets, acknowledge-
   ment packets, or control packets.

   Even though packet-dropping cannot be used to subvert end-to-end con-
   gestion control, there *are* non-ECN-based methods for subverting
   end-to-end congestion control that a broken or malicious router could
   use.  For example, a broken router could duplicate data packets, thus
   effectively negating the effects of end-to-end congestion control
   along some portion of the path.  (For a router that duplicated pack-
   ets within an IPsec tunnel, the security administrator can cause the
   duplicate packets to be discarded by configuring anti-replay protec-
   tion for the tunnel.)  This duplication of packets within the network
   would have similar implications for the network and for the subverted
   flow as those described in Sections 18.1.1 and 18.1.4 above.

20.  The Motivation for the ECT bit.

   The need for the ECT bit is motivated by the fact that ECN will be
   deployed incrementally in an Internet where some transport protocols
   and routers understand ECN and some do not. With the ECT bit, the
   router can drop packets from flows that are not ECN-capable, but can
   *instead* set the CE bit in packets that *are* ECN-capable. Because
   the ECT bit allows an end node to have the CE bit set in a packet
   *instead* of having the packet dropped, an end node might have some
   incentive to deploy ECN.

   If there was no ECT indication, then the router would have to set the



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   CE bit for packets from both ECN-capable and non-ECN-capable flows.
   In this case, there would be no incentive for end-nodes to deploy
   ECN, and no viable path of incremental deployment from a non-ECN
   world to an ECN-capable world.  Consider the first stages of such an
   incremental deployment, where a subset of the flows are ECN-capable.
   At the onset of congestion, when the packet dropping/marking rate
   would be low, routers would only set CE bits, rather than dropping
   packets.  However, only those flows that are ECN-capable would under-
   stand and respond to CE packets. The result is that the ECN-capable
   flows would back off, and the non-ECN-capable flows would be unaware
   of the ECN signals and would continue to open their congestion win-
   dows.

   In this case, there are two possible outcomes: (1) the ECN-capable
   flows back off, the non-ECN-capable flows get all of the bandwidth,
   and congestion remains mild, or (2) the ECN-capable flows back off,
   the non-ECN-capable flows don't, and congestion increases until the
   router transitions from setting the CE bit to dropping packets.
   While this second outcome evens out the fairness, the ECN-capable
   flows would still receive little benefit from being ECN-capable,
   because the increased congestion would drive the router to packet-
   dropping behavior.

   A flow that advertised itself as ECN-Capable but does not respond to
   CE bits is functionally equivalent to a flow that turns off conges-
   tion control, as discussed earlier in this document.

   Thus, in a world when a subset of the flows are ECN-capable, but
   where ECN-capable flows have no mechanism for indicating that fact to
   the routers, there would be less effective and less fair congestion
   control in the Internet, resulting in a strong incentive for end
   nodes not to deploy ECN.

21.  Why use Two Bits in the IP Header?

   Given the need for an ECT indication in the IP header, there still
   remains the question of whether the ECT (ECN-Capable Transport) and
   CE (Congestion Experienced) indications should have been overloaded
   on a single bit.  This overloaded-one-bit alternative, explored in
   [Floyd94], would have involved a single bit with two values.  One
   value, "ECT and not CE", would represent an ECN-Capable Transport,
   and the other value, "CE or not ECT", would represent either Conges-
   tion Experienced or a non-ECN-Capable transport.

   One difference between the one-bit and two-bit implementations con-
   cerns packets that traverse multiple congested routers.  Consider a
   CE packet that arrives at a second congested router, and is selected
   by the active queue management at that router for either marking or



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   dropping.  In the one-bit implementation, the second congested router
   has no choice but to drop the CE packet, because it cannot distin-
   guish between a CE packet and a non-ECT packet.  In the two-bit
   implementation, the second congested router has the choice of either
   dropping the CE packet, or of leaving it alone with the CE bit set.

   Another difference between the one-bit and two-bit implementations
   comes from the fact that with the one-bit implementation, receivers
   in a single flow cannot distinguish between CE and non-ECT packets.
   Thus, in the one-bit implementation an ECN-capable data sender would
   have to unambiguously indicate to the receiver or receivers whether
   each packet had been sent as ECN-Capable or as non-ECN-Capable.  One
   possibility would be for the sender to indicate in the transport
   header whether the packet was sent as ECN-Capable.  A second possi-
   bility that would involve a functional limitation for the one- bit
   implementation would be for the sender to unambiguously indicate that
   it was going to send *all* of its packets as ECN-Capable or as non-
   ECN-Capable.  For a multicast transport protocol, this unambiguous
   indication would have to be apparent to receivers joining an on-going
   multicast session.

   Another concern that was described earlier (and recommended in this
   document) is that transports (particularly TCP) should not mark pure
   ACK packets or retransmitted packets as being ECN-Capable.  A pure
   ACK packet from a non-ECN-capable transport could be dropped, without
   necessarily having an impact on the transport from a congestion con-
   trol perspective (because subsequent ACKs are cumulative).  An ECN-
   capable transport reacting to the CE bit set in a pure ACK packet by
   reducing the window would be at a disadvantage in comparison to a
   non-ECN-capable transport. For this reason (and for reasons described
   earlier in relation to retransmitted packets), it is desirable to
   have the ECN-Capable bit indication on a per-packet basis.

   Another advantage of the two-bit approach is that it is somewhat more
   robust.  The most critical issue, discussed in Section 8, is that the
   default indication should be that of a non-ECN-Capable transport.  In
   a two-bit implementation, this requirement for the default value sim-
   ply means that the ECT bit should be `OFF' by default.  In the one-
   bit implementation, this means that the single overloaded bit should
   by default be in the "CE or not ECT" position.  This is less clear
   and straightforward, and possibly more open to incorrect implementa-
   tions either in the end nodes or in the routers.

   In summary, while the one-bit implementation could be a possible
   implementation, it has the following significant limitations relative
   to the two-bit implementation.  First, the one-bit implementation has
   more limited functionality for the treatment of CE packets at a sec-
   ond congested router.  Second, the one-bit implementation requires



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   either that extra information be carried in the transport header of
   packets from ECN-Capable flows (to convey the functionality of the
   second bit elsewhere, namely in the transport header), or that
   senders in ECN-Capable flows accept the limitation that receivers
   must be able to determine a priori which packets are ECN-Capable and
   which are not ECN-Capable. Third, the one-bit implementation is pos-
   sibly more open to errors from faulty implementations that choose the
   wrong default value for the ECN bit.  We believe that the use of the
   extra bit in the IP header for the ECT-bit is extremely valuable to
   overcome these limitations.

22.  Historical Definitions for the IPv4 TOS Octet

   RFC 791 [RFC791] defined the ToS (Type of Service) octet in the IP
   header.  In RFC 791, bits 6 and 7 of the ToS octet are listed as
   "Reserved for Future Use", and are shown set to zero.  The first two
   fields of the ToS octet were defined as the Precedence and Type of
   Service (TOS) fields.

            0     1     2     3     4     5     6     7
         +-----+-----+-----+-----+-----+-----+-----+-----+
         |   PRECEDENCE    |       TOS       |  0  |  0  |  RFC 791
         +-----+-----+-----+-----+-----+-----+-----+-----+

   RFC 1122 included bits 6 and 7 in the TOS field, though it did not
   discuss any specific use for those two bits:

            0     1     2     3     4     5     6     7
         +-----+-----+-----+-----+-----+-----+-----+-----+
         |   PRECEDENCE    |       TOS                   |  RFC 1122
         +-----+-----+-----+-----+-----+-----+-----+-----+

   The IPv4 TOS octet was redefined in RFC 1349 [RFC1349] as follows:

            0     1     2     3     4     5     6     7
         +-----+-----+-----+-----+-----+-----+-----+-----+
         |   PRECEDENCE    |       TOS             | MBZ |  RFC 1349
         +-----+-----+-----+-----+-----+-----+-----+-----+


   Bit 6 in the TOS field was defined in RFC 1349 for "Minimize Monetary
   Cost".  In addition to the Precedence and Type of Service (TOS)
   fields, the last field, MBZ (for "must be zero") was defined as cur-
   rently unused.  RFC 1349 stated that "The originator of a datagram
   sets [the MBZ] field to zero (unless participating in an Internet
   protocol experiment which makes use of that bit)."

   RFC 1455 [RFC 1455] defined an experimental standard that used all



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   four bits in the TOS field to request a guaranteed level of link
   security.

   RFC 1349 and RFC 1455 have been obsoleted by "Definition of the Dif-
   ferentiated Services Field (DS Field) in the IPv4 and IPv6 Headers"
   [RFC2474] in which bits 6 and 7 of the DS field are listed as Cur-
   rently Unused (CU).  RFC 2780 [RFC2780] specified ECN as an experi-
   mental use of the two-bit CU field.  RFC 2780 updated the definition
   of the DS Field to only encompass the first six bits of this octet
   rather than all eight bits; these first six bits are defined as the
   Differentiated Services CodePoint (DSCP):

            0     1     2     3     4     5     6     7
         +-----+-----+-----+-----+-----+-----+-----+-----+
         |               DSCP                |    CU     |  RFCs 2474,
   2780
         +-----+-----+-----+-----+-----+-----+-----+-----+

   Because of this unstable history, the definition of the ECN field in
   this document cannot be guaranteed to be backwards compatible with
   all past uses of these two bits.

   Prior to RFC 2474, routers were not permitted to modify bits in
   either the DSCP or ECN field of packets forwarded through them, and
   hence routers that comply only with RFCs prior to 2474 should have no
   effect on ECN.  For end nodes, bit 7 (the ECN CE bit) must be trans-
   mitted as zero for any implementation compliant only with RFCs prior
   to 2474.  Such nodes may transmit bit 6 (the ECN ECT bit) as one for
   the "Minimize Monetary Cost" provision of RFC 1349 or the experiment
   authorized by RFC 1455; neither this aspect of RFC 1349 nor the
   experiment in RFC 1455 were widely implemented or used.  The damage
   that could be done by a broken, non-conformant router would be to
   "erase" the CE bit for an ECN- capable packet that arrived at the
   router with the CE bit set, or set the CE bit even in the absence of
   congestion.  This has been discussed in the section on "Non-compli-
   ance in the Network".

   The damage that could be done in an ECN-capable environment by a non-
   ECN-capable end-node transmitting packets with the ECT bit set has
   been discussed in the section on "Non-compliance by the End Nodes".

23.  IANA Considerations

   The bits for ECT and CE in the ECN Field of the IP header and the
   bits for CWR and ECE in the TCP header are specified by the Standards
   Action of this RFC, as is required by RFC 2780.  We would note that
   this RFC does not define the codepoint of (ECT=0, CE=1) for the ECT
   and CE bits.



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   IANA allocated the IPSEC Security Association Attribute value 10 for
   the ECN Tunnel use described in Section 9.2.1.2 above at the request
   of David Black in November 1999.  If this draft is approved for pub-
   lication as an RFC, IANA should change the Reference for this alloca-
   tion from David Black's request to this RFC based on its RFC number.

   AUTHORS' ADDRESSES


      K. K. Ramakrishnan
      TeraOptic Networks, Inc.
      Phone: +1 (408) 666-8650
      Email: kk@teraoptic.com

      Sally Floyd
      Phone: +1 (510) 666-2989
      ACIRI
      Email: floyd@aciri.org
      URL: http://www.aciri.org/floyd/

      David L. Black
      EMC Corporation
      42 South St.
      Hopkinton, MA  01748
      Phone:  +1 (508) 435-1000 x75140
      Email: black_david@emc.com


      This draft was created in January 2001.
      It expires July 2001.





















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