Internet DRAFT - draft-iab-model
draft-iab-model
E. Rescorla
RTFM, Inc.
INTERNET-DRAFT IAB
draft-iab-model-03.txt February 2005 (Expires September 2005)
Writing Protocol Models
Status of this Memo
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Copyright Notice
Copyright (C) The Internet Society (1999-2004). All Rights Reserved.
Abstract
The IETF process depends on peer review. However, IETF documents
are generally written to be useful for implementors, not for
reviewers. In particular, while great care is generally taken to
provide a complete description of the state machines and bits on
the wire, this level of detail tends to get in the way of initial
understanding. This document describes an approach for providing
protocol "models" that allow reviewers to quickly grasp the essence
of a system.
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Contents
1. Introduction
The IETF process depends on peer review. However, in many cases,
the documents submitted for publication are extremely difficult to
review. Since reviewers have only limited amounts of time, this
leads to extremely long review times, inadequate reviews, or both.
In my view, a large part of the problem is that most documents fail
to present an architectural model for how the protocol operates,
opting instead to simply describe the protocol and let the reviewer
figure it out.
This is acceptable when documenting a protocol for implementors,
because they need to understand the protocol in any case, but
dramatically increases the strain on reviewers. Reviewers
necessarily need to get the big picture of the system and then
focus on particular points. They simply do not have time to give
the entire document the attention an implementor would.
One way to reduce this load is to present the reviewer with a
MODEL--a short description of the system in overview form. This
provides the reviewer with the context to identify the important or
difficult pieces of the system and focus on them for review. As a
side benefit, if the model is done first, it can be serve as an aid
to the detailed protocol design and a focus for early review prior
to protocol completion. The intention is that the model would
either be the first section of the protocol document or be a
separate document provided with the protocol.
2. The Purpose of a Protocol Model
A protocol model needs to answer three basic questions:
1. What problem is the protocol trying to achieve?
2. What messages are being transmitted and what do they
mean?
3. What are the important but un-obvious features of the
protocol?
The basic idea is to provide enough information that the reader
could design a protocol which was roughly isomorphic to the
protocol being described. This doesn't, of course, mean that the
protocol would be identical, but merely that it would share most
important features. For instance, the decision to use a KDC-based
authentication model is an essential feature of Kerberos
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[KERBEROS]. By constrast, the use of ASN.1 is a simple
implementation decision. S-expressions--or XML, had it existed at
the time--would have served equally well.
The purpose of a protocol model is explicitly not to provide a
complete or alternate description of the protocol being discussed.
Instead, it is to provide a big picture overview of the protocol so
that readers can quickly understand the essential elements of how
it works.
3. Basic Principles
In this section we discuss basic principles that should guide your
presentation.
3.1. Less is more
Humans are only capable of keeping a very small number of pieces of
information in their head at once. Since we're interested in
ensuring that people get the big picture, we therefore have to
dispense with a lot of detail. That's good, not bad. The simpler
you can make things the better.
3.2. Abstraction is good
A key technique for representing complex systems is to try to
abstract away pieces. For instance, maps are better than
photographs for finding out where you want to go because they
provide an abstract, stylized, view of the information you're
interested in. Don't be afraid to compress multiple protocol
elements into a single abstract piece for pedagogical purposes.
3.3. A few well-chosen details sometimes helps
The converse of the previous principle is that sometimes details
help to bring a description into focus. Many people work better
when given examples. Thus, it's often a good approach to talk about
the material in the abstract and then provide a concrete
description of one specific piece to bring it into focus. Authors
should focus on the normal path. Error cases and corner cases
should only be discussed where they help illustrate some important
point.
4. Writing Protocol Models
Our experience indicates that it's easiest to grasp protocol models
when they're presented in visual form. We recommend a presentation
format that is centered around a few key diagrams with explanatory
text for each. These diagrams should be simple and typically
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consist of "boxes and arrows"--boxes representing the major
components, arrows representing their relationships and labels
indicating important features.
We recommend a presentation structured in three parts to match the
three questions mentioned in the previous sections. Each part
should contain 1-3 diagrams intended to illustrate the relevant
points.
4.1. Describe the problem you're trying to solve
The absolutely most critical task that a protocol model must
perform is to explain what the protocol is trying to achieve. This
provides crucial context for understanding how the protocol works
and whether it meets its goals. Given the desired goals, in most
cases an experienced reviewer will have an idea of how they would
approach the problem and be able to compare that to the approach
taken by the protocol under review.
The "Problem" section of the model should start out with a short
statement of the environments in which the protocol is expected to
be used. This section should describe the relevant entities and the
likely scenarios under which they participate in the protocol. The
Problem section should feature a diagram showing the major
communicating parties and their inter-relationships. It is
particularly important to lay out the trust relationships between
the various parties as these are often un-obvious.
4.1.1. Example: STUN (RFC 3489)
[STUN] is a UNilateral Self-Address Fixing (UNSAF) [UNSAF] protocol
which allows a machine located behind a NAT to determine what its
external apparent IP address is. Unfortunately, although STUN
provides a complete and thorough description of the operation of
the protocol, it does not provide a brief, up-front overview
suitable for a quick understanding of its operation. The rest of
this section shows what a suitable overview might look like.
Network Address Translation (NAT) makes it difficult to run a
number of classes of service from behind the NAT gateway. This is a
particular problem when protocols need to advertise address/port
pairs as part of the application layer protocol. Although the NAT
can be configured to accept data destined for that port, address
translation means that the address that the application knows about
is not the same as the one that it is reachable on.
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Consider the scenario represented in the figure below. A SIP client
is initiating a session with a SIP server in which it wants the SIP
server to send it some media. In its Session Description Protocol
(SDP) [SDP] request it provides the IP address and port on which it
is listening. However, unbeknownst to the client, a NAT is in the
way. It translates the IP address in the header, but unless it is
SIP aware, it doesn't change the address in the request. The result
is that the media goes into a black hole.
+-----------+
| SIP |
| Server |
| |
+-----------+
^
| [FROM: 198.203.2.1:8954]
| [MSG: SEND MEDIA TO 10.0.10.5:6791]
|
|
+-----------+
| |
| NAT |
--------------+ Gateway +----------------
| |
+-----------+
^
| [FROM: 10.0.10.5:6791]
| [MSG: SEND MEDIA TO 10.0.10.5:6791]
|
10.0.10.5
+-----------+
| SIP |
| Client |
| |
+-----------+
The purpose of STUN [STUN] is to allow clients to detect this
situation and determine the address mapping. They can then place the
appropriate address in their application-level messages. This is done
by making use of an external STUN server. That server is able to
determine the translated address and tell the STUN client, as shown
below.
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+-----------+
| STUN |
| Server |
| |
+-----------+
^ |
[IP HDR FROM: 198.203.2.1:8954] | | [IP HDR TO: 198.203.2.1:8954]
[MSG: WHAT IS MY ADDRESS?] | | [MSG: YOU ARE 198.203.2.1:8954]
| v
+-----------+
| |
| NAT |
--------------+ Gateway +----------------
| |
+-----------+
^ |
[IP HDR FROM: 10.0.10.5:6791] | | [IP HDR TO: 10.0.10.5:6791]
[MSG: WHAT IS MY ADDRESS?] | | [MSG: YOU ARE 198.203.2.1:8954]
| v
10.0.10.5
+-----------+
| SIP |
| Client |
| |
+-----------+
4.2. Describe the protocol in broad overview
Once you've described the problem, the next task is to describe the
protocol in broad overview. This means showing, either in "ladder
diagram" or "boxes and arrows" form, the protocol messages that
flow between the various networking agents. This diagram should be
accompanied with explanatory text that describes the purpose of
each message and the MAJOR data elements.
This section SHOULD NOT contain detailed descriptions of the
protocol messages or of each data element. In particular, bit
diagrams, ASN.1 modules and XML schema SHOULD NOT be shown. The
purpose of this section is explicitly not to provide a complete
description of the protocol. Instead, it is to provide enough of a
map so that a person reading the full protocol document can see
where each specific piece fits.
In certain cases, it may be helpful to provide a state machine
description of the behavior of network elements. However, such
state machines should be kept as minimal as possible. Remember that
the purpose is to promote high-level comprehension, not complete
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understanding.
4.2.1. Example: DCCP
Datagram Congestion Control Protocol [DCCP] is a protocol for
providing datagram transport with network friendly congestion
avoidance behavior. The DCCP base protocol document is over 100
pages long and the congestion control mechanisms themselves are
separate. It is therefore very helpful to have a an architectural
overview of DCCP that abstracts away the details. The remainder of
this section is an attempt to do so.
NOTE: The author of this document was on the [DCCP] review team and
his experience with that document was one of the motivating factors
for this document. In the time since, the DCCP authors have added
some overview material, some of which derives from earlier versions
of this document.
Although DCCP [DCCP] is datagram oriented like UDP, it is stateful
like TCP. Connections go through the following phases:
1. Initiation
2. Feature negotiation
3. Data transfer
4. Termination
4.2.1.1. Initiation
As with TCP, the initiation phase of DCCP involves a three-way
handshake, shown below.
Client Server
------ ------
DCCP-Request ->
[Ports, Service,
Features]
<- DCCP-Response
[Features,
Cookie]
DCCP-Ack ->
[Features,
Cookie]
DCCP 3-way handshake
In the DCCP-Request message, the client tells the server the name
of the service it wants to talk to and the ports it wants to
communicate on. Note that ports are not tightly bound to services
the way they are in TCP or UDP common practice. It also starts
feature negotiation. For pedagogical reasons, we will present
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feature negotiation separately in the next section. However,
realize that the early phases of feature negotiation happen
concurrently with initiation.
In the DCCP-Response message, the server tells the client that it
is willing to accept the connection and continues feature
negotiation. In order to prevent SYN-flood style DOS attacks, DCCP
incorporates an IKE-style cookie exchange. The server can provide
the client with a cookie that contains all the negotiation state.
This cookie must be echoed by the client in the DCCP-Ack, thus
removing the need for the server to keep state.
In the DCCP-Ack message, the client acknowledges the DCCP-Response
and returns the cookie to permit the server to complete its side of
the connection. As indicated above this message may also include
feature negotiation messages.
4.2.1.2. Feature Negotiation
In DCCP, feature negotiation is performed by attaching options to
other DCCP packets. Thus feature negotiation can be piggybacked on
any other DCCP message. This allows feature negotiation during
connection initiation as well as feature renegotiation during data
flow.
Somewhat unusually, DCCP features are one-sided. Thus, it's
possible to have a different congestion control regime for data
sent from client to server than from server to client.
Feature negotiation is done with the Change and Confirm options.
There are four feature negotiation options in all: Change L,
Confirm L, Change R, and Confirm R. The "L" options are sent by the
feature location, where the feature is maintained, and the "R"
options are sent by the feature remote.
A Change R message says to the peer "change this option setting on
your side". The peer can respond with a Confirm L, meaning "I've
changed it". Some features allow Change R options to contain
multiple values, sorted in preference order. For example:
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Client Server
------ ------
Change R(CCID, 2) -->
<-- Confirm L(CCID, 2)
* agreement that CCID/Server = 2 *
Change R(CCID, 3 4) -->
<-- Confirm L(CCID, 4, 4 2)
* agreement that CCID/Server = 4 *
<- Confirm(CC,2)
In the second exchange, the client requests that the server use
either CCID 3 or CCID 4, with 3 preferred. The server chooses 4 and
supplies its preference list, "4 2".
The Change L and Confirm R options are used for feature
negotiations initiated by the feature location. In the following
example, the server requests that CCID/Server be set to 3 or 2,
with 3 preferred, and the client agrees.
Client Server
------ ------
<-- Change L(CCID, 3 2)
Confirm R(CCID, 3, 3 2) -->
* agreement that CCID/Server = 3 *
4.2.1.3. Data Transfer
Rather than have a single congestion control regime as in TCP, DCCP
offers a variety of negotiable congestion control regimes. The DCCP
documents describe two congestion control regimes: additive
increase, multiplicative decrease (CCID-2 [CCID2]) and TCP-friendly
rate control (CCID-3 [CCID3]). CCID-2 is intended for applications
which want maximum throughput. CCID-3 is intended for real-time
applications which want smooth response to congestion.
4.2.1.3.1. CCID-2
CCID-2's congestion control is extremely similar to that of TCP.
The sender maintains a congestion window and sends packets until
that window is full. Packets are Acked by the receiver. Dropped
packets and ECN [ECN] are used to indicate congestion. The response
to congestion is to halve the congestion window. One subtle
diference between DCCP and TCP is that the Acks in DCCP must
contain the sequence numbers of all received packets (within a
given window) not just the highest sequence number as in TCP.
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4.2.1.3.2. CCID-3
CCID-3 is an equation-based form of rate control which is intended
to provide smoother response to congestion than CCID-2. The sender
maintains a "transmit rate". The receiver sends ACK packets which
also contain information about the receiver's estimate of packet
loss. The sender uses this information to update its transmit rate.
Although CCID-3 behaves somewhat differently from TCP in its short-
term congestion response, it is designed to operate fairly with TCP
over the long term.
4.2.1.4. Termination
Connection termination in DCCP is initiated by sending a Close
message. Either side can send a Close message. The peer then
responds with a Reset message, at which point the connection is
closed. The side that sent the Close message must quietly preserve
the socket in TIMEWAIT state for 2MSL.
Client Server
------ ------
Close ->
<- Reset
[Remains in TIMEWAIT]
Note that the server may wish to close the connection but not
remain in TIMEWAIT (e.g., due to a desire to minimize server-side
state.) In order to accomplish this, the server can elicit a Close
from the client by sending a CloseReq message and thus keeping the
TIMEWAIT state on the client.
4.3. Describe any important protocol features
The final section (if there is one) should contain an explanation
of any important protocol features which are not obvious from the
previous sections. In the best case, all the important features of
the protocol would be obvious from the message flow. However, this
isn't always the case. This section is an opportunity for the
author to explain those features. Authors should think carefully
before writing this section. If there are no important points to be
made they should not populate this section.
Examples of the kind of feature that belongs in this section
include: high-level security considerations, congestion control
information and overviews of the algorithms that the network
elements are intended to follow. For instance, if you have a
routing protocol you might use this section to sketch out the
algorithm that the router uses to determine the appropriate routes
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from protocol messages.
4.3.1. Example: WebDAV COPY and MOVE
The WebDAV standard [WEBDAV] is in general fairly terse, preferring
to define the required behaviors and let the reader work out the
implications. In some situations, explanatory material detailing
those implications can be helpful to give the reader a sense of the
overall model. The rest of this section describes one such issue.
WebDAV [WEBDAV] includes both a COPY method and a MOVE method.
While a MOVE can be thought of as a COPY followed by DELETE,
COPY+DELETE and MOVE aren't entirely equivalent.
The use of COPY+DELETE as a MOVE substitute is problematic because
of the creation of the intermediate file. Consider the case where
the user is approaching some quota boundary. A COPY+DELETE should
be forbidden because it would temporarily exceed the quota.
However, a simple rename should work in this situation.
The second issue is permissions. The WebDAV permissions model
allows the server to grant users permission to rename files but not
to create new ones--this is unusual in ordinary filesystems but
nothing prevents it in WebDAV. This is clearly not possible if a
client uses COPY+DELETE to do a MOVE.
Finally, a COPY+DELETE does not produce the same logical result as
would be expected with a MOVE. Because COPY creates a new resource,
it is permitted (but not required) to use the time of new file
creation as the creation date property. By contrast, the
expectation for move is that the renamed file will have the same
properties as the original.
5. Formatting Issues
The requirement that Internet-Drafts and RFCs be renderable in
ASCII is a significant obstacle when writing the sort of graphics-
heavy document being described here. Authors may find it more
convenient to do a separate protocol model document in Postscript
or PDF and simply make it available at review time--though an
archival version would certainly be handy.
6. A Complete Example: Internet Key Exchange (IKE)
Internet Key Exchange (IKE) [IKE] is one of the most complicated
security protocols ever designed by the IETF. Although the basic
IKE core is a fairly straightforward Diffie-Hellman-based
handshake, this can often be difficult for new readers to
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understand abstractly apart from the protocol details. The
remainder of this section provides overview of IKE suitable for
those new readers.
6.1. Operating Environment
Internet key Exchange (IKE) [IKE] is a key establishment and
parameter negotiation protocol for Internet protocols. Its primary
application is for establishing security associations (SAs) [IPSEC]
for IPsec AH [AH] and ESP [ESP].
+--------------------+ +--------------------+
| | | |
| +------------+ | | +------------+ |
| | Key | | IKE | | Key | |
| | Management | <-+-----------------------+-> | Management | |
| | Process | | | | Process | |
| +------------+ | | +------------+ |
| ^ | | ^ |
| | | | | |
| v | | v |
| +------------+ | | +------------+ |
| | IPsec | | AH/ESP | | IPsec | |
| | Stack | <-+-----------------------+-> | Stack | |
| | | | | | | |
| +------------+ | | +------------+ |
| | | |
| | | |
| Initiator | | Responder |
+--------------------+ +--------------------+
The general deployment model for IKE is shown in Figure 1. The
IPsec engines and IKE engines typically are separate modules. When
a packet needs to be processed (either sent or received) for which
no security association exists, the IPsec engine contacts the IKE
engine and asks it to establish an appropriate SA. The IKE engine
contacts the appropriate peer and uses IKE to establish the SA.
Once the IKE handshake is finished it registers the SA with the
IPsec engine.
In addition, IKE traffic between the peers can be used to refresh
keying material or adjust operating parameters such as algorithms.
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6.1.1. Initiator and Responder
Although IPsec is basically symmetrical, IKE is not. The party who
sends the first message is called the INITIATOR. The other party is
called the RESPONDER. In the case of TCP connections the INITIATOR
will typically be the peer doing the active open (i.e. the client).
6.1.2. Perfect Forward Secrecy
One of the major concerns in IKE design was that traffic be
protected even if they keying material of the nodes was later
compromised, provided that the session in question had terminated
and so the session-specific keying material was gone. This property
is often called PERFECT FORWARD SECRECY (PFS) or BACK TRAFFIC
PROTECTION.
6.1.3. Denial of Service Resistance
Since IKE allows arbitrary peers to initiate computationally
expensive cryptographic operations, it potentially allows resource
consumption denial of service attacks to be mounted against the IKE
engine. IKE includes countermeasures designed to minimize this
risk.
6.1.4. Keying Assumptions
Because Security Associations are essentially symmetric, both sides
must in general be authenticated. Because IKE needs to be able to
establish SAs between a broad range of peers with various kinds of
prior relationships, IKE supports a very flexible keying model.
Peers can authenticate via shared keys, digital signatures
(typically from keys vouched for by certificates), or encryption
keys.
6.1.5. Identity Protection
Although IKE requires the peers to authenticate to each other, it
was considered desirable by the working group to provide some
identity protection for the communicating peers. In particular, the
peers should be able to hide their identity from passive observers
and one peer should be able to require the author to authenticate
before they self-identity. In this case, the designers chose to
make the party who speaks first (the INITIATOR) identify first.
6.2. Protocol Overview
At a very high level, there are two kinds of IKE handshake:
(1) Those which establish an IKE security association.
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(2) Those which establish an AH or ESP security association.
When two peers which have never communicated before need to
establish an AH/ESH SA, they must first establish an IKE SA. This
allows them to exchange an arbitrary amount of protected IKE
traffic. They can then use that SA to do a second handshake to
establish SAs for AH and ESP. This process is shown in schematic
form below. The notation E(SA,XXXX) is used to indicate that
traffic is encrypted under a given SA.
Initiator Responder
--------- ---------
Handshake MSG -> \ Stage 1:
<- Handshake MSG \ Establish IKE
/ SA (IKEsa)
[...] /
Stage 2:
E(IKEsa, Handshake MSG) -> \ Establish AH/ESP
<- E(IKEsa, Handshake MSG) / SA
The two kinds of IKE handshake
IKE terminology is somewhat confusing, referring under different
circumstances to "phases" and "modes". For maximal clarity we will
refer to the Establishment of the IKE SA as "Stage 1" and the
Establishment of AH/ESP SAs as "Stage 2". Note that it's quite
possible for there to be more than one Stage 2 handshake, once
Stage 1 has been finished. This might be useful if you wanted to
establish multiple AH/ESP SAs with different cryptographic
properties.
The Stage 1 and Stage 2 handshakes are actually rather different,
because the Stage 2 handshake can of course assume that its traffic
is being protected with an IKE SA. Accordingly, we will first
discuss Stage 1 and then Stage 2.
6.2.1. Stage 1
There are a large number of variants of the IKE Stage 1 handshake,
necessitated by use of different authentication mechanisms.
However, broadly speaking they fall into one of two basic
categories: MAIN MODE, which provides identity protection and DoS
resistance, and AGGRESSIVE MODE, which does not. We will cover MAIN
MODE first.
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6.2.1.1. Main Mode
Main Mode is a six message (3 round trip) handshake which offers
identity protection and DoS resistance. An overview of the
handshake is below.
Initiator Responder
--------- ---------
CookieI, Algorithms -> \ Parameter
<- CookieR, Algorithms / Establishment
CookieR,
Nonce, Key Exchange ->
<- Nonce, Key Exchange\ Establish
/ Shared key
E(IKEsa, Auth Data) ->
<- E(IKEsa, Auth data)\ Authenticate
/ Peers
IKE Main Mode handshake (stage 1)
In the first round trip, the Initiator offers a set of algorithms
and parameters. The Responder picks out the single set that it
likes and responds with that set. It also provides CookieR, which
will be used to prevent DoS attacks. At this point, there is no
secure association but the peers have tentatively agreed upon
parameters. These parameters include a Diffie-Hellman group, which
will be used in the second round trip.
In the second round trip, the Initiator sends the key exchange
information. This generally consists of the Initiator's Diffie-
Hellman public share (Yi). He also supplies CookieR, which was
provided by the responder. The Responder replies with his own DH
share (Yr). At this point, both Initiator and Responder can compute
the shared DH key (ZZ). However, there has been no authentication
and so they don't know with any certainty that the connection
hasn't been attacked. Note that as long as the peers generate fresh
DH shares for each handshake than PFS will be provided.
Before we move on, let's take a look at the cookie exchange. The
basic anti-DoS measure used by IKE is to force the peer to
demonstrate that they can receive traffic from you. This foils
blind attacks like SYN floods [SYNFLOOD] and also makes it somewhat
easier to track down attackers. The cookie exchange serves this
role in IKE. The Responder can verify that the Initiator supplied a
valid CookieR before doing the expensive DH key agreement. This
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does not totally eliminate DoS attacks, since an attacker who was
willing to reveal his location could still consume server
resources, but it does protect against a certain class of blind
attack.
In the final round trip, the peers establish their identities.
Since they share an (unauthenticated) key, they can send their
identities encrypted, thus providing identity protection from
eavesdroppers. The exact method of proving identity depends on what
form of credential is being used (signing key, encryption key,
shared secret, etc.), but in general you can think of it as a
signature over some subset of the handshake messages. So, each side
would supply its certificate and then sign using the key associated
with that certificate. If shared keys are used, the authentication
data would be a key id and a MAC. Authentication using public key
encryption follows similar principles but is more complicated.
Refer to the IKE document for more details.
At the end of the Main Mode handshake, the peers share:
(1) A set of algorithms for encryption of further IKE traffic.
(2) Traffic encryption and authentication keys.
(3) Mutual knowledge of the peer's identity.
6.2.1.2. Aggressive Mode
Although IKE Main Mode provides the required services, there was
concern that the large number of round trips required added
excessive latency. Accordingly, an Aggressive Mode was defined.
Aggressive mode packs more data into fewer messages and thus
reduces latency. However, it does not provide protection against
DoS or identity protection.
Initiator Responder
--------- ---------
Algorithms, Nonce,
Key Exchange, ->
<- Algorithms, Nonce,
Key Exchange, Auth Data
Auth Data ->
IKE Aggressive Mode handshake (stage 1)
After the first round trip, the peers have all the required
properties except that the Initiator has not authenticated to the
Responder. The third message closes the loop by authenticating the
Initiator. Note that since the authentication data is sent in the
clear, no identity protection is provided and since the Responder
does the DH key agreement without a round trip to the Initiator,
Rescorla [Page 16] Internet-Draft Writing Protocol Models 9/2004
there is no DoS protection
6.2.2. Stage 2
Stage 1 on its own isn't very useful. The purpose of IKE, after
all, is to establish associations to be used to protect other
traffic, not just to establish IKE SAs. Stage 2 (what IKE calls
"Quick Mode") is used for this purpose. The basic Stage 2 handshake
is shown below.
Initiator Responder
--------- ---------
AH/ESP parameters,
Algorithms, Nonce,
Handshake Hash ->
<- AH/ESP parameters,
Algorithms, Nonce,
Handshake Hash
Handshake Hash ->
The basic IKE Quick Mode (stage 2)
As with quick mode, the first two messages establish the algorithms
and parameters while the final message is a check over the previous
messages. In this case, the parameters also include the transforms
to be applied to the traffic (AH or ESP) and the kinds of traffic
which are to be protected. Note that there is no key exchange
information shown in these messages.
In this version of Quick Mode, the peers use the pre-existing Stage
1 keying material to derive fresh keying material for traffic
protection (with the nonces to ensure freshness). Quick mode also
allows for a new Diffie-Hellman handshake for per-traffic key PFS.
In that case, the first two messages shown above would also include
Key Exchange payloads, as shown below.
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Initiator Responder
--------- ---------
AH/ESP parameters,
Algorithms, Nonce,
Key Exchange, ->
Handshake Hash
<- AH/ESP parameters,
Algorithms, Nonce,
Key Exchange,
Handshake Hash
Handshake Hash ->
A variant of Quick Mode with PFS (stage 2)
6.3. Other Considerations
There are a number of features of IKE that deserve special
consideration. These are discussed here.
6.3.1. Cookie Generation
As mentioned previously, IKE uses cookies as a partial defense
against DoS attacks. When the responder receives Main Mode message
3 containing the Key Exchange data and the cookie, it verifies that
the cookie is correct. However, this verification must not involve
having a list of valid cookies. Otherwise, an attacker could
potentially consume arbitrary amounts of memory by repeatedly
requesting cookies from a responder. The recommended way to
generate a cookie, suggested by Phil Karn, is by having a single
master key and compute a hash of the secret and the initiator's
address information. This cookie can be verified by recomputing the
cookie value based on information in the third message and seeing
if it matches.
6.3.2. Endpoint Identities
So far we have been rather vague about what sorts of endpoint
identities are used. In principle, there are three ways a peer
might be identified: by a shared key, a pre-configured public key,
and a certificate.
6.3.2.1. Shared Key
In a shared key scheme, the peers share some symmetric key. This
key is associated with a key identifier which is known to both
parties. It is assumed that the party verifying that identity also
has some sort of table that indicates what sorts of traffic (e.g.
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what addresses) that identity is allowed to negotiate SAs for.
6.3.2.2. Pre-configured public key
A pre-configured public key scheme is the same as a shared key
scheme except that the verifying party has the authenticating
party's public key instead of a shared key.
6.3.2.3. Certificate
In a certificate scheme, authenticating party presents a
certificate containing their public key. It's straightforward to
establish that that certificate matches the authentication data
provided by the peer. What's less straightforward is to determine
whether a given peer is entitled to negotiate for a given class of
traffic. In theory, one might be able to determine this from the
name in the certificate (e.g. the subject name contains an IP
address that matches the ostensible IP address). In practice, this
is not clearly specified in IKE and therefore not really
interoperable. The more likely case at the moment is that there is
a configuration table mapping certificates to policies, as with the
other two authentication schemes.
Normative References
There are no normative references for this document.
Informative References
[AH] Kent, S., and Atkinson, R., "IP Authentication Header",
RFC 2402, November 1998.
[CCID2] Floyd, S., Kohler, E., "Profile for DCCP Congestion Control ID 2:
TCP-like Congestion Control", draft-ietf-dccp-ccid2-04.txt,
October 2003.
[CCID3] Floyd, S., Kohler, E., Padhye, J. "Profile for DCCP Congestion
Control ID 3: TFRC Congestion Control",
draft-ietf-dccp-ccid3-05.txt, February 2004.
[DCCP] Kohler, E., Handley, M., Floyd, S., "Datagram Congestion
Control Protocol (DCCP)", draft-ietf-dccp-spec-09.txt,
November, 2004.
[ECN] Ramakrishnan, K. Floyd, S., Black D., "The Addition of
Explicit Congestion Notification (ECN) to IP",
RFC 3168, September 2001.
[ESP] Kent, S., and Atkinson, R., "IP Encapsulating Security
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Payload (ESP)", RFC 2406, November 1998.
[IKE] Harkins, D., Carrel, D., "The Internet Key Exchange (IKE)",
RFC 2409, November 1998.
[IPSEC] Kent, S., Atkinson, R., "Security Architecture for the Internet
Protocol", RFC 2401, November 1998.
[KERBEROS] Kohl, J., Neuman, C., "The Kerberos Network Authentication
Service (V5)", RFC 1510, September 1993.
[SDP] Handley, M., Jacobson, V., "SDP: Session Description Protocol"
RFC 2327, April 1998.
[STUN] Rosenberg, J., Weinberger, J., Huitema, C., Mahy, R.,
"STUN - Simple Traversal of User Datagram Protocol (UDP)",
RFC 3489, March 2003.
[UNSAF] Daigle, L., Editor, "IAB Considerations for UNilateral Self-
Address Fixing (UNSAF) Across Network Address Translation", RFC
3424, November 2002.
[WEBDAV] Goland, Y., Whitehead, E., Faizi, A., Carter, S., Jensen, D.
"HTTP Extensions for Distributed Authoring -- WEBDAV",
RFC 2518, February 1999.
Security Considerations
This document does not define any protocols and therefore has no
security considerations.
Full Copyright Statement
The IETF takes no position regarding the validity or scope of any
Intellectual Property Rights 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; nor does it represent that it has
made any independent effort to identify any such rights. Information
on the procedures with respect to rights in RFC documents can be
found in BCP 78 and BCP 79.
Copies of IPR disclosures made to the IETF Secretariat 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 implementers or users of this
specification can be obtained from the IETF on-line IPR repository at
Rescorla [Page 20] Internet-Draft Writing Protocol Models 9/2004
http://www.ietf.org/ipr.
The IETF invites any interested party to bring to its attention any
copyrights, patents or patent applications, or other proprietary
rights that may cover technology that may be required to implement
this standard. Please address the information to the IETF at ietf-
ipr@ietf.org.
Copyright Notice
Copyright (C) The Internet Society (2003). This document is subject
to the rights, licenses and restrictions contained in BCP 78, and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
"AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS
OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
ENGINEERING TASK FORCE DISCLAIM 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.
Author's Address
Eric Rescorla <ekr@rtfm.com>
RTFM, Inc.
2064 Edgewood Drive
Palo Alto, CA 94303
Phone: (650)-320-8549
Internet Architecture Board <iab@iab.org>
IAB
Appendix A. IAB Members at the time of this writing
Bernard Aboba
Harald Alvestrand
Rob Austein
Leslie Daigle
Patrik Falstrom
Sally Floyd
Jun-ichiro Itojun Hagino
Mark Handley
Bob Hinden
Geoff Huston
Eric Rescorla
Pete Resnick
Jonathan Rosenberg
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