One document matched: draft-ietf-aaa-transport-12.txt-88564.txt
Differences from 12.txt-11.txt
AAA Working Group Bernard Aboba
INTERNET-DRAFT Microsoft
Category: Standards Track Jonathan Wood
<draft-ietf-aaa-transport-12.txt> Sun Microsystems, Inc.
7 January 2003
Authentication, Authorization and Accounting (AAA) Transport Profile
This document is an Internet-Draft and is in full conformance with all
provisions of Section 10 of RFC 2026.
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.
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time. It is inappropriate to use Internet-Drafts as reference material
or to cite them other than as "work in
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.
Copyright Notice
Copyright (C) The Internet Society (2002). All Rights Reserved.
Abstract
This document discusses transport issues that arise with 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.
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Table of Contents
1. Introduction ........................................... 3
1.1 Requirements language ............................ 3
1.2 Terminology ...................................... 3
2. Issues in transport usage .............................. 5
2.1 Application-driven versus network-driven.......... 6
2.2 Slow failover .................................... 7
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 retransmit ................. 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
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 .... 19
3.7 Inability to use fast re-transmit ................ 21
3.8 Head of line blocking ............................ 21
3.9 Congestion avoidance ............................. 23
3.10 Premature failover ............................... 23
4. Security considerations ................................ 24
5. IANA considerations .................................... 24
6. Normative references ................................... 25
7. Informative references ................................. 25
Appendix A - Detailed Watchdog Algorithm Description .......... 26
Appendix B - AAA agents ....................................... 32
B.1 Relays and proxies ............................... 32
B.2 Re-directs ....................................... 34
B.3 Store and forward proxies ........................ 35
B.4 Transport layer proxies .......................... 37
Acknowledgments ............................................... 38
Author addresses .............................................. 38
Intellectual property statement ............................... 38
Full copyright statement ...................................... 39
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1. Introduction
This document discusses transport issues that arise with protocols for
Authentication, Authorization and Accounting (AAA). It also provides
recommendations on 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.
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
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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.
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
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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 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
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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, much larger than the Round Trip Time (RTT) in most networks.
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
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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 ("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
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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 [RFC2414] of 2, then the 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 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.
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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 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
[RFC2414]. 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
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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.
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
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individual AAA client-server conversations to self-clock, and this
technique appears impractical for use in AAA.
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 worsening the 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 the
lost packet can be successfully re-transmitted. This can be an issue for
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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 a NAS reboots, or where connectivity is restored between a 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.
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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 adress 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 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 downstream proxy or
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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 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.
Watchdog packets are not retransmitted by the AAA protocol, since
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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 oustanding 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
problem. Without transport layer visibility, the application is
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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-transmisions 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 occuring 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 is
received to a watchdog message 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 so 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.
[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
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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
minumum 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 compared,
providing insight into the cause of the problem (transport or
application layer).
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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 to 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.
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.
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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 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 can be returned.
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
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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 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.
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.
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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.
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
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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 has 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:
+------+ +------+ +------+
| | | | | |
| 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
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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.
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
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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 is also may 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 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 draft does not create any new number spaces for IANA
administration.
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6. Normative references
[RFC793] Postel, J., "Transmission Control Protocol", STD 7, RFC 793,
September 1981.
[RFC896] Nagle, J., "Congestion Control in IP/TCP", RFC 896, January
1984.
[RFC1750] Eastlake, D., Crocker, S., and Schiller, J., "Randomness
Recommendations for Security", RFC 1750, December 1994.
[RFC2018] Mathis, M., Mahdavi, J., Floyd, S., Romanow, A., "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.
[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., Romanow, A.,
"An Extension to the Selective Acknowledgment (SACK) Option
for TCP", RFC 2883, July 2000.
[RFC2960] R. Stewart et al., "Stream Control Transmission Protocol", RFC
2960, October 2000.
[RFC2988] Paxson, V., Allman, M., "Computing TCP's Retransmission
Timer", RFC 2988, November 2000.
[RFC3042] Allman, M., Balakrishnan H., Floyd, S., "Enhancing TCP's Loss
Recovery Using Limited Transmit", RFC 3042, January 2001.
[RFC3074] Volz, B., Gonczi, S., Lemon, T., Stevens, R., "DHC Load
Balancing Algorithm", RFC 3074, February 2001.
[RFC3124] Balakrishnan, H., Seshan, S., "The Congestion Manager", RFC
3124, June 2001.
7. Informative references
[RFC2246] Dierks, T., Allen, C., "The TLS Protocol Version 1.0", RFC
2246, November 1998.
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[RFC2401] Atkinson, R., Kent, S., "Security Architecture for the
Internet Protocol", RFC 2401, November 1998
[RFC2409] Harkins, D., Carrel, D., "The Internet Key Exchange (IKE)",
RFC 2409, November 1998
[RFC2414] Allman, M., Floyd, S. and C. Partridge, "Increasing TCP's
Initial Window", RFC 2414, September 1998.
[RFC2607] Aboba, B., Vollbrecht, J., "Proxy Chaining and Policy
Implementation in Roaming", RFC 2607, June 1999.
[RFC2861] Handley, M., Padhye, J., Floyd, S., "TCP Congestion Window
Validation", RFC 2861, June 2000.
[RFC2865] Rigney, C., Willens, S., Rubens, A., Simpson, W., "Remote
Authentication Dial In User Service (RADIUS)", RFC 2865, June
2000.
[RFC2866] Rigney, C., "RADIUS Accounting", RFC 2866, June 2000.
[RFC2914] Floyd, S., "Congestion Control Principles", RFC 2914,
September 2000.
[RFC2975] Aboba, B., Arkko, J., "Introduction to Accounting Management",
RFC 2975, June 2000.
[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.
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.
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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)
{
if (msgType == DWA) {
Pending = FALSE;
}
switch (pcb->Status){
case OKAY:
SetWatchdog();
break;
case SUSPECT:
pcb->Status = OKAY;
Failback(pcb);
SetWatchdog();
break;
case REOPEN:
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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;
FailOver(pcb);
SetWatchdog();
break ;
case SUSPECT:
pcb->status = DOWN;
CloseConnection(pcb);
SetWatchdog();
break;
case INITIAL:
case DOWN:
AttemptOpen(pcb);
SetWatchdog();
break;
case REOPEN:
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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);
SetWatchdog();
Pending = TRUE;
break;
default:
error("Shouldn't be here!);
break;
}
}
/*
OnConnectionDown() is called whenever a connection goes down
*/
OnConnectionDown(pcb)
{
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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
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
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STATE Event Actions New State
===== ------ ------- ----------
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
*/
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Appendix B - AAA agents
As described in [RFC2865],[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.
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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 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 +
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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.
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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 storage before it has
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been accepted by the accounting server. The 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
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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 conection 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
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Tsr = Server request processing time
TpD = Proxy delayed ack processing time
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.
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
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Expiration Date
This memo is filed as <draft-ietf-aaa-transport-12.txt>, and expires
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| PAFTECH AB 2003-2026 | 2026-04-22 06:41:44 |