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Network Working Group                                        E. RescorlaRequest for Comments: 4101                                    RTFM, Inc.Category: Informational                                              IAB                                                               June 2005Writing Protocol ModelsStatus 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.Rescorla                     Informational                      [Page 1]

RFC 4101                Writing Protocol Models                June 2005   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.Rescorla                     Informational                      [Page 2]

RFC 4101                Writing Protocol Models                June 20053.2.  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 majorRescorla                     Informational                      [Page 3]

RFC 4101                Writing Protocol Models                June 2005   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.Rescorla                     Informational                      [Page 4]

RFC 4101                Writing Protocol Models                June 2005                   +-----------+                   |    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.Rescorla                     Informational                      [Page 5]

RFC 4101                Writing Protocol Models                June 2005                               +-----------+                               |   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.Rescorla                     Informational                      [Page 6]

RFC 4101                Writing Protocol Models                June 20054.2.1.  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. Termination4.2.1.1.  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 negotiationRescorla                     Informational                      [Page 7]

RFC 4101                Writing Protocol Models                June 2005   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 *Rescorla                     Informational                      [Page 8]

RFC 4101                Writing Protocol Models                June 2005   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.Rescorla                     Informational                      [Page 9]

RFC 4101                Writing Protocol Models                June 20054.2.1.4.  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.Rescorla                     Informational                     [Page 10]

RFC 4101                Writing Protocol Models                June 2005   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.Rescorla                     Informational                     [Page 11]

RFC 4101                Writing Protocol Models                June 20056.1.  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).Rescorla                     Informational                     [Page 12]

RFC 4101                Writing Protocol Models                June 20056.1.2.  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.Rescorla                     Informational                     [Page 13]

RFC 4101                Writing Protocol Models                June 20056.2.  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.Rescorla                     Informational                     [Page 14]

RFC 4101                Writing Protocol Models                June 20056.2.1.  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.Rescorla                     Informational                     [Page 15]

RFC 4101                Writing Protocol Models                June 2005   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)Rescorla                     Informational                     [Page 16]

RFC 4101                Writing Protocol Models                June 2005   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 protection6.2.2.  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.Rescorla                     Informational                     [Page 17]

RFC 4101                Writing Protocol Models                June 2005      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.Rescorla                     Informational                     [Page 18]

RFC 4101                Writing Protocol Models                June 20056.3.2.2.  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.Rescorla                     Informational                     [Page 19]

RFC 4101                Writing Protocol Models                June 2005A.  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 RosenbergNormative References   There are no normative references for this document.Informative References   [AH]       Kent, S., and R. Atkinson, "IP Authentication Header",RFC2402, 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.Rescorla                     Informational                     [Page 20]

RFC 4101                Writing Protocol Models                June 2005   [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.orgRescorla                     Informational                     [Page 21]

RFC 4101                Writing Protocol Models                June 2005Full Copyright Statement   Copyright (C) The Internet Society (2005).   This document is subject to the rights, licenses and restrictions   contained inBCP 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.Intellectual Property   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 inBCP 78 andBCP 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 athttp://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.Acknowledgement   Funding for the RFC Editor function is currently provided by the   Internet Society.Rescorla                     Informational                     [Page 22]