Network Working Group                                          T. Ylonen
Internet-Draft                                                T. Kivinen
Expires: January 18, 2002               SSH Communications Security Corp
                                                             M. Saarinen
                                                 University of Jyvaskyla
                                                                T. Rinne
                                                             S. Lehtinen
                                        SSH Communications Security Corp
                                                           July 20, 2001

                       SSH Protocol Architecture

Status of this Memo

      This document is an Internet-Draft and is in full conformance with
      all provisions of Section 10 of RFC2026.

      Internet-Drafts are working documents of the Internet Engineering
      Task Force (IETF), its areas, and its working groups.  Note that
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      Internet-Drafts are draft documents valid for a maximum of six
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      as reference material or to cite them other than as "work in

      The list of current Internet-Drafts can be accessed at

      The list of Internet-Draft Shadow Directories can be accessed at

      This Internet-Draft will expire on January 18, 2002.

Copyright Notice

      Copyright (C) The Internet Society (2001).  All Rights Reserved.


      SSH is a protocol for secure remote login and other secure network
      services over an insecure network.  This document describes the
      architecture of the SSH protocol, as well as the notation and
      terminology used in SSH protocol documents.  It also discusses the
      SSH algorithm naming system that allows local extensions.  The SSH

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      protocol consists of three major components: The Transport Layer
      Protocol provides server authentication, confidentiality, and
      integrity with perfect forward secrecy.  The User Authentication
      Protocol authenticates the client to the server.  The Connection
      Protocol multiplexes the encrypted tunnel into several logical
      channels.  Details of these protocols are described in separate

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Specification of Requirements  . . . . . . . . . . . . . . . .  3
   3.  Architecture . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.1 Host Keys  . . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.2 Extensibility  . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.3 Policy Issues  . . . . . . . . . . . . . . . . . . . . . . . .  5
   3.4 Security Properties  . . . . . . . . . . . . . . . . . . . . .  6
   3.5 Packet Size and Overhead . . . . . . . . . . . . . . . . . . .  6
   3.6 Localization and Character Set Support . . . . . . . . . . . .  7
   4.  Data Type Representations Used in the SSH Protocols  . . . . .  8
   5.  Algorithm Naming . . . . . . . . . . . . . . . . . . . . . . . 10
   6.  Message Numbers  . . . . . . . . . . . . . . . . . . . . . . . 11
   7.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 11
   8.  Security Considerations  . . . . . . . . . . . . . . . . . . . 12
   9.  Trademark Issues . . . . . . . . . . . . . . . . . . . . . . . 12
   10. Additional Information . . . . . . . . . . . . . . . . . . . . 12
       References . . . . . . . . . . . . . . . . . . . . . . . . . . 12
       Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 14
       Full Copyright Statement . . . . . . . . . . . . . . . . . . . 15

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   1. Introduction

      SSH is a protocol for secure remote login and other secure network
      services over an insecure network.  It consists of three major
      o  The Transport Layer Protocol [SSH-TRANS] provides server
         authentication, confidentiality, and integrity.  It may
         optionally also provide compression.  The transport layer will
         typically be run over a TCP/IP connection, but might also be
         used on top of any other reliable data stream.
      o  The User Authentication Protocol [SSH-USERAUTH] authenticates
         the client-side user to the server.  It runs over the transport
         layer protocol.
      o  The Connection Protocol [SSH-CONNECT] multiplexes the encrypted
         tunnel into several logical channels.  It runs over the user
         authentication protocol.

      The client sends a service request once a secure transport layer
      connection has been established.  A second service request is sent
      after user authentication is complete.  This allows new protocols
      to be defined and coexist with the protocols listed above.

      The connection protocol provides channels that can be used for a
      wide range of purposes.  Standard methods are provided for setting
      up secure interactive shell sessions and for forwarding
      ("tunneling") arbitrary TCP/IP ports and X11 connections.

   2. Specification of Requirements

      All documents related to the SSH protocols shall use the keywords
      "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" to describe
      requirements.  They are to be interpreted as described in [RFC-

   3. Architecture

   3.1 Host Keys

      Each server host SHOULD have a host key.  Hosts MAY have multiple
      host keys using multiple different algorithms.  Multiple hosts MAY
      share the same host key.  If a host has keys at all, it MUST have
      at least one key using each REQUIRED public key algorithm
      (currently DSS [FIPS-186]).

      The server host key is used during key exchange to verify that the
      client is really talking to the correct server.  For this to be
      possible, the client must have a priori knowledge of the server's

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      public host key.

      Two different trust models can be used:
      o  The client has a local database that associates each host name
         (as typed by the user) with the corresponding public host key.
         This method requires no centrally administered infrastructure,
         and no third-party coordination.  The downside is that the
         database of name-to-key associations may become burdensome to
      o  The host name-to-key association is certified by some trusted
         certification authority.  The client only knows the CA root
         key, and can verify the validity of all host keys certified by
         accepted CAs.

         The second alternative eases the maintenance problem, since
         ideally only a single CA key needs to be securely stored on the
         client.  On the other hand, each host key must be appropriately
         certified by a central authority before authorization is
         possible.  Also, a lot of trust is placed on the central

      The protocol provides the option that the server name - host key
      association is not checked when connecting to the host for the
      first time.  This allows communication without prior communication
      of host keys or certification.  The connection still provides
      protection against passive listening; however, it becomes
      vulnerable to active man-in-the-middle attacks.  Implementations
      SHOULD NOT normally allow such connections by default, as they
      pose a potential security problem.  However, as there is no widely
      deployed key infrastructure available on the Internet yet, this
      option makes the protocol much more usable during the transition
      time until such an infrastructure emerges, while still providing a
      much higher level of security than that offered by older solutions
      (e.g.  telnet [RFC-854] and rlogin [RFC-1282]).

      Implementations SHOULD try to make the best effort to check host
      keys.  An example of a possible strategy is to only accept a host
      key without checking the first time a host is connected, save the
      key in a local database, and compare against that key on all
      future connections to that host.

      Implementations MAY provide additional methods for verifying the
      correctness of host keys, e.g.  a hexadecimal fingerprint derived
      from the SHA-1 hash of the public key.  Such fingerprints can
      easily be verified by using telephone or other external
      communication channels.

      All implementations SHOULD provide an option to not accept host

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      keys that cannot be verified.

      We believe that ease of use is critical to end-user acceptance of
      security solutions, and no improvement in security is gained if
      the new solutions are not used.  Thus, providing the option not to
      check the server host key is believed to improve the overall
      security of the Internet, even though it reduces the security of
      the protocol in configurations where it is allowed.

   3.2 Extensibility

      We believe that the protocol will evolve over time, and some
      organizations will want to use their own encryption,
      authentication and/or key exchange methods.  Central registration
      of all extensions is cumbersome, especially for experimental or
      classified features.  On the other hand, having no central
      registration leads to conflicts in method identifiers, making
      interoperability difficult.

      We have chosen to identify algorithms, methods, formats, and
      extension protocols with textual names that are of a specific
      format.  DNS names are used to create local namespaces where
      experimental or classified extensions can be defined without fear
      of conflicts with other implementations.

      One design goal has been to keep the base protocol as simple as
      possible, and to require as few algorithms as possible.  However,
      all implementations MUST support a minimal set of algorithms to
      ensure interoperability (this does not imply that the local policy
      on all hosts would necessary allow these algorithms).  The
      mandatory algorithms are specified in the relevant protocol

      Additional algorithms, methods, formats, and extension protocols
      can be defined in separate drafts.  See Section Algorithm Naming
      (Section 5) for more information.

   3.3 Policy Issues

      The protocol allows full negotiation of encryption, integrity, key
      exchange, compression, and public key algorithms and formats.
      Encryption, integrity, public key, and compression algorithms can
      be different for each direction.

      The following policy issues SHOULD be addressed in the
      configuration mechanisms of each implementation:
      o  Encryption, integrity, and compression algorithms, separately
         for each direction.  The policy MUST specify which is the

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         preferred algorithm (e.g.  the first algorithm listed in each
      o  Public key algorithms and key exchange method to be used for
         host authentication.  The existence of trusted host keys for
         different public key algorithms also affects this choice.
      o  The authentication methods that are to be required by the
         server for each user.  The server's policy MAY require multiple
         authentication for some or all users.  The required algorithms
         MAY depend on the location where the user is trying to log in
      o  The operations that the user is allowed to perform using the
         connection protocol.  Some issues are related to security; for
         example, the policy SHOULD NOT allow the server to start
         sessions or run commands on the client machine, and MUST NOT
         allow connections to the authentication agent unless forwarding
         such connections has been requested.  Other issues, such as
         which TCP/IP ports can be forwarded and by whom, are clearly
         issues of local policy.  Many of these issues may involve
         traversing or bypassing firewalls, and are interrelated with
         the local security policy.

   3.4 Security Properties

      The primary goal of the SSH protocol is improved security on the
      Internet.  It attempts to do this in a way that is easy to deploy,
      even at the cost of absolute security.
      o  All encryption, integrity, and public key algorithms used are
         well-known, well-established algorithms.
      o  All algorithms are used with cryptographically sound key sizes
         that are believed to provide protection against even the
         strongest cryptanalytic attacks for decades.
      o  All algorithms are negotiated, and in case some algorithm is
         broken, it is easy to switch to some other algorithm without
         modifying the base protocol.

      Specific concessions were made to make wide-spread fast deployment
      easier.  The particular case where this comes up is verifying that
      the server host key really belongs to the desired host; the
      protocol allows the verification to be left out (but this is NOT
      RECOMMENDED).  This is believed to significantly improve usability
      in the short term, until widespread Internet public key
      infrastructures emerge.

   3.5 Packet Size and Overhead

      Some readers will worry about the increase in packet size due to
      new headers, padding, and MAC.  The minimum packet size is in the
      order of 28 bytes (depending on negotiated algorithms).  The

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      increase is negligible for large packets, but very significant for
      one-byte packets (telnet-type sessions).  There are, however,
      several factors that make this a non-issue in almost all cases:
      o  The minimum size of a TCP/IP header is 32 bytes.  Thus, the
         increase is actually from 33 to 51 bytes (roughly).
      o  The minimum size of the data field of an Ethernet packet is 46
         bytes [RFC-894].  Thus, the increase is no more than 5 bytes.
         When Ethernet headers are considered, the increase is less than
         10 percent.
      o  The total fraction of telnet-type data in the Internet is
         negligible, even with increased packet sizes.

      The only environment where the packet size increase is likely to
      have a significant effect is PPP [RFC-1134] over slow modem lines
      (PPP compresses the TCP/IP headers, emphasizing the increase in
      packet size).  However, with modern modems, the time needed to
      transfer is in the order of 2 milliseconds, which is a lot faster
      than people can type.

      There are also issues related to the maximum packet size.  To
      minimize delays in screen updates, one does not want excessively
      large packets for interactive sessions.  The maximum packet size
      is negotiated separately for each channel.

   3.6 Localization and Character Set Support

      For the most part, the SSH protocols do not directly pass text
      that would be displayed to the user.  However, there are some
      places where such data might be passed.  When applicable, the
      character set for the data MUST be explicitly specified.  In most
      places, ISO 10646 with UTF-8 encoding is used [RFC-2279].  When
      applicable, a field is also provided for a language tag [RFC-

      One big issue is the character set of the interactive session.
      There is no clear solution, as different applications may display
      data in different formats.  Different types of terminal emulation
      may also be employed in the client, and the character set to be
      used is effectively determined by the terminal emulation.  Thus,
      no place is provided for directly specifying the character set or
      encoding for terminal session data.  However, the terminal
      emulation type (e.g.  "vt100") is transmitted to the remote site,
      and it implicitly specifies the character set and encoding.
      Applications typically use the terminal type to determine what
      character set they use, or the character set is determined using
      some external means.  The terminal emulation may also allow
      configuring the default character set.  In any case, the character
      set for the terminal session is considered primarily a client

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      local issue.

      Internal names used to identify algorithms or protocols are
      normally never displayed to users, and must be in US-ASCII.

      The client and server user names are inherently constrained by
      what the server is prepared to accept.  They might, however,
      occasionally be displayed in logs, reports, etc.  They MUST be
      encoded using ISO 10646 UTF-8, but other encodings may be required
      in some cases.  It is up to the server to decide how to map user
      names to accepted user names.  Straight bit-wise binary comparison

      For localization purposes, the protocol attempts to minimize the
      number of textual messages transmitted.  When present, such
      messages typically relate to errors, debugging information, or
      some externally configured data.  For data that is normally
      displayed, it SHOULD be possible to fetch a localized message
      instead of the transmitted message by using a numerical code.  The
      remaining messages SHOULD be configurable.

   4. Data Type Representations Used in the SSH Protocols

         A byte represents an arbitrary 8-bit value (octet) [RFC-1700].
         Fixed length data is sometimes represented as an array of
         bytes, written byte[n], where n is the number of bytes in the

         A boolean value is stored as a single byte.  The value 0
         represents FALSE, and the value 1 represents TRUE.  All non-
         zero values MUST be interpreted as TRUE; however, applications
         MUST NOT store values other than 0 and 1.

         Represents a 32-bit unsigned integer.  Stored as four bytes in
         the order of decreasing significance (network byte order).  For
         example, the value 699921578 (0x29b7f4aa) is stored as 29 b7 f4

         Represents a 64-bit unsigned integer.  Stored as eight bytes in
         the order of decreasing significance (network byte order).

         Arbitrary length binary string.  Strings are allowed to contain
         arbitrary binary data, including null characters and 8-bit

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         characters.  They are stored as a uint32 containing its length
         (number of bytes that follow) and zero (= empty string) or more
         bytes that are the value of the string.  Terminating null
         characters are not used.

         Strings are also used to store text.  In that case, US-ASCII is
         used for internal names, and ISO-10646 UTF-8 for text that
         might be displayed to the user.  The terminating null character
         SHOULD NOT normally be stored in the string.

         For example, the US-ASCII string "testing" is represented as 00
         00 00 07 t e s t i n g.  The UTF8 mapping does not alter the
         encoding of US-ASCII characters.

         Represents multiple precision integers in two's complement
         format, stored as a string, 8 bits per byte, MSB first.
         Negative numbers have the value 1 as the most significant bit
         of the first byte of the data partition.  If the most
         significant bit would be set for a positive number, the number
         MUST be preceded by a zero byte.  Unnecessary leading bytes
         with the value 0 or 255 MUST NOT be included.  The value zero
         MUST be stored as a string with zero bytes of data.

         By convention, a number that is used in modular computations in
         Z_n SHOULD be represented in the range 0 &lt= x &lt n.

       value (hex)        representation (hex)
       0                  00 00 00 00
       9a378f9b2e332a7    00 00 00 08 09 a3 78 f9 b2 e3 32 a7
       80                 00 00 00 02 00 80
       -1234              00 00 00 02 ed cc
       -deadbeef          00 00 00 05 ff 21 52 41 11


         A string containing a comma separated list of names.  A name
         list is represented as a uint32 containing its length (number
         of bytes that follow) followed by a comma-separated list of
         zero or more names.  A name MUST be non-zero length, and it
         MUST NOT contain a comma (',').  Context may impose additional
         restrictions on the names; for example, the names in a list may
         have to be valid algorithm identifier (see Algorithm Naming
         below), or [RFC-1766] language tags.  The order of the names in

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         a list may or may not be significant, also depending on the
         context where the list is is used.  Terminating NUL characters
         are not used, neither for the individual names, nor for the
         list as a whole.

       value              representation (hex)
       (), the empty list 00 00 00 00
       ("zlib")           00 00 00 04 7a 6c 69 62
       ("zlib", "none")   00 00 00 09 7a 6c 69 62 2c 6e 6f 6e 65

   5. Algorithm Naming

      The SSH protocols refer to particular hash, encryption, integrity,
      compression, and key exchange algorithms or protocols by names.
      There are some standard algorithms that all implementations MUST
      support.  There are also algorithms that are defined in the
      protocol specification but are OPTIONAL.  Furthermore, it is
      expected that some organizations will want to use their own

      In this protocol, all algorithm identifiers MUST be printable US-
      ASCII non-empty strings no longer than 64 characters.  Names MUST
      be case-sensitive.

      There are two formats for algorithm names:
      o  Names that do not contain an at-sign (@) are reserved to be
         assigned by IETF consensus (RFCs).  Examples include `3des-
         cbc', `sha-1', `hmac-sha1', and `zlib' (the quotes are not part
         of the name).  Names of this format MUST NOT be used without
         first registering them.  Registered names MUST NOT contain an
         at-sign (@) or a comma (,).
      o  Anyone can define additional algorithms by using names in the
         format name@domainname, e.g.  "".  The
         format of the part preceding the at sign is not specified; it
         MUST consist of US-ASCII characters except at-sign and comma.
         The part following the at-sign MUST be a valid fully qualified
         internet domain name [RFC-1034] controlled by the person or
         organization defining the name.  It is up to each domain how it
         manages its local namespace.

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   6. Message Numbers

      SSH packets have message numbers in the range 1 to 255.  These
      numbers have been allocated as follows:

     Transport layer protocol:

       1 to 19    Transport layer generic (e.g. disconnect, ignore, debug,
       20 to 29   Algorithm negotiation
       30 to 49   Key exchange method specific (numbers can be reused for
                  different authentication methods)

     User authentication protocol:

       50 to 59   User authentication generic
       60 to 79   User authentication method specific (numbers can be
                  reused for different authentication methods)

     Connection protocol:

       80 to 89   Connection protocol generic
       90 to 127  Channel related messages

     Reserved for client protocols:

       128 to 191 Reserved

     Local extensions:

       192 to 255 Local extensions

   7. IANA Considerations

      Allocation of the following types of names in the SSH protocols is
      assigned by IETF consensus:
      o  encryption algorithm names,
      o  MAC algorithm names,
      o  public key algorithm names (public key algorithm also implies
         encoding and signature/encryption capability),
      o  key exchange method names, and
      o  protocol (service) names.

      These names MUST be printable US-ASCII strings, and MUST NOT
      contain the characters at-sign ('@'), comma (','), or whitespace

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      or control characters (ASCII codes 32 or less).  Names are case-
      sensitive, and MUST NOT be longer than 64 characters.

      Names with the at-sign ('@') in them are allocated by the owner of
      DNS name after the at-sign (hierarchical allocation in [RFC-
      2343]), otherwise the same restrictions as above.

      Each category of names listed above has a separate namespace.
      However, using the same name in multiple categories SHOULD be
      avoided to minimize confusion.

      Message numbers (see Section Message Numbers (Section 6)) in the
      range of 0..191 should be allocated via IETF consensus; message
      numbers in the 192..255 range (the "Local extensions" set) are
      reserved for private use.

   8. Security Considerations

      Special care should be taken to ensure that all of the random
      numbers are of good quality.  The random numbers SHOULD be
      produced with safe mechanisms discussed in [RFC-1750].

      When displaying text, such as error or debug messages to the user,
      the client software SHOULD replace any control characters (except
      tab, carriage return and newline) with safe sequences to avoid
      attacks by sending terminal control characters.

      Not using MAC or encryption SHOULD be avoided.  The user
      authentication protocol is subject to man-in-the-middle attacks if
      the encryption is disabled.  The SSH protocol does not protect
      against message alteration if no MAC is used.

   9. Trademark Issues

      As of this writing, SSH Communications Security Oy claims ssh as
      its trademark.  As with all IPR claims the IETF takes no position
      regarding the validity or scope of this trademark claim.

   10. Additional Information

      The current document editor is: Darren.Moffat@Sun.COM.  Comments
      on this internet draft should be sent to the IETF SECSH working
      group, details at:


      [FIPS-186]      Federal Information Processing Standards

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                      Publication, ., "FIPS PUB 186, Digital Signature
                      Standard", May 1994.

      [RFC0854]       Postel, J. and J. Reynolds, "Telnet Protocol
                      Specification", STD 8, RFC 854, May 1983.

      [RFC0894]       Hornig, C., "Standard for the transmission of IP
                      datagrams over Ethernet networks", STD 41, RFC
                      894, Apr 1984.

      [RFC1034]       Mockapetris, P., "Domain names - concepts and
                      facilities", STD 13, RFC 1034, Nov 1987.

      [RFC1134]       Perkins, D., "Point-to-Point Protocol: A proposal
                      for multi-protocol transmission of datagrams over
                      Point-to-Point links", RFC 1134, Nov 1989.

      [RFC1282]       Kantor, B., "BSD Rlogin", RFC 1282, December 1991.

      [RFC1700]       Reynolds, J. and J. Postel, "Assigned Numbers",
                      STD 2, RFC 1700, October 1994.

      [RFC1750]       Eastlake, D., Crocker, S. and J. Schiller,
                      "Randomness Recommendations for Security", RFC
                      1750, December 1994.

      [RFC1766]       Alvestrand, H., "Tags for the Identification of
                      Languages", RFC 1766, March 1995.

      [RFC2119]       Bradner, S., "Key words for use in RFCs to
                      Indicate Requirement Levels", BCP 14, RFC 2119,
                      March 1997.

      [RFC2279]       Yergeau, F., "UTF-8, a transformation format of
                      ISO 10646", RFC 2279, January 1998.

      [RFC2434]       Narten, T. and H. Alvestrand, "Guidelines for
                      Writing an IANA Considerations Section in RFCs",
                      BCP 26, RFC 2434, October 1998.

      [SSH-ARCH]      Ylonen, T., "SSH Protocol Architecture", I-D
                      draft-ietf-architecture-09.txt, July 2001.

      [SSH-TRANS]     Ylonen, T., "SSH Transport Layer Protocol", I-D
                      draft-ietf-transport-11.txt, July 2001.

      [SSH-USERAUTH]  Ylonen, T., "SSH Authentication Protocol", I-D
                      draft-ietf-userauth-11.txt, July 2001.

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      [SSH-CONNECT]   Ylonen, T., "SSH Connection Protocol", I-D draft-
                      ietf-connect-11.txt, July 2001.

Authors' Addresses

   Tatu Ylonen
   SSH Communications Security Corp
   Fredrikinkatu 42
   HELSINKI  FIN-00100


   Tero Kivinen
   SSH Communications Security Corp
   Fredrikinkatu 42
   HELSINKI  FIN-00100


   Markku-Juhani O. Saarinen
   University of Jyvaskyla

   Timo J. Rinne
   SSH Communications Security Corp
   Fredrikinkatu 42
   HELSINKI  FIN-00100


   Sami Lehtinen
   SSH Communications Security Corp
   Fredrikinkatu 42
   HELSINKI  FIN-00100


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Full Copyright Statement

      Copyright (C) The Internet Society (2001).  All Rights Reserved.

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      explain it or assist in its implementation may be prepared,
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      restriction of any kind, provided that the above copyright notice
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      The limited permissions granted above are perpetual and will not
      be revoked by the Internet Society or its successors or assigns.

      This document and the information contained herein is provided on


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

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