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rfc:rfc3539

Network Working Group B. Aboba Request for Comments: 3539 Microsoft Category: Standards Track J. Wood

                                                Sun Microsystems, Inc.
                                                             June 2003
Authentication, Authorization and Accounting (AAA) Transport Profile

Status of this Memo

 This document specifies an Internet standards track protocol for the
 Internet community, and requests discussion and suggestions for
 improvements.  Please refer to the current edition of the "Internet
 Official Protocol Standards" (STD 1) for the standardization state
 and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

 Copyright (C) The Internet Society (2003).  All Rights Reserved.

Abstract

 This document discusses transport issues that arise within protocols
 for Authentication, Authorization and Accounting (AAA).  It also
 provides recommendations on the use of transport by AAA protocols.
 This includes usage of standards-track RFCs as well as experimental
 proposals.

Table of Contents

 1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  2
     1.1.  Requirements Language. . . . . . . . . . . . . . . . . .  2
     1.2.  Terminology. . . . . . . . . . . . . . . . . . . . . . .  2
 2.  Issues in Transport Usage. . . . . . . . . . . . . . . . . . .  5
     2.1.  Application-driven Versus Network-driven . . . . . . . .  5
     2.2.  Slow Failover. . . . . . . . . . . . . . . . . . . . . .  6
     2.3.  Use of Nagle Algorithm . . . . . . . . . . . . . . . . .  7
     2.4.  Multiple Connections . . . . . . . . . . . . . . . . . .  7
     2.5.  Duplicate Detection. . . . . . . . . . . . . . . . . . .  8
     2.6.  Invalidation of Transport Parameter Estimates. . . . . .  8
     2.7.  Inability to use Fast Re-Transmit. . . . . . . . . . . .  9
     2.8.  Congestion Avoidance . . . . . . . . . . . . . . . . . .  9
     2.9.  Delayed Acknowledgments. . . . . . . . . . . . . . . . . 11
     2.10. Premature Failover . . . . . . . . . . . . . . . . . . . 11
     2.11. Head of Line Blocking. . . . . . . . . . . . . . . . . . 11
     2.12. Connection Load Balancing. . . . . . . . . . . . . . . . 12

Aboba & Wood Standards Track [Page 1] RFC 3539 AAA Transport Profile June 2003

 3.  AAA Transport Profile. . . . . . . . . . . . . . . . . . . . . 12
     3.1.  Transport Mappings . . . . . . . . . . . . . . . . . . . 12
     3.2.  Use of Nagle Algorithm . . . . . . . . . . . . . . . . . 12
     3.3.  Multiple Connections . . . . . . . . . . . . . . . . . . 13
     3.4.  Application Layer Watchdog . . . . . . . . . . . . . . . 13
     3.5.  Duplicate Detection. . . . . . . . . . . . . . . . . . . 19
     3.6.  Invalidation of Transport Parameter Estimates. . . . . . 20
     3.7.  Inability to use Fast Re-Transmit. . . . . . . . . . . . 21
     3.8.  Head of Line Blocking. . . . . . . . . . . . . . . . . . 22
     3.9.  Congestion Avoidance . . . . . . . . . . . . . . . . . . 23
     3.10. Premature Failover . . . . . . . . . . . . . . . . . . . 24
 4.  Security Considerations. . . . . . . . . . . . . . . . . . . . 24
 5.  IANA Considerations. . . . . . . . . . . . . . . . . . . . . . 25
 6.  References . . . . . . . . . . . . . . . . . . . . . . . . . . 25
     6.1.  Normative References . . . . . . . . . . . . . . . . . . 25
     6.2.  Informative References . . . . . . . . . . . . . . . . . 26
 Appendix A - Detailed Watchdog Algorithm Description . . . . . . . 28
 Appendix B - AAA Agents. . . . . . . . . . . . . . . . . . . . . . 33
     B.1.  Relays and Proxies . . . . . . . . . . . . . . . . . . . 33
     B.2.  Re-directs . . . . . . . . . . . . . . . . . . . . . . . 35
     B.3.  Store and Forward Proxies. . . . . . . . . . . . . . . . 36
     B.4.  Transport Layer Proxies. . . . . . . . . . . . . . . . . 38
 Intellectual Property Statement. . . . . . . . . . . . . . . . . . 39
 Acknowledgments. . . . . . . . . . . . . . . . . . . . . . . . . . 39
 Author Addresses . . . . . . . . . . . . . . . . . . . . . . . . . 40
 Full Copyright Statement . . . . . . . . . . . . . . . . . . . . . 41

1. Introduction

 This document discusses transport issues that arise within protocols
 for Authentication, Authorization and Accounting (AAA).  It also
 provides recommendations on the use of transport by AAA protocols.
 This includes usage of standards-track RFCs as well as experimental
 proposals.

1.1. Requirements Language

 In this document, the key words "MAY", "MUST, "MUST NOT", "optional",
 "recommended", "SHOULD", and "SHOULD NOT", are to be interpreted as
 described in [RFC2119].

1.2. Terminology

 Accounting
           The act of collecting information on resource usage for the
           purpose of trend analysis, auditing, billing, or cost
           allocation.

Aboba & Wood Standards Track [Page 2] RFC 3539 AAA Transport Profile June 2003

 Administrative Domain
           An internet, or a collection of networks, computers, and
           databases under a common administration.
 Agent     A AAA agent is an intermediary that communicates with AAA
           clients and servers.  Several types of AAA agents exist,
           including Relays, Re-directs, and Proxies.
 Application-driven transport
           Transport behavior is said to be "application-driven" when
           the rate at which messages are sent is limited by the rate
           at which the application generates data, rather than by the
           size of the congestion window.  In the most extreme case,
           the time between transactions exceeds the round-trip time
           between sender and receiver, implying that the application
           operates with an effective congestion window of one.  AAA
           transport is typically application driven.
 Attribute Value Pair (AVP)
           The variable length concatenation of a unique Attribute
           (represented by an integer) and a Value containing the
           actual value identified by the attribute.
 Authentication
           The act of verifying a claimed identity, in the form of a
           pre-existing label from a mutually known name space, as the
           originator of a message (message authentication) or as the
           end-point of a channel (entity authentication).
 Authorization
           The act of determining if a particular right, such as
           access to some resource, can be granted to the presenter of
           a particular credential.
 Billing   The act of preparing an invoice.
 Network Access Identifier
           The Network Access Identifier (NAI) is the userID submitted
           by the host during network access authentication.  In
           roaming, the purpose of the NAI is to identify the user as
           well as to assist in the routing of the authentication
           request.  The NAI may not necessarily be the same as the
           user's e-mail address or the user-ID submitted in an
           application layer authentication.

Aboba & Wood Standards Track [Page 3] RFC 3539 AAA Transport Profile June 2003

 Network Access Server (NAS)
           A Network Access Server (NAS) is a device that hosts
           connect to in order to get access to the network.
 Proxy     In addition to forwarding requests and responses, proxies
           enforce policies relating to resource usage and
           provisioning.  This is typically accomplished by tracking
           the state of NAS devices.  While proxies typically do not
           respond to client Requests prior to receiving a Response
           from the server, they may originate Reject messages in
           cases where policies are violated.  As a result, proxies
           need to understand the semantics of the messages passing
           through them, and may not support all extensions.
 Local Proxy
           A Local Proxy is a proxy that exists within the same
           administrative domain as the network device (e.g. NAS) that
           issued the AAA request.  Typically a local proxy is used to
           multiplex AAA messages to and from a large number of
           network devices, and may implement policy.
 Store and forward proxy
           Store and forward proxies distinguish themselves from other
           proxy species by sending a reply to the NAS prior to
           proxying the request to the server.  As a result, store and
           forward proxies need to implement AAA client and server
           functionality for the messages that they handle.  Store and
           Forward proxies also typically keep state on conversations
           in progress in order to assure delivery of proxied Requests
           and Responses.  While store and forward proxies are most
           frequently deployed for accounting, they also can be used
           to implement authentication/authorization policy.
 Network-driven transport
           Transport behavior is said to be "network driven" when the
           rate at which messages are sent is limited by the
           congestion window, not by the rate at which the application
           can generate data.  File transfer is an example of an
           application where transport is network driven.
 Re-direct Rather than forwarding Requests and Responses between
           clients and servers, Re-directs refer clients to servers
           and allow them to communicate directly.  Since Re-directs
           do not sit in the forwarding path, they do not alter any
           AVPs transitting between client and server.  Re-directs do
           not originate messages and are capable of handling any
           message type.  A Re-direct may be configured only to re-
           direct messages of certain types, while acting as a Relay

Aboba & Wood Standards Track [Page 4] RFC 3539 AAA Transport Profile June 2003

           or Proxy for other types.  As with Relays, re-directs do
           not keep state with respect to conversations or NAS
           resources.
 Relay     Relays forward requests and responses based on routing-
           related AVPs and domain forwarding table entries.  Since
           relays do not enforce policies, they do not examine or
           alter non-routing AVPs.  As a result, relays never
           originate messages, do not need to understand the semantics
           of messages or non-routing AVPs, and are capable of
           handling any extension or message type.  Since relays make
           decisions based on information in routing AVPs and domain
           forwarding tables they do not keep state on NAS resource
           usage or conversations in progress.

2. Issues in AAA Transport Usage

 Issues that arise in AAA transport usage include:
    Application-driven versus network-driven
    Slow failover
    Use of Nagle Algorithm
    Multiple connections
    Duplicate detection
    Invalidation of transport parameter estimates
    Inability to use fast re-transmit
    Congestion avoidance
    Delayed acknowledgments
    Premature Failover
    Head of line blocking
    Connection load balancing
 We discuss each of these issues in turn.

2.1. Application-driven versus Network-driven

 AAA transport behavior is typically application rather than network
 driven.  This means that the rate at which messages are sent is
 typically limited by how quickly they are generated by the
 application, rather than by the size of the congestion window.
 For example, let us assume a 48-port NAS with an average session time
 of 20 minutes.  This device will, on average, send only 144
 authentication/authorization requests/hour, and an equivalent number
 of accounting requests.  This represents an average inter-packet
 spacing of 25 seconds, which is much larger than the Round Trip Time
 (RTT) in most networks.

Aboba & Wood Standards Track [Page 5] RFC 3539 AAA Transport Profile June 2003

 Even on much larger NAS devices, the inter-packet spacing is often
 larger than the RTT.  For example, consider a 2048-port NAS with an
 average session time of 10 minutes.  It will on average send 3.4
 authentication/authorization requests/second, and an equivalent
 number of accounting requests.  This translates to an average inter-
 packet spacing of 293 ms.
 However, even where transport behavior is largely application-driven,
 periods of network-driven behavior can occur.  For example, after a
 NAS reboot, previously stored accounting records may be sent to the
 accounting server in rapid succession.  Similarly, after recovery
 from a power failure, users may respond with a large number of
 simultaneous logins.  In both cases, AAA messages may be generated
 more quickly than the network will allow them to be sent, and a queue
 will build up.
 Network congestion can occur when transport behavior is network-
 driven or application-driven.  For example, while a single NAS may
 not send substantial AAA traffic, many NASes may communicate with a
 single AAA proxy or server.  As a result, routers close to a heavily
 loaded proxy or server may experience congestion, even though traffic
 from each individual NAS is light.  Such "convergent congestion" can
 result in dropped packets in routers near the AAA server, or even
 within the AAA server itself.
 Let us consider what happens when 10,000 48-ports NASes, each with an
 average session time of 20 minutes, are configured with the same AAA
 agent or server.  The unfortunate proxy or server would receive 400
 authentication/authorization requests/second and an equivalent number
 of accounting requests.  For 1000 octet requests, this would generate
 6.4 Mbps of incoming traffic at the AAA agent or server.
 While this transaction load is within the capabilities of the fastest
 AAA agents and servers, implementations exist that cannot handle such
 a high load.  Thus high queuing delays and/or dropped packets may be
 experienced at the agent or server, even if routers on the path are
 not congested.  Thus, a well designed AAA protocol needs to be able
 to handle congestion occurring at the AAA server, as well as
 congestion experienced within the network.

2.2. Slow Failover

 Where TCP [RFC793] is used as the transport, AAA implementations will
 experience very slow fail over times if they wait until a TCP
 connection times out before resending on another connection.  This is
 not an issue for SCTP [RFC2960], which supports endpoint and path
 failure detection.  As described in section 8 of [RFC2960], when the
 number of retransmissions exceeds the maximum

Aboba & Wood Standards Track [Page 6] RFC 3539 AAA Transport Profile June 2003

 ("Association.Max.Retrans"), the peer endpoint is considered
 unreachable, the association enters the CLOSED state, and the failure
 is reported to the application.  This enables more rapid failure
 detection.

2.3. Use of Nagle Algorithm

 AAA protocol messages are often smaller than the maximum segment size
 (MSS).  While exceptions occur when certificate-based authentication
 messages are issued or where a low path MTU is found, typically AAA
 protocol messages are less than 1000 octets.  Therefore, when using
 TCP [RFC793], the total packet count and associated network overhead
 can be reduced by combining multiple AAA messages within a single
 packet.
 Where AAA runs over TCP and transport behavior is network-driven,
 such as after a reboot when many users login simultaneously, or many
 stored accounting records need to be sent, the Nagle algorithm will
 result in "transport layer batching" of AAA messages.  While this
 does not reduce the work required by the application in parsing
 packets and responding to the messages, it does reduce the number of
 packets processed by routers along the path.  The Nagle algorithm is
 not used with SCTP.
 Where AAA transport is application-driven, the NAS will typically
 receive a reply from the home server prior to having another request
 to send.  This implies, for example, that accounting requests will
 typically be sent individually rather than being batched by the
 transport layer.  As a result, within the application-driven regime,
 the Nagle algorithm [RFC896] is ineffective.

2.4. Multiple Connections

 Since the RADIUS [RFC2865] Identifier field is a single octet, a
 maximum of 256 requests can be in progress between two endpoints
 described by a 5-tuple: (Client IP address, Client port, UDP, Server
 IP address, Server port).  In order to get around this limitation,
 RADIUS clients have utilized more than one sending port, sometimes
 even going to the extreme of using a different UDP source port for
 each NAS port.
 Were this behavior to be extended to AAA protocols operating over
 reliable transport, the result would be multiplication of the
 effective slow-start ramp-up by the number of connections.  For
 example, if a AAA client had ten connections open to a AAA agent, and
 used a per-connection initial window [RFC3390] of 2, then the

Aboba & Wood Standards Track [Page 7] RFC 3539 AAA Transport Profile June 2003

 effective initial window would be 20.  This is inappropriate, since
 it would permit the AAA client to send a large burst of packets into
 the network.

2.5. Duplicate Detection

 Where a AAA client maintains connections to multiple AAA agents or
 servers, and where failover/failback or connection load balancing is
 supported, it is possible for multiple agents or servers to receive
 duplicate copies of the same transaction.  A transaction may be sent
 on another connection before expiration of the "time wait" interval
 necessary to guarantee that all packets sent on the original
 connection have left the network.  Therefore it is conceivable that
 transactions sent on the alternate connection will arrive before
 those sent on the failed connection.  As a result, AAA agents and
 servers MUST be prepared to handle duplicates, and MUST assume that
 duplicates can arrive on any connection.
 For example, in billing, it is necessary to be able to weed out
 duplicate accounting records, based on the accounting session-id,
 event-timestamp and NAS identification information.  Where
 authentication requests are always idempotent, the resultant
 duplicate responses from multiple servers will presumably be
 identical, so that little harm will result.
 However, there are situations where the response to an authentication
 request will depend on a previously established state, such as when
 simultaneous usage restrictions are being enforced.  In such cases,
 authentication requests will not be idempotent.  For example, while
 an initial request might elicit an Accept response, a duplicate
 request might elicit a Reject response from another server, if the
 user were already presumed to be logged in, and only one simultaneous
 session were permitted.  In these situations, the AAA client might
 receive both Accept and Reject responses to the same duplicate
 request, and the outcome will depend on which response arrives first.

2.6. Invalidation of Transport Parameter Estimates

 Congestion control principles [Congest],[RFC2914] require the ability
 of a transport protocol to respond effectively to congestion, as
 sensed via increasing delays, packet loss, or explicit congestion
 notification.
 With network-driven applications, it is possible to respond to
 congestion on a timescale comparable to the round-trip time (RTT).
 However, with AAA protocols, the time between sends may be longer
 than the RTT, so that the network conditions can not be assumed to

Aboba & Wood Standards Track [Page 8] RFC 3539 AAA Transport Profile June 2003

 persist between sends.  For example, the congestion window may grow
 during a period in which congestion is being experienced because few
 packets are sent, limiting the opportunity for feedback.  Similarly,
 after congestion is detected, the congestion window may remain small,
 even though the network conditions that existed at the time of
 congestion no longer apply by the time when the next packets are
 sent.  In addition, due to the low sampling interval, estimates of
 RTT and RTO made via the procedure described in [RFC2988] may become
 invalid.

2.7. Inability to Use Fast Re-transmit

 When congestion window validation [RFC2861] is implemented, the
 result is that AAA protocols operate much of the time in slow-start
 with an initial congestion window set to 1 or 2, depending on the
 implementation [RFC3390].  This implies that AAA protocols gain
 little benefit from the windowing features of reliable transport.
 Since the congestion window is so small, it is generally not possible
 to receive enough duplicate ACKs (3) to trigger fast re-transmit.  In
 addition, since AAA traffic is two-way, ACKs including data will not
 count as part of the duplicate ACKs necessary to trigger fast re-
 transmit.  As a result, dropped packets will require a retransmission
 timeout (RTO).

2.8. Congestion Avoidance

 The law of conservation of packets [Congest] suggests that a client
 should not send another packet into the network until it can be
 reasonably sure that a packet has exited the network on the same
 path.  In the case of a AAA client, the law suggests that it should
 not retransmit to the same server or choose another server until it
 can be reasonably sure that a packet has exited the network on the
 same path.  If the client advances the window as responses arrive,
 then the client will "self clock", adjusting its transmission rate to
 the available bandwidth.
 While a AAA client using a reliable transport such as TCP [RFC793] or
 SCTP [RFC2960] will self-clock when communicating directly with a
 AAA-server, end-to-end self-clocking is not assured when AAA agents
 are present.
 As described in the Appendix, AAA agents include Relays, Proxies,
 Re-directs, Store and Forward proxies, and Transport proxies.  Of
 these agents, only Transport proxies and Re-directs provide a direct
 transport connection between the AAA client and server, allowing
 end-to-end self-clocking to occur.

Aboba & Wood Standards Track [Page 9] RFC 3539 AAA Transport Profile June 2003

 With Relays, Proxies or Store and Forward proxies, two separate and
 de-coupled transport connections are used.  One connection operates
 between the AAA client and agent, and another between the agent and
 server.  Since the two transport connections are de-coupled,
 transport layer ACKs do not flow end-to-end, and self-clocking does
 not occur.
 For example, consider what happens when the bottleneck exists between
 a AAA Relay and a AAA server.  Self-clocking will occur between the
 AAA client and AAA Relay, causing the AAA client to adjust its
 sending rate to the rate at which transport ACKs flow back from the
 AAA Relay.  However, since this rate is higher than the bottleneck
 bandwidth, the overall system will not self-clock.
 Since there is no direct transport connection between the AAA client
 and AAA server, the AAA client does not have the ability to estimate
 end-to-end transport parameters and adjust its sending rate to the
 bottleneck bandwidth between the Relay and server.  As a result, the
 incoming rate at the AAA Relay can be higher than the rate at which
 packets can be sent to the AAA server.
 In this case, the end-to-end performance will be determined by
 details of the agent implementation.  In general, the end-to-end
 transport performance in the presence of Relays, Proxies or Store and
 Forward proxies will always be worse in terms of delay and packet
 loss than if the AAA client and server were communicating directly.
 For example, if the agent operates with a large receive buffer, it is
 possible that a large queue will develop on the receiving side, since
 the AAA client is able to send packets to the AAA agent more rapidly
 than the agent can send them to the AAA server.  Eventually, the
 buffer will overflow, causing wholesale packet loss as well as high
 delay.
 Methods to induce fine-grained coupling between the two transport
 connections are difficult to implement.  One possible solution is for
 the AAA agent to operate with a receive buffer that is no larger than
 its send buffer.  If this is done, "back pressure" (closing of the
 receive window) will cause the agent to reduce the AAA client sending
 rate when the agent send buffer fills.  However, unless multiple
 connections exist between the AAA client and AAA agent, closing of
 the receive window will affect all traffic sent by the AAA client,
 even traffic destined to AAA servers where no bottleneck exists.
 Since multiple connections between a AAA client and agent result in
 multiplication of the effective slow-start ramp rate, this is not
 recommended.  As a result, use of "back pressure" cannot enable
 individual AAA client-server conversations to self-clock, and this
 technique appears impractical for use in AAA.

Aboba & Wood Standards Track [Page 10] RFC 3539 AAA Transport Profile June 2003

2.9. Delayed Acknowledgments

 As described in Appendix B, ACKs may comprise as much as half of the
 traffic generated in a AAA exchange.  This occurs because AAA
 conversations are typically application-driven, and therefore there
 is frequently not enough traffic to enable ACK piggybacking.  As a
 result, AAA protocols running over TCP or SCTP transport may
 experience a doubling of traffic as compared with implementations
 utilizing UDP transport.
 It is typically not possible to address this issue via the sockets
 API.  ACK parameters (such as the value of the delayed ACK timer) are
 typically fixed by TCP and SCTP implementations and are therefore not
 tunable by the application.

2.10. Premature Failover

 RADIUS failover implementations are typically based on the concept of
 primary and secondary servers, in which all traffic flows to the
 primary server unless it is unavailable.  However, the failover
 algorithm was not specified in [RFC2865] or [RFC2866].  As a result,
 RADIUS failover implementations vary in quality, with some failing
 over prematurely, violating the law of "conservation of packets".
 Where a Relay, Proxy or Store and Forward proxy is present, the AAA
 client has no direct connection to a AAA server, and is unable to
 estimate the end-to-end transport parameters.  As a result, a AAA
 client awaiting an application-layer response from the server has no
 transport-based mechanism for determining an appropriate failover
 timer.
 For example, if the path between the AAA agent and server includes a
 high delay link, or if the AAA server is very heavily loaded, it is
 possible that the NAS will failover to another agent while packets
 are still in flight.  This violates the principle of "conservation of
 packets", since the AAA client will inject additional packets into
 the network before having evidence that a previously sent packet has
 left the network.  Such behavior can result in a worse situation on
 an already congested link, resulting in congestive collapse
 [Congest].

2.11. Head of Line Blocking

 Head of line blocking occurs during periods of packet loss where the
 time between sends is shorter than the re-transmission timeout value
 (RTO).  In such situations, packets back up in the send queue until

Aboba & Wood Standards Track [Page 11] RFC 3539 AAA Transport Profile June 2003

 the lost packet can be successfully re-transmitted.  This can be an
 issue for SCTP when using ordered delivery over a single stream, and
 for TCP.
 Head of line blocking is typically an issue only on larger NASes.
 For example, a 48-port NAS with an average inter-packet spacing of 25
 seconds is unlikely to have an RTO greater than this, unless severe
 packet loss has been experienced.  However, a 2048-port NAS with an
 average inter-packet spacing of 293 ms may experience head-of-line
 blocking since the inter-packet spacing is less than the minimum RTO
 value of 1 second [RFC2988].

2.12. Connection Load Balancing

 In order to lessen queuing delays and address head of line blocking,
 a AAA implementation may wish to load balance between connections to
 multiple destinations.  While it is possible to employ dynamic load
 balancing techniques, this level of sophistication may not be
 required.  In many situations, adequate reliability and load
 balancing can be achieved via static load balancing, where traffic is
 distributed between destinations based on static "weights".

3. AAA Transport Profile

 In order to address AAA transport issues, it is recommended that AAA
 protocols make use of standards track as well as experimental
 techniques.  More details are provided in the sections that follow.

3.1. Transport Mappings

 AAA Servers MUST support TCP and SCTP.  AAA clients SHOULD support
 SCTP, but MUST support TCP if SCTP is not available.  As support for
 SCTP improves, it is possible that SCTP support will be required on
 clients at some point in the future.  AAA agents inherit all the
 obligations of Servers with respect to transport support.

3.2. Use of Nagle Algorithm

 While AAA protocols typically operate in the application-driven
 regime, there are circumstances in which they are network driven.
 For example, where an NAS reboots, or where connectivity is restored
 between an NAS and a AAA agent, it is possible that multiple packets
 will be available for sending.
 As a result, there are circumstances where the transport-layer
 batching provided by the Nagle Algorithm (12) is useful, and as a
 result, AAA implementations running over TCP MUST enable the Nagle
 algorithm, [RFC896].  The Nagle algorithm is not used with SCTP.

Aboba & Wood Standards Track [Page 12] RFC 3539 AAA Transport Profile June 2003

3.3. Multiple Connections

 AAA protocols SHOULD use only a single persistent connection between
 a AAA client and a AAA agent or server.  They SHOULD provide for
 pipelining of requests, so that more than one request can be in
 progress at a time.  In order to minimize use of inactive connections
 in roaming situations, a AAA client or agent MAY bring down a
 connection to a AAA agent or server if the connection has been
 unutilized (discounting the watchdog) for a certain period of time,
 which MUST NOT be less than BRINGDOWN_INTERVAL (5 minutes).
 While a AAA client/agent SHOULD only use a single persistent
 connection to a given AAA agent or server, it MAY have connections to
 multiple AAA agents or servers.  A AAA client/agent connected to
 multiple agents/servers can treat them as primary/secondary or
 balance load between them.

3.4. Application Layer Watchdog

 In order to enable AAA implementations to more quickly detect
 transport and application-layer failures, AAA protocols MUST support
 an application layer watchdog message.
 The application layer watchdog message enables failover from a peer
 that has failed, either because it is unreachable or because its
 applications functions have failed.  This is distinct from the
 purpose of the SCTP heartbeat, which is to enable failover between
 interfaces.  The SCTP heartbeat may enable a failover to another path
 to reach the same server, but does not address the situation where
 the server system or the application service has failed.  Therefore
 both mechanisms MAY be used together.
 The watchdog is used in order to enable a AAA client or agent to
 determine when to resend on another connection.  It operates on all
 open connections and is used to suspend and eventually close
 connections that are experiencing difficulties.  The watchdog is also
 used to re-open and validate connections that have returned to
 health.  The watchdog may be utilized either within primary/secondary
 or load balancing configurations.  However, it is not intended as a
 cluster heartbeat mechanism.
 The application layer watchdog is designed to detect failures of the
 immediate peer, and not to be affected by failures of downstream
 proxies or servers.  This prevents instability in downstream AAA
 components from propagating upstream.  While the receipt of any AAA
 Response from a peer is taken as evidence that the peer is up, lack
 of a Response is insufficient to conclude that the peer is down.
 Since the lack of Response may be the result of problems with a

Aboba & Wood Standards Track [Page 13] RFC 3539 AAA Transport Profile June 2003

 downstream proxy or server, only after failure to respond to the
 watchdog message can it be determined that the peer is down.
 Since the watchdog algorithm takes any AAA Response into account in
 determining peer liveness, decreases in the watchdog timer interval
 do not significantly increase the level of watchdog traffic on
 heavily loaded networks.  This is because watchdog messages do not
 need to be sent where other AAA Response traffic serves as a constant
 reminder of peer liveness.  Watchdog traffic only increases when AAA
 traffic is light, and therefore a AAA Response "signal" is not
 present.  Nevertheless, decreasing the timer interval TWINIT does
 increase the probability of false failover significantly, and so this
 decision should be made with care.

3.4.1. Algorithm Overview

 The watchdog behavior is controlled by an algorithm defined in this
 section.  This algorithm is appropriate for use either within
 primary/secondary or load balancing configurations.  Implementations
 SHOULD implement this algorithm, which operates as follows:
 [1] Watchdog behavior is controlled by a single timer (Tw).  The
     initial value of Tw, prior to jittering is Twinit.  The default
     value of Twinit is 30 seconds.  This value was selected because
     it minimizes the probability that failover will be initiated due
     to a routing flap, as noted in [Paxson].
     While Twinit MAY be set as low as 6 seconds (not including
     jitter), it MUST NOT be set lower than this.  Note that setting
     such a low value for Twinit is likely to result in an increased
     probability of duplicates, as well as an increase in spurious
     failover and failback attempts.
     In order to avoid synchronization behaviors that can occur with
     fixed timers among distributed systems, each time the watchdog
     interval is calculated with a jitter by using the Twinit value
     and randomly adding a value drawn between -2 and 2 seconds.
     Alternative calculations to create jitter MAY be used.  These
     MUST be pseudo-random, generated by a PRNG seeded as per
     [RFC1750].
 [2] When any AAA message is received, Tw is reset.  This need not be
     a response to a watchdog request.  Receiving a watchdog response
     from a peer constitutes activity, and Tw should be reset.  If the
     watchdog timer expires and no watchdog response is pending, then
     a watchdog message is sent.  On sending a watchdog request, Tw is
     reset.

Aboba & Wood Standards Track [Page 14] RFC 3539 AAA Transport Profile June 2003

     Watchdog packets are not retransmitted by the AAA protocol, since
     AAA protocols run over reliable transports that will handle all
     retransmissions internally.  As a result, a watchdog request is
     only sent when there is no watchdog response pending.
 [3] If the watchdog timer expires and a watchdog response is pending,
     then failover is initiated.  In order for a AAA client or agent
     to perform failover procedures, it is necessary to maintain a
     pending message queue for a given peer.  When an answer message
     is received, the corresponding request is removed from the queue.
     The Hop-by-Hop Identifier field MAY be used to match the answer
     with the queued request.
     When failover is initiated, all messages in the queue are sent to
     an alternate agent, if available.  Multiple identical requests or
     answers may be received as a result of a failover.  The
     combination of an end-to-end identifier and the origin host MUST
     be used to identify duplicate messages.
     Note that where traffic is heavy, the application layer watchdog
     can take as long as 2Tw to determine that a peer has gone down.
     For peers receiving a high volume of AAA Requests, AAA Responses
     will continually reset the timer, so that after a failure it will
     take Tw for the lack of traffic to be noticed, and for the
     watchdog message to be sent.  Another Tw will elapse before
     failover is initiated.
     On a lightly loaded network without much AAA Response traffic,
     the watchdog timer will typically expire without being reset, so
     that a watchdog response will be outstanding and failover will be
     initiated after only a single timer interval has expired.
 [4] The client MUST NOT close the primary connection until the
     primary's watchdog timer has expired at least twice without a
     response (note that the watchdog is not sent a second time,
     however).  Once this has occurred, the client SHOULD cause a
     transport reset or close to be done on the connection.
     Once the primary connection has failed, subsequent requests are
     sent to the alternate server until the watchdog timer on the
     primary connection is reset.
     Suspension of the primary connection prevents flapping between
     primary and alternate connections, and ensures that failover
     behavior remains consistent.  The application may not receive a
     response to the watchdog request message due to a connectivity
     problem, in which case a transport layer ACK will not have been
     received, or the lack of response may be due to an application

Aboba & Wood Standards Track [Page 15] RFC 3539 AAA Transport Profile June 2003

     problem.  Without transport layer visibility, the application is
     unable to tell the difference, and must behave conservatively.
     In situations where no transport layer ACK is received on the
     primary connection after multiple re-transmissions, the RTO will
     be exponentially backed off as described in [RFC2988].  Due to
     Karn's algorithm as implemented in SCTP and TCP, the RTO
     estimator will not be reset until another ACK is received in
     response to a non-re-transmitted request.  Thus, in cases where
     the problem occurs at the transport layer, after the client fails
     over to the alternate server, the RTO of the primary will remain
     at a high value unless an ACK is received on the primary
     connection.
     In the case where the problem occurs at the transport layer,
     subsequent requests sent on the primary connection will not
     receive the same service as was originally provided.  For
     example, instead of failover occurring after 3 retransmissions,
     failover might occur without even a single retransmission if RTO
     has been sufficiently backed off.  Of course, if the lack of a
     watchdog response was due to an application layer problem, then
     RTO will not have been backed off.  However, without transport
     layer visibility, there is no way for the application to know
     this.
     Suspending use of the primary connection until a response to a
     watchdog message is received guarantees that the RTO timer will
     have been reset before the primary connection is reused.  If no
     response is received after the second watchdog timer expiration,
     then the primary connection is closed and the suspension becomes
     permanent.
 [5] While the connection is in the closed state, the AAA client MUST
     NOT attempt to send further watchdog messages on the connection.
     However, after the connection is closed, the AAA client continues
     to periodically attempt to reopen the connection.
     The AAA client SHOULD wait for the transport layer to report
     connection failure before attempting again, but MAY choose to
     bound this wait time by the watchdog interval, Tw.  If the
     connection is successfully opened, then the watchdog message is
     sent.  Once three watchdog messages have been sent and responded
     to, the connection is returned to service, and transactions are
     once again sent over it.  Connection validation via receipt of
     multiple watchdogs is not required when a connection is initially
     brought up -- in this case, the connection can immediately be put
     into service.

Aboba & Wood Standards Track [Page 16] RFC 3539 AAA Transport Profile June 2003

 [6] When using SCTP as a transport, it is not necessary to disable
     SCTP's transport-layer heartbeats.  However, if AAA
     implementations have access to SCTP's heartbeat parameters, they
     MAY chose to ensure that SCTP's heartbeat interval is longer than
     the AAA watchdog interval, Tw.  This will ensure that alternate
     paths are still probed by SCTP, while the primary path has a
     minimum of heartbeat redundancy.

3.4.2. Primary/Secondary Failover Support

 The watchdog timer MAY be integrated with primary/secondary style
 failover so as to provide improved reliability and basic load
 balancing.  In order to balance load among multiple AAA servers, each
 AAA server is designated the primary for a portion of the clients,
 and designated as secondaries of varying priority for the remainder.
 In this way, load can be balanced among the AAA servers.
 Within primary/secondary configurations, the watchdog timer operates
 as follows:
 [1] Assume that each client or agent is initially configured with a
     single primary agent or server, and one or more secondary
     connections.
 [2] The watchdog mechanism is used to suspend and eventually close
     primary connections that are experiencing difficulties.  It is
     also used to re-open and validate connections that have returned
     to health.
 [3] Once a secondary is promoted to primary status, either on a
     temporary or permanent basis, the next server on the list of
     secondaries is promoted to fill the open secondary slot.
 [4] The client or agent periodically attempts to re-open closed
     connections, so that it is possible that a previously closed
     connection can be returned to service and become eligible for use
     again.  Implementations will typically retain a limit on the
     number of connections open at a time, so that once a previously
     closed connection is brought online again, the lowest priority
     secondary connection will be closed.  In order to prevent
     periodic closing and re-opening of secondary connections, it is
     recommended that functioning connections remain open for a
     minimum of 5 minutes.
 [5] In order to enable diagnosis of failover behavior, it is
     recommended that a table of failover events be kept within the
     MIB.  These failover events SHOULD include appropriate
     transaction identifiers so that client and server data can be

Aboba & Wood Standards Track [Page 17] RFC 3539 AAA Transport Profile June 2003

     compared, providing insight into the cause of the problem
     (transport or application layer).

3.4.3. Connection Load Balancing

 Primary/secondary failover is capable of providing improved
 resilience and basic load balancing.  However, it does not address
 TCP head of line blocking, since only a single connection is in use
 at a time.
 A AAA client or agent maintaining connections to multiple agents or
 servers MAY load balance between them.  Establishing connections to
 multiple agents or servers reduces, but does not eliminate, head of
 line blocking issues experienced on TCP connections.  This issue does
 not exist with SCTP connections utilizing multiple streams.
 In connection load balancing configurations, the application watchdog
 operates as follows:
 [1] Assume that each client or agent is initially configured with
     connections to multiple AAA agents or servers, with one
     connection between a given client/agent and an agent/server.
 [2] In static load balancing, transactions are apportioned among the
     connections based on the total number of connections and a
     "weight" assigned to each connection.  Pearson's hash [RFC3074]
     applied to the NAI [RFC2486] can be used to determine which
     connection will handle a given transaction.  Hashing on the NAI
     provides highly granular load balancing, while ensuring that all
     traffic for a given conversation will be sent to the same agent
     or server.  In dynamic load balancing, the value of the "weight"
     can vary based on conditions such as AAA server load.  Such
     techniques, while sophisticated, are beyond the scope of this
     document.
 [3] Transactions are distributed to connections based on the total
     number of available connections and their weights.  A change in
     the number of available connections forces recomputation of the
     hash table.  In order not to cause conversations in progress to
     be switched to new destinations, on recomputation, a transitional
     period is required in which both old and new hash tables are
     needed in order to permit aging out of conversations in progress.
     Note that this requires a way to easily determine whether a
     Request represents a new conversation or the continuation of an
     existing conversation.  As a result, removing and adding of
     connections is an expensive operation, and it is recommended that
     the hash table only be recomputed once a connection is closed or
     returned to service.

Aboba & Wood Standards Track [Page 18] RFC 3539 AAA Transport Profile June 2003

     Suspended connections, although they are not used, do not force
     hash table reconfiguration until they are closed.  Similarly,
     re-opened connections not accumulating sufficient watchdog
     responses do not force a reconfiguration until they are returned
     to service.
     While a connection is suspended, transactions that were to have
     been assigned to it are instead assigned to the next available
     server.  While this results in a momentary imbalance, it is felt
     that this is a relatively small price to pay in order to reduce
     hash table thrashing.
 [4] In order to enable diagnosis of load balancing behavior, it is
     recommended that in addition to a table of failover events, a
     table of statistics be kept on each client, indexed by a AAA
     server.  That way, the effectiveness of the load balancing
     algorithm can be evaluated.

3.5. Duplicate Detection

 Multiple facilities are required to enable duplicate detection.
 These include session identifiers as well as hop-by-hop and end-to-
 end message identifiers.  Hop-by-hop identifiers whose value may
 change at each hop are not sufficient, since a AAA server may receive
 the same message from multiple agents.  For example, a AAA client can
 send a request to Agent1, then failover and resend the request to
 Agent2; both agents forward the request to the home AAA server, with
 different hop-by-hop identifiers.  A Session Identifier is
 insufficient as it does not distinguish different messages for the
 the same session.
 Proper treatment of the end-to-end message identifier ensures that
 AAA operations are idempotent.  For example, without an end-to-end
 identifier, a AAA server keeping track of simultaneous logins might
 send an Accept in response to an initial Request, and then a Reject
 in response to a duplicate Request (where the user was allowed only
 one simultaneous login).  Depending on which Response arrived first,
 the user might be allowed access or not.
 However, if the server were to store the end-to-end message
 identifier along with the simultaneous login information, then the
 duplicate Request (which utilizes the same end-to-end message
 identifier) could be identified and the correct response could be
 returned.

Aboba & Wood Standards Track [Page 19] RFC 3539 AAA Transport Profile June 2003

3.6. Invalidation of Transport Parameter Estimates

 In order to address invalidation of transport parameter estimates,
 AAA protocol implementations MAY utilize Congestion Window Validation
 [RFC2861] and RTO validation when using TCP.  This specification also
 recommends a procedure for RTO validation.
 [RFC2581] and [RFC2861] both recommend that a connection go into
 slow-start after a period where no traffic has been sent within the
 RTO interval.  [RFC2861] recommends only increasing the congestion
 window if it was full when the ACK arrived.  The congestion window is
 reduced by half once every RTO interval if no traffic is received.
 When Congestion Window Validation is used, the congestion window will
 not build during application-driven periods, and instead will be
 decayed.  As a result, AAA applications operating within the
 application-driven regime will typically run with a congestion window
 equal to the initial window much of the time, operating in "perpetual
 slowstart".
 During periods in which AAA behavior is application-driven this will
 have no effect.  Since the time between packets will be larger than
 RTT, AAA will operate with an effective congestion window equal to
 the initial window.  However, during network-driven periods, the
 effect will be to space out sending of AAA packets.  Thus instead of
 being able to send a large burst of packets into the network, a
 client will need to wait several RTTs as the congestion window builds
 during slow-start.
 For example, a client operating over TCP with an initial window of 2,
 with 35 AAA requests to send would take approximately 6 RTTs to send
 them, as the congestion window builds during slow start: 2, 3, 3, 6,
 9, 12.  After the backlog is cleared, the implementation will once
 again be application-driven and the congestion window size will
 decay.  If the client were using SCTP, the number of RTTs needed to
 transmit all requests would usually be less, and would depend on the
 size of the requests, since SCTP tracks the progress for the opening
 of the congestion window by bytes, not segments.
 Note that [RFC2861] and [RFC2988] do not address the issue of RTO
 validation.  This is also a problem, particularly when the Congestion
 Manager [RFC3124] is implemented.  During periods of high packet
 loss, the RTO may be repeatedly increased via exponential back-off,
 and may attain a high value.  Due to lack of timely feedback on RTT
 and RTO during application-driven periods, the high RTO estimate may
 persist long after the conditions that generated it have dissipated.

Aboba & Wood Standards Track [Page 20] RFC 3539 AAA Transport Profile June 2003

 RTO validation MAY be used to address this issue for TCP, via the
 following procedure:
    After the congestion window is decayed according to [RFC2861],
    reset the estimated RTO to 3 seconds.  After the next packet comes
    in, re-calculate RTTavg, RTTdev, and RTO according to the method
    described in [RFC2581].
 To address this issue for SCTP, AAA implementations SHOULD use SCTP
 heartbeats.  [RFC2960] states that heartbeats should be enabled by
 default, with an interval of 30 seconds.  If this interval proves to
 be too long to resolve this issue, AAA implementations MAY reduce the
 heartbeat interval.

3.7. Inability to Use Fast Re-Transmit

 When Congestion Window Validation [RFC2861] is used, AAA
 implementations will operate with a congestion window equal to the
 initial window much of the time.  As a result, the window size will
 often not be large enough to enable use of fast re-transmit for TCP.
 In addition, since AAA traffic is two-way, ACKs carrying data will
 not count towards triggering fast re-transmit.  SCTP is less likely
 to encounter this issue, so the measures described below apply to
 TCP.
 To address this issue, AAA implementations SHOULD support selective
 acknowledgement as described in [RFC2018] and [RFC2883].  AAA
 implementations SHOULD also implement Limited Transmit for TCP, as
 described in [RFC3042].  Rather than reducing the number of duplicate
 ACKs required for triggering fast recovery, which would increase the
 number of inappropriate re-transmissions, Limited Transmit enables
 the window size be increased, thus enabling the sending of additional
 packets which in turn may trigger fast re-transmit without a change
 to the algorithm.
 However, if congestion window validation [RFC2861] is implemented,
 this proposal will only have an effect in situations where the time
 between packets is less than the estimated retransmission timeout
 (RTO).  If the time between packets is greater than RTO, additional
 packets will typically not be available for sending so as to take
 advantage of the increased window size.  As a result, AAA protocols
 will typically operate with the lowest possible congestion window
 size, resulting in a re-transmission timeout for every lost packet.

Aboba & Wood Standards Track [Page 21] RFC 3539 AAA Transport Profile June 2003

3.8. Head of Line Blocking

 TCP inherently does not provide a solution to the head-of-line
 blocking problem, although its effects can be lessened by
 implementation of Limited Transmit [RFC3042], and connection load
 balancing.

3.8.1. Using SCTP Streams to Prevent Head of Line Blocking

 Each AAA node SHOULD distribute its messages evenly across the range
 of SCTP streams that it and its peer have agreed upon.  (A lost
 message in one stream will not cause any other streams to block.)  A
 trivial and effective implementation of this simply increments a
 counter for the stream ID to send on.  When the counter reaches the
 maximum number of streams for the association, it resets to 0.
 AAA peers MUST be able to accept messages on any stream.  Note that
 streams are used *solely* to prevent head-of-the-line blocking.  All
 identifying information is carried within the Diameter payload.
 Messages distributed across multiple streams may not be received in
 the order they are sent.
 SCTP peers can allocate up to 65535 streams for an association.  The
 cost for idle streams may or may not be zero, depending on the
 implementation, and the cost for non-idle streams is always greater
 than 0.  So administrators may wish to limit the number of possible
 streams on their diameter nodes according to the resources (i.e.
 memory, CPU power, etc.) of a particular node.
 On a Diameter client, the number of streams may be determined by the
 maximum number of peak users on the NAS.  If a stream is available
 per user, then this should be sufficient to prevent head-of-line
 blocking.  On a Diameter proxy, the number of streams may be
 determined by the maximum number of peak sessions in progress from
 that proxy to each downstream AAA server.
 Stream IDs do not need to be preserved by relay agents.  This
 simplifies implementation, as agents can easily handle forwarding
 between two associations with different numbers of streams.  For
 example, consider the following case, where a relay server DRL
 forwards messages between a NAS and a home server, HMS.  The NAS and
 DRL have agreed upon 1000 streams for their association, and DRL and
 HMS have agreed upon 2000 streams for their association.  The
 following figure shows the message flow from NAS to HMS via DRL, and
 the stream ID assignments for each message:

Aboba & Wood Standards Track [Page 22] RFC 3539 AAA Transport Profile June 2003

 +------+                   +------+                   +------+
 |      |                   |      |                   |      |
 | NAS  |    --------->     | DRL  |     --------->    | HMS  |
 |      |                   |      |                   |      |
 +------+   1000 streams    +------+    2000 streams   +------+
            msg 1: str id 0             msg 1: str id 0
            msg 2: str id 1             msg 2: str id 1
            ...
            msg 1000: str id 999        msg 1000: str id 999
            msg 1001: str id 0          msg 1001: str id 1000
 DRL can forward messages 1 through 1000 to HMS using the same stream
 ID that NAS used to send to DRL.  However, since the NAS / DRL
 association has only 1000 streams, NAS wraps around to stream ID 0
 when sending message 1001.  The DRL / HMS association, on the other
 hand, has 2000 streams, so DRL can reassign message 1001 to stream ID
 1000 when forwarding it on to HMS.
 This distribution scheme acts like a hash table.  It is possible, yet
 unlikely, that two messages will end up in the same stream, and even
 less likely that there will be message loss resulting in blocking
 when this happens.  If it does turn out to be a problem, local
 administrators can increase the number of streams on their nodes to
 improve performance.

3.9. Congestion Avoidance

 In order to improve upon default timer estimates, AAA implementations
 MAY implement the Congestion Manager (CM) [RFC3124].  CM is an end-
 system module that:
     (i) Enables an ensemble of multiple concurrent streams from a
         sender destined to the same receiver and sharing the same
         congestion properties to perform proper congestion avoidance
         and control, and
    (ii) Allows applications to easily adapt to network congestion.
 The CM helps integrate congestion management across all applications
 and transport protocols.  The CM maintains congestion parameters
 (available aggregate and per-stream bandwidth, per-receiver round-
 trip times, etc.) and exports an API that enables applications to
 learn about network characteristics, pass information to the CM,
 share congestion information with each other, and schedule data
 transmissions.

Aboba & Wood Standards Track [Page 23] RFC 3539 AAA Transport Profile June 2003

 The CM enables the AAA application to access transport parameters
 (RTTavg, RTTdev) via callbacks.  RTO estimates are currently not
 available via the callback interface, though they probably should be.
 Where available, transport parameters SHOULD be used to improve upon
 default timer values.

3.10. Premature Failover

 Premature failover is prevented by the watchdog functionality
 described above.  If the next hop does not return a reply, the AAA
 client will send a watchdog message to it to verify liveness.  If a
 watchdog reply is received, then the AAA client will know that the
 next hop server is functioning at the application layer.  As a
 result, it is only necessary to provide terminal error messages, such
 as the following:
    "Busy": agent/Server too busy to handle additional requests, NAS
    should failover all requests to another agent/server.
    "Can't Locate": agent can't locate the AAA server for the
    indicated realm; NAS should failover that request to another
    proxy.
    "Can't Forward": agent has tried both primary and secondary AAA
    servers with no response; NAS should failover the request to
    another agent.
 Note that these messages differ in their scope.  The "Busy" message
 tells the NAS that the agent/server is too busy for ANY request.  The
 "Can't Locate" and "Can't Forward" messages indicate that the
 ultimate destination cannot be reached or isn't responding, implying
 per-request failover.

4. Security Considerations

 Since AAA clients, agents and servers serve as network access
 gatekeepers, they are tempting targets for attackers.  General
 security considerations concerning TCP congestion control are
 discussed in [RFC2581].  However, there are some additional
 considerations that apply to this specification.
 By enabling failover between AAA agents, this specification improves
 the resilience of AAA applications.  However, it may also open
 avenues for denial of service attacks.
 The failover algorithm is driven by lack of response to AAA requests
 and watchdog packets.  On a lightly loaded network where AAA
 responses would not be received prior to expiration of the watchdog

Aboba & Wood Standards Track [Page 24] RFC 3539 AAA Transport Profile June 2003

 timer, an attacker can swamp the network, causing watchdog packets to
 be dropped.  This will cause the AAA client to switch to another AAA
 agent, where the attack can be repeated.  By causing the AAA client
 to cycle between AAA agents, service can be denied to users desiring
 network access.
 Where TLS [RFC2246] is being used to provide AAA security, there will
 be a vulnerability to spoofed reset packets, as well as other
 transport layer denial of service attacks (e.g. SYN flooding).  Since
 SCTP offers improved denial of service resilience compared with TCP,
 where AAA applications run over SCTP, this can be mitigated to some
 extent.
 Where IPsec [RFC2401] is used to provide security, it is important
 that IPsec policy require IPsec on incoming packets.  In order to
 enable a AAA client to determine what security mechanisms are in use
 on an agent or server without prior knowledge, it may be tempting to
 initiate a connection in the clear, and then to have the AAA agent
 respond with IKE [RFC2409].  While this approach minimizes required
 client configuration, it increases the vulnerability to denial of
 service attack, since a connection request can now not only tie up
 transport resources, but also resources within the IKE
 implementation.

5. IANA Considerations

 This document does not create any new number spaces for IANA
 administration.

References

6.1. Normative References

 [RFC793]  Postel, J., "Transmission Control Protocol", STD 7, RFC
           793, September 1981.
 [RFC896]  Nagle, J., "Congestion Control in IP/TCP internetworks",
           RFC 896, January 1984.
 [RFC1750] Eastlake, D., Crocker, S. and J. Schiller, "Randomness
           Recommendations for Security", RFC 1750, December 1994.
 [RFC2018] Mathis, M., Mahdavi, J., Floyd, S. and A. Romanow, "TCP
           Selective Acknowledgment Options", RFC 2018, October 1996.
 [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
           Requirement Levels", BCP 14, RFC 2119, March 1997.

Aboba & Wood Standards Track [Page 25] RFC 3539 AAA Transport Profile June 2003

 [RFC2486] Aboba, B. and M. Beadles, "The Network Access Identifier",
           RFC 2486, January 1999.
 [RFC2581] Allman, M., Paxson, V. and W. Stevens, "TCP Congestion
           Control", RFC 2581, April 1999.
 [RFC2883] Floyd, S., Mahdavi, J., Mathis, M., Podolsky, M. and A.
           Romanow, "An Extension to the Selective Acknowledgment
           (SACK) Option for TCP", RFC 2883, July 2000.
 [RFC2960] Stewart, R., Xie, Q., Morneault, K., Sharp, C.,
           Schwarzbauer, H., Taylor, T., Rytina, I., Kalla, M., Zhang,
           L. and V. Paxson, "Stream Control Transmission Protocol",
           RFC 2960, October 2000.
 [RFC2988] Paxson, V. and M. Allman, "Computing TCP's Retransmission
           Timer", RFC 2988, November 2000.
 [RFC3042] Allman, M., Balakrishnan H. and S. Floyd, "Enhancing TCP's
           Loss Recovery Using Limited Transmit", RFC 3042, January
           2001.
 [RFC3074] Volz, B., Gonczi, S., Lemon, T. and R. Stevens, "DHC Load
           Balancing Algorithm", RFC 3074, February 2001.
 [RFC3124] Balakrishnan, H. and S. Seshan, "The Congestion Manager",
           RFC 3124, June 2001.

6.2. Informative References

 [RFC2246] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
           RFC 2246, January 1999.
 [RFC2401] Atkinson, R. and S. Kent, "Security Architecture for the
           Internet Protocol", RFC 2401, November 1998.
 [RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
           (IKE)", RFC 2409, November 1998.
 [RFC2607] Aboba, B. and J. Vollbrecht, "Proxy Chaining and Policy
           Implementation in Roaming", RFC 2607, June 1999.
 [RFC2861] Handley, M., Padhye, J. and S. Floyd, "TCP Congestion
           Window Validation", RFC 2861, June 2000.
 [RFC2865] Rigney, C., Willens, S., Rubens, A. and W. Simpson, "Remote
           Authentication Dial In User Service (RADIUS)", RFC 2865,
           June 2000.

Aboba & Wood Standards Track [Page 26] RFC 3539 AAA Transport Profile June 2003

 [RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
 [RFC2914] Floyd, S., "Congestion Control Principles", BCP 41, RFC
           2914, September 2000.
 [RFC2975] Aboba, B., Arkko, J. and D. Harrington, "Introduction to
           Accounting Management", RFC 2975, June 2000.
 [RFC3390] Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's
           Initial Window", RFC 3390, October 2002.
 [Congest] Jacobson, V., "Congestion Avoidance and Control", Computer
           Communication Review, vol. 18, no. 4, pp. 314-329, Aug.
           1988.  ftp://ftp.ee.lbl.gov/papers/congavoid.ps.Z
 [Paxson]  Paxson, V., "Measurement and Analysis of End-to-End
           Internet Dynamics", Ph.D. Thesis, Computer Science
           Division, University of California, Berkeley, April 1997.

Aboba & Wood Standards Track [Page 27] RFC 3539 AAA Transport Profile June 2003

Appendix A - Detailed Watchdog Algorithm

 In this Appendix, the memory control structure that contains all
 information regarding a specific peer is referred to as a Peer
 Control Block, or PCB.  The PCB contains the following fields:
 Status:
   OKAY:       The connection is up
   SUSPECT:    Failover has been initiated on the connection.
   DOWN:       Connection has been closed.
   REOPEN:     Attempting to reopen a closed connection
   INITIAL:    The initial state of the pcb when it is first created.
               The pcb has never been opened.
 Variables:
   Pending:    Set to TRUE if there is an outstanding unanswered
               watchdog request
   Tw:         Watchdog timer value
   NumDWA:     Number of DWAs received during REOPEN
 Tw is the watchdog timer, measured in seconds.  Every  second, Tw  is
 decremented.  When it reaches 0, the OnTimerElapsed event (see below)
 is invoked.  Pseudo-code for the algorithm is included on the
 following pages.
 SetWatchdog()
 {
 /*
  SetWatchdog() is called whenever it is necessary
  to reset the watchdog timer Tw.  The value of the
  watchdog timer is calculated based on the default
  initial value TWINIT and a jitter ranging from
  -2 to 2 seconds.  The default for TWINIT is 30 seconds,
  and MUST NOT be set lower than 6 seconds.
 */
     Tw=TWINIT -2.0 + 4.0 * random() ;
     SetTimer(Tw) ;
     return ;
 }
 /*
  OnReceive() is called whenever a message
  is received from the peer.  This message MAY
  be a request or an answer, and can include
  DWR and DWA messages.  Pending is assumed to
  be a global variable.
 */
 OnReceive(pcb, msgType)

Aboba & Wood Standards Track [Page 28] RFC 3539 AAA Transport Profile June 2003

 {
    if (msgType == DWA) {
         Pending = FALSE;
         }
    switch (pcb->Status){
    case OKAY:
         SetWatchdog();
         break;
    case SUSPECT:
         pcb->Status = OKAY;
         Failback(pcb);
         SetWatchdog();
         break;
    case REOPEN:
         if (msgType == DWA) {
            NumDWA++;
            if (NumDWA == 3) {
               pcb->status = OKAY;
               Failback();
            }
         } else {
            Throwaway(received packet);
         }
         break;
    case INITIAL:
    case DOWN:
         Throwaway(received packet);
         break;
    default:
         Error("Shouldn't be here!");
         break;
    }
 }
 /*
 OnTimerElapsed() is called whenever Tw reaches zero (0).
 */
 OnTimerElapsed(pcb)
 {
     switch (pcb->status){
        case OKAY:
           if (!Pending) {
              SendWatchdog(pcb);
              SetWatchdog();
              Pending = TRUE;
              break;
           }
           pcb->status = SUSPECT;

Aboba & Wood Standards Track [Page 29] RFC 3539 AAA Transport Profile June 2003

           FailOver(pcb);
           SetWatchdog();
           break ;
        case SUSPECT:
           pcb->status = DOWN;
           CloseConnection(pcb);
           SetWatchdog();
           break;
        case INITIAL:
        case DOWN:
           AttemptOpen(pcb);
           SetWatchdog();
           break;
        case REOPEN:
           if (!Pending) {
              SendWatchdog(pbc);
              SetWatchdog();
              Pending = TRUE;
              break;
           }
           if (NumDWA < 0) {
              pcb->status = DOWN;
              CloseConnection(pcb);
           } else {
              NumDWA = -1;
           }
           SetWatchdog();
           break;
        default:
           error("Shouldn't be here!);
           break;
        }
 }
 /*
 OnConnectionUp() is called whenever a connection comes up
 */
 OnConnectionUp(pcb)
 {
     switch (pcb->status){
        case INITIAL:
           pcb->status = OKAY;
           SetWatchdog();
           break;
        case DOWN:
           pcb->status = REOPEN;
           NumDWA = 0;
           SendWatchdog(pcb);

Aboba & Wood Standards Track [Page 30] RFC 3539 AAA Transport Profile June 2003

           SetWatchdog();
           Pending = TRUE;
           break;
        default:
           error("Shouldn't be here!);
           break;
        }
 }
 /*
 OnConnectionDown() is called whenever a connection goes down
 */
 OnConnectionDown(pcb)
 {
     pcb->status = DOWN;
     CloseConnection();
     switch (pcb->status){
        case OKAY:
           Failover(pcb);
           SetWatchdog();
           break;
        case SUSPECT:
        case REOPEN:
           SetWatchdog();
           break;
        default:
           error("Shouldn't be here!);
           break;
        }
 }
 /*  Here is the state machine equivalent to the above code:
 STATE         Event                Actions              New State
 =====         ------               -------              ----------
 OKAY          Receive DWA          Pending = FALSE
                                    SetWatchdog()        OKAY
 OKAY          Receive non-DWA      SetWatchdog()        OKAY
 SUSPECT       Receive DWA          Pending = FALSE
                                    Failback()
                                    SetWatchdog()        OKAY
 SUSPECT       Receive non-DWA      Failback()
                                    SetWatchdog()        OKAY
 REOPEN        Receive DWA &        Pending = FALSE
               NumDWA == 2          NumDWA++
                                    Failback()           OKAY
 REOPEN        Receive DWA &        Pending = FALSE
               NumDWA < 2           NumDWA++             REOPEN

Aboba & Wood Standards Track [Page 31] RFC 3539 AAA Transport Profile June 2003

 STATE         Event                Actions              New State
 =====         ------               -------              ----------
 REOPEN        Receive non-DWA      Throwaway()          REOPEN
 INITIAL       Receive DWA          Pending = FALSE
                                    Throwaway()          INITIAL
 INITIAL       Receive non-DWA      Throwaway()          INITIAL
 DOWN          Receive DWA          Pending = FALSE
                                    Throwaway()          DOWN
 DOWN          Receive non-DWA      Throwaway()          DOWN
 OKAY          Timer expires &      SendWatchdog()
               !Pending             SetWatchdog()
                                    Pending = TRUE       OKAY
 OKAY          Timer expires &      Failover()
               Pending              SetWatchdog()        SUSPECT
 SUSPECT       Timer expires        CloseConnection()
                                    SetWatchdog()        DOWN
 INITIAL       Timer expires        AttemptOpen()
                                    SetWatchdog()        INITIAL
 DOWN          Timer expires        AttemptOpen()
                                    SetWatchdog()        DOWN
 REOPEN        Timer expires &      SendWatchdog()
               !Pending             SetWatchdog()
                                    Pending = TRUE       REOPEN
 REOPEN        Timer expires &      CloseConnection()
               Pending &            SetWatchdog()
               NumDWA < 0                                DOWN
 REOPEN        Timer expires &      NumDWA = -1
               Pending &            SetWatchdog()
               NumDWA >= 0                               REOPEN
 INITIAL       Connection up        SetWatchdog()        OKAY
 DOWN          Connection up        NumDWA = 0
                                    SendWatchdog()
                                    SetWatchdog()
                                    Pending = TRUE       REOPEN
 OKAY          Connection down      CloseConnection()
                                    Failover()
                                    SetWatchdog()        DOWN
 SUSPECT       Connection down      CloseConnection()
                                    SetWatchdog()        DOWN
 REOPEN        Connection down      CloseConnection()
                                    SetWatchdog()        DOWN
 */

Aboba & Wood Standards Track [Page 32] RFC 3539 AAA Transport Profile June 2003

Appendix B - AAA Agents

 As described in [RFC2865] and [RFC2607], AAA agents have become
 popular in order to support services such as roaming and shared use
 networks.  Such agents are used both for
 authentication/authorization, as well as accounting [RFC2975].
 AAA agents include:
    Relays
    Proxies
    Re-directs
    Store and Forward proxies
    Transport layer proxies
 The transport layer behavior of each of these agents is described
 below.

B.1 Relays and Proxies

 While the application-layer behavior of relays and proxies are
 different, at the transport layer the behavior is similar.  In both
 cases, two connections are established: one from the AAA client (NAS)
 to the relay/proxy, and another from the relay/proxy to the AAA
 server.  The relay/proxy does not respond to a client request until
 it receives a response from the server.  Since the two connections
 are de-coupled, the end-to-end conversation between the client and
 server may not self clock.
 Since AAA transport is typically application-driven, there is
 frequently not enough traffic to enable ACK piggybacking.  As a
 result, the Nagle algorithm is rarely triggered, and delayed ACKs may
 comprise nearly half the traffic.  Thus AAA protocols running over
 reliable transport will see packet traffic nearly double that
 experienced with UDP transport.  Since ACK parameters (such as the
 value of the delayed ACK timer) are typically fixed by the TCP
 implementation and are not tunable by the application, there is
 little that can be done about this.

Aboba & Wood Standards Track [Page 33] RFC 3539 AAA Transport Profile June 2003

 A typical trace of a conversation between a NAS, proxy and server is
 shown below:
 Time            NAS           Relay/Proxy           Server
 ------          ---           -----------           ------
 0               Request
                 ------->
 OTTnp + Tpr                     Request
                                 ------->
 OTTnp + TdA                     Delayed ACK
                                 <-------
 OTTnp + OTTps +                                 Reply/ACK
 Tpr + Tsr                                       <-------
 OTTnp + OTTps +
 Tpr + Tsr +                     Reply
 OTTsp + TpR                     <-------
 OTTnp + OTTps +
 Tpr + Tsr +                     Delayed ACK
 OTTsp + TdA                     ------->
 OTTnp + OTTps +
 OTTsp + OTTpn +
 Tpr + Tsr +      Delayed ACK
 TpR + TdA        ------->
 Key
 ---
 OTT   = One-way Trip Time
 OTTnp = One-way trip time (NAS to Relay/Proxy)
 OTTpn = One-way trip time (Relay/Proxy to NAS)
 OTTps = One-way trip time (Relay/Proxy to Server)
 OTTsp = One-way trip time (Server to Relay/Proxy)
 TdA   = Delayed ACK timer
 Tpr   = Relay/Proxy request processing time
 TpR   = Relay/Proxy reply processing time
 Tsr   = Server request processing time
 At time 0, the NAS sends a request to the relay/proxy.  Ignoring the
 serialization time, the request arrives at the relay/proxy at time
 OTTnp, and the relay/proxy takes an additional Tpr in order to
 forward the request toward the home server.  At time TdA after

Aboba & Wood Standards Track [Page 34] RFC 3539 AAA Transport Profile June 2003

 receiving the request, the relay/proxy sends a delayed ACK.  The
 delayed ACK is sent, rather than being piggybacked on the reply, as
 long as TdA < OTTps + OTTsp + Tpr + Tsr + TpR.
 Typically Tpr < TdA, so that the delayed ACK is sent after the
 relay/proxy forwards the request toward the server, but before the
 relay/proxy receives the reply from the server.  However, depending
 on the TCP implementation on the relay/proxy and when the request is
 received, it is also possible for the delayed ACK to be sent prior to
 forwarding the request.
 At time OTTnp + OTTps + Tpr, the server receives the request, and Tsr
 later, it generates the reply.  Where Tsr < TdA, the reply will
 contain a piggybacked ACK.  However, depending on the server
 responsiveness and TCP implementation, the ACK and reply may be sent
 separately.  This can occur, for example, where a slow database or
 storage system must be accessed prior to sending the reply.
 At time OTTnp + OTTps + OTTsp + Tpr + Tsr the reply/ACK reaches the
 relay/proxy, which then takes TpR additional time to forward the
 reply to the NAS.  At TdA after receiving the reply, the relay/proxy
 generates a delayed ACK.  Typically TpR < TdA so that the delayed ACK
 is sent to the server after the relay/proxy forwards the reply to the
 NAS.  However, depending on the circumstances and the relay/proxy TCP
 implementation, the delayed ACK may be sent first.
 As with a delayed ACK sent in response to a request, which may be
 piggybacked if the reply can be received quickly enough, piggybacking
 of the ACK sent in response to a reply from the server is only
 possible if additional request traffic is available.  However, due to
 the high inter-packet spacings in typical AAA scenarios, this is
 unlikely unless the AAA protocol supports a reply ACK.
 At time OTTnp + OTTps + OTTsp + OTTpn + Tpr + Tsr + TpR the NAS
 receives the reply.  TdA later, a delayed ACK is generated.

B.2 Re-directs

 Re-directs operate by referring a NAS to the AAA server, enabling the
 NAS to talk to the AAA server directly.  Since a direct transport
 connection is established, the end-to-end connection will self-clock.
 With re-directs, delayed ACKs are less frequent than with
 application-layer proxies since the Re-direct and Server will
 typically piggyback replies with ACKs.

Aboba & Wood Standards Track [Page 35] RFC 3539 AAA Transport Profile June 2003

 The sequence of events is as follows:
 Time            NAS             Re-direct       Server
 ------          ---             ---------       ------
 0               Request
                 ------->
 OTTnp + Tpr                     Redirect/ACK
                                 <-------
 OTTnp + Tpr +   Request
 OTTpn + Tnr     ------->
 OTTnp + OTTpn +
 Tpr + Tsr +                                     Reply/ACK
 OTTns                                           <-------
 OTTnp + OTTpn +
 OTTns + OTTsn +
 Tpr + Tsr +      Delayed ACK
 TdA              ------->
 Key
 ---
 OTT   = One-way Trip Time
 OTTnp = One-way trip time (NAS to Re-direct)
 OTTpn = One-way trip time (Re-direct to NAS)
 OTTns = One-way trip time (NAS to Server)
 OTTsn = One-way trip time (Server to NAS)
 TdA   = Delayed ACK timer
 Tpr   = Re-direct processing time
 Tnr   = NAS re-direct processing time
 Tsr   = Server request processing time

B.3 Store and Forward Proxies

 With a store and forward proxy, the proxy may send a reply to the NAS
 prior to forwarding the request to the server.  While store and
 forward proxies are most frequently deployed for accounting
 [RFC2975], they also can be used to implement
 authentication/authorization policy, as described in [RFC2607].
 As noted in [RFC2975], store and forward proxies can have a negative
 effect on accounting reliability.  By sending a reply to the NAS
 without receiving one from the accounting server, store and forward
 proxies fool the NAS into thinking that the accounting request had
 been accepted by the accounting server when this is not the case.  As
 a result, the NAS can delete the accounting packet from non-volatile

Aboba & Wood Standards Track [Page 36] RFC 3539 AAA Transport Profile June 2003

 storage before it has been accepted by the accounting server.  That
 leaves the proxy responsible for delivering accounting packets.  If
 the proxy involves moving parts (e.g. a disk drive) while the NAS
 does not, overall system reliability can be reduced.  As a result,
 store and forward proxies SHOULD NOT be used.
 The sequence of events is as follows:
 Time            NAS             Proxy           Server
 ------          ---             -----           ------
 0               Request
                 ------->
 OTTnp + TpR                     Reply/ACK
                                 <-------
 OTTnp + Tpr                     Request
                                 ------->
 OTTnp + OTTph +                                 Reply/ACK
 Tpr + Tsr                                       <-------
 OTTnp + OTTph +
 Tpr + Tsr +                     Reply
 OTThp + TpR                     <-------
 OTTnp + OTTph +
 Tpr + Tsr +                     Delayed ACK
 OTThp + TdA                     ------->
 OTTnp + OTTph +
 OTThp + OTTpn +
 Tpr + Tsr +      Delayed ACK
 TpR + TdA        ------->
 Key
 ---
 OTT   = One-way Trip Time
 OTTnp = One-way trip time (NAS to Proxy)
 OTTpn = One-way trip time (Proxy to NAS)
 OTTph = One-way trip time (Proxy to Home server)
 OTThp = One-way trip time (Home Server to Proxy)
 TdA   = Delayed ACK timer
 Tpr   = Proxy request processing time
 TpR   = Proxy reply processing time
 Tsr   = Server request processing time

Aboba & Wood Standards Track [Page 37] RFC 3539 AAA Transport Profile June 2003

B.4 Transport Layer Proxies

 In addition to acting as proxies at the application layer, transport
 layer proxies forward transport ACKs between the AAA client and
 server.  This splices together the client-proxy and proxy-server
 connections into a single connection that behaves as though it
 operates end-to-end, exhibiting self-clocking.  However, since
 transport proxies operate at the transport layer, they cannot be
 implemented purely as applications and they are rarely deployed.
 With a transport proxy, the sequence of events is as follows:
 Time            NAS             Proxy           Home Server
 ------          ---             -----           -----------
 0               Request
                 ------->
 OTTnp + Tpr                     Request
                                 ------->
 OTTnp + OTTph +                                 Reply/ACK
 Tpr + Tsr                                       <-------
 OTTnp + OTTph +
 Tpr + Tsr +                     Reply/ACK
 OTThp + TpR                     <-------
 OTTnp + OTTph +
 OTThp + OTTpn +
 Tpr + Tsr +      Delayed ACK
 TpR + TdA        ------->
 OTTnp + OTTph +
 OTThp + OTTpn +
 Tpr + Tsr +                     Delayed ACK
 TpR + TpD                       ------->
 Key
 ---
 OTT   = One-way Trip Time
 OTTnp = One-way trip time (NAS to Proxy)
 OTTpn = One-way trip time (Proxy to NAS)
 OTTph = One-way trip time (Proxy to Home server)
 OTThp = One-way trip time (Home Server to Proxy)
 TdA   = Delayed ACK timer
 Tpr   = Proxy request processing time
 TpR   = Proxy reply processing time

Aboba & Wood Standards Track [Page 38] RFC 3539 AAA Transport Profile June 2003

 Tsr   = Server request processing time
 TpD   = Proxy delayed ack processing time

Intellectual Property Statement

 The IETF takes no position regarding the validity or scope of any
 intellectual property or other rights that might be claimed to
 pertain to the implementation or use of the technology described in
 this document or the extent to which any license under such rights
 might or might not be available; neither does it represent that it
 has made any effort to identify any such rights.  Information on the
 IETF's procedures with respect to rights in standards-track and
 standards-related documentation can be found in BCP-11.  Copies of
 claims of rights made available for publication and any assurances of
 licenses to be made available, or the result of an attempt made to
 obtain a general license or permission for the use of such
 proprietary rights by implementors or users of this specification can
 be obtained from the IETF Secretariat.
 The IETF invites any interested party to bring to its attention any
 copyrights, patents or patent applications, or other proprietary
 rights which may cover technology that may be required to practice
 this standard.  Please address the information to the IETF Executive
 Director.

Acknowledgments

 Thanks to Allison Mankin of AT&T, Barney Wolff of Databus, Steve Rich
 of Cisco, Randy Bush of AT&T, Bo Landarv of IP Unplugged, Jari Arkko
 of Ericsson, and Pat Calhoun of Blackstorm Networks for fruitful
 discussions relating to AAA transport.

Aboba & Wood Standards Track [Page 39] RFC 3539 AAA Transport Profile June 2003

Authors' Addresses

 Bernard Aboba
 Microsoft Corporation
 One Microsoft Way
 Redmond, WA 98052
 Phone: +1 425 706 6605
 Fax:   +1 425 936 7329
 EMail: bernarda@microsoft.com
 Jonathan Wood
 Sun Microsystems, Inc.
 901 San Antonio Road
 Palo Alto, CA 94303
 EMail: jonwood@speakeasy.net

Aboba & Wood Standards Track [Page 40] RFC 3539 AAA Transport Profile June 2003

Full Copyright Statement

 Copyright (C) The Internet Society (2003).  All Rights Reserved.
 This document and translations of it may be copied and furnished to
 others, and derivative works that comment on or otherwise explain it
 or assist in its implementation may be prepared, copied, published
 and distributed, in whole or in part, without restriction of any
 kind, provided that the above copyright notice and this paragraph are
 included on all such copies and derivative works.  However, this
 document itself may not be modified in any way, such as by removing
 the copyright notice or references to the Internet Society or other
 Internet organizations, except as needed for the purpose of
 developing Internet standards in which case the procedures for
 copyrights defined in the Internet Standards process must be
 followed, or as required to translate it into languages other than
 English.
 The limited permissions granted above are perpetual and will not be
 revoked by the Internet Society or its successors or assigns.
 This document and the information contained herein is provided on an
 "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
 TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
 BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
 HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
 MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

 Funding for the RFC Editor function is currently provided by the
 Internet Society.

Aboba & Wood Standards Track [Page 41]

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