Network Working Group E. Rescorla
Request for Comments: 4101 RTFM, Inc.
Category: Informational IAB
June 2005
Writing Protocol Models
Status of This Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2005).
Abstract
The IETF process depends on peer review. However, IETF documents are
generally written to be useful for implementors, not 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.
1. Introduction
The IETF process depends on peer review. However, in many cases, the
documents submitted for publication are extremely difficult to
review. Because reviewers have only limited amounts of time, this
leads to extremely long review times, inadequate reviews, or both.
In our 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 it
dramatically increases the strain on reviewers. Reviewers 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.
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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 unobvious, 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. Of course, this doesn't 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 [KERBEROS]. By contrast, 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. Because we're interested in
ensuring that people get the big picture, we have to dispense with a
lot of detail. That's good, not bad. The simpler you can make
things the better.
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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 help
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 an important point.
4. Writing Protocol Models
Our experience indicates that it is easiest to grasp protocol models
when they are presented in visual form. We recommend a presentation
format centered around a few key diagrams, with explanatory text for
each. These diagrams should be simple and typically 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 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, an experienced reviewer
will usually have an idea of how they would approach the problem and,
thus, be able to compare that approach with the approach taken by the
protocol under review.
The "Problem" section of the model should start 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 would participate in the protocol.
The Problem section should feature a diagram of the major
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communicating parties and their inter-relationships. It is
particularly important to lay out the trust relationships between the
various parties, as these are often unobvious.
4.1.1. Example: STUN (RFC 3489)
STUN [STUN] is a UNilateral Self-Address Fixing (UNSAF) [UNSAF]
protocol that allows a machine located behind a NAT to determine what
its external apparent IP address is. 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
particularly a 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 the address that the application knows about is not
the same as the one on which it is reachable.
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. The NAT 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.
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+-----------+
| 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 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 using
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 the problem has been described, the next task is to give a broad
overview of the protocol. 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 not to provide a complete
description of the protocol, but to provide enough of a
map 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
understanding.
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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. Therefore, it is 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. Since the review, the DCCP authors have added
some overview material, some of which derives from earlier versions
of this document.
Although 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, as they are
in TCP or UDP common practice. It also starts feature negotiation.
For pedagogical reasons, we will present feature negotiation
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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 of 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 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:
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 *
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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
that are initiated by the feature location. In the following
example, the server requests that CCID/Server be set to 3 or 2 (with
3 being 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 that
want maximum throughput. CCID-3 is intended for real-time
applications that 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 difference
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.
4.2.1.3.2. CCID-3
CCID-3 is an equation-based form of rate control, intended to provide
smoother response to congestion than CCID-2. The sender maintains a
"transmit rate". The receiver sends Ack packets that 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 than TCP in its short-term congestion
response, it is designed to operate fairly with TCP over the long
term.
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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, keep 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 that 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 from protocol
messages.
4.3.1. Example: WebDAV COPY and MOVE
The WebDAV standard [WEBDAV] is fairly terse, preferring to define
the required behaviors and let the reader work out the implications.
In some situations, explanatory material that details those
implications can help the reader understand the overall model. The
rest of this section describes one such case.
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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 substitute for MOVE is problematic
because of the creation of the intermediate file. Consider the case
where the user is approaching a 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 understand abstractly,
apart from the protocol details. The remainder of this section
provides overview of IKE suitable for those new readers.
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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 above. The IPsec
engines and IKE engines typically are separate modules. When no
security association exists for a packet that needs to be processed
(either sent or received), 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.
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).
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6.1.2. Perfect Forward Secrecy
One of the major concerns in IKE design was that traffic be protected
even if the 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
Because IKE allows arbitrary peers to initiate computationally-
expensive cryptographic operations, it potentially allows resource
consumption denial of service (DoS) 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.
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6.2. Protocol Overview
At a very high level, there are two kinds of IKE handshake:
(1) Those that establish an IKE security association.
(2) Those that establish an AH or ESP security association.
When two peers that 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 for establishing 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.
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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 Stage 1 handshakes 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.
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 (DH) 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, therefore, 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, PFS will be provided.
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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 it 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 does not
totally eliminate DoS attacks, because 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.
Because 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 identity protection or
protection against DoS.
Initiator Responder
--------- ---------
Algorithms, Nonce,
Key Exchange, ->
<- Algorithms, Nonce,
Key Exchange, Auth Data
Auth Data ->
IKE Aggressive Mode Handshake (Stage 1)
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After the first round trip, the peers have all the required
properties, but 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 because the Responder does the
DH key agreement without a round trip to the Initiator, 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
merely 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
that 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 preexisting 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. They 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, as suggested by Phil Karn, is to have 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 kinds 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, or a
certificate.
6.3.2.1. Shared Key
In a shared key scheme, the peers share a 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 a table
that indicates for which traffic (i.e., what addresses) that identity
is allowed to negotiate SAs.
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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, the authenticating party presents a
certificate containing their public key. It is straightforward to
establish that this certificate matches the authentication data
provided by the peer. What is 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, is not really
interoperable. Currently, it is likely that a configuration table
maps certificates to policies, as in the other two authentication
schemes.
7. Security Considerations
This document does not define any protocols and therefore has no
security issues.
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RFC 4101 Writing Protocol Models June 2005
A. Appendix: 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
Normative References
There are no normative references for this document.
Informative References
[AH] Kent, S., and R. Atkinson, "IP Authentication Header", RFC
2402, November 1998.
[CCID2] Floyd, S. and E. Kohler, "Profile for DCCP Congestion
Control ID 2: TCP-like Congestion Control", Work in
Progress, October 2003.
[CCID3] Floyd, S., Kohler, E., and J. Padhye, "Profile for DCCP
Congestion Control ID 3: TFRC Congestion Control", Work in
Progress, February 2004.
[DCCP] Kohler, E., Handley, M., and S. Floyd, "Datagram
Congestion Control Protocol (DCCP)", Work in Progress,
November 2004.
[ECN] Ramakrishnan, K. Floyd, S., and D. Black, "The Addition of
Explicit Congestion Notification (ECN) to IP", RFC 3168,
September 2001.
[ESP] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.
[IKE] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
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[IPSEC] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[KERBEROS] Kohl, J. and C. Neuman, "The Kerberos Network
Authentication Service (V5)", RFC 1510, September 1993.
[SDP] Handley, M. and V. Jacobson, "SDP: Session Description
Protocol" RFC 2327, April 1998.
[STUN] Rosenberg, J., Weinberger, J., Huitema, C., and R. Mahy,
"STUN - Simple Traversal of User Datagram Protocol (UDP)",
RFC 3489, March 2003.
[SYNFLOOD] CERT Advisory CA-1996-21 TCP SYN Flooding and IP Spoofing
Attacks <http://www.cert.org/advisories/CA-1996-21.html>,
September 19, 1996.
[UNSAF] Daigle, L. and IAB, "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., and D.
Jensen, "HTTP Extensions for Distributed Authoring --
WEBDAV", RFC 2518, February 1999.
Authors' Addresses
Eric Rescorla
RTFM, Inc.
2064 Edgewood Drive
Palo Alto, CA 94303
Phone: (650)-320-8549
EMail: ekr@rtfm.com
Internet Architecture Board
IAB
EMail: iab@iab.org
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