Network Working Group                                          S. Venaas
Internet-Draft                                                   UNINETT
Intended status: Informational                        September 18, Standards Track                        November 3, 2008
Expires: March 22, May 7, 2009

                        Multicast Ping Protocol
                      draft-ietf-mboned-ssmping-05
                      draft-ietf-mboned-ssmping-06

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Abstract

   The Multicast Ping Protocol specified in this document allows for
   checking whether an endpoint can receive multicast, both Source-
   Specific Multicast (SSM) and Any-Source Multicast (ASM).  It can also
   be used to obtain additional multicast related information like
   multicast tree setup time etc.  This protocol is based on an
   implementation of tools called ssmping and asmping.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Architecture . . . . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Protocol specification . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Option format  . . . . . . . . . . . . . . . . . . . . . .  5
     3.2.  Defined Options  . . . . . . . . . . . . . . . . . . . . .  6
   4.  Packet Format  . . . . . . . . . . . . . . . . . . . . . . . . 10
   5.  Message types and options  . . . . . . . . . . . . . . . . . . 11
   6.  Client Behaviour . . . . . . . . . . . . . . . . . . . . . . . 12 13
   7.  Server Behaviour . . . . . . . . . . . . . . . . . . . . . . . 14
   8.  Recommendations for Implementers . . . . . . . . . . . . . . . 15
   9.  Acknowledgements  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . . 16
   10. IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 16
   11. Security Considerations  . . . . . . . . . . . . . . . . . . . 16 17
   12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17 18
     12.1. Normative References . . . . . . . . . . . . . . . . . . . 17 18
     12.2. Informative References . . . . . . . . . . . . . . . . . . 17 18
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17 18
   Intellectual Property and Copyright Statements . . . . . . . . . . 18 20

1.  Introduction

   The Multicast Ping Protocol specified in this document allows for
   checking multicast connectivity.  In addition to checking reception
   of multicast (SSM or ASM), the protocol can provide related
   information like multicast tree setup time, the number of hops the
   packets have traveled, as well as packet delay and loss.  This
   functionality resembles, in part, the ICMP Echo Request/Reply
   mechanism, but uses UDP (RFC RFC 768 [2] and RFC 2460 [3]) and requires both a client and a
   server implementing this protocol.  Intermediate routers are not
   required to support this protocol.  They forward Protocol Messages
   and data traffic as usual.

   The protocol here specified is based on the actual implementation of
   the ssmping and asmping tools [5] [4] which are widely used by the
   Internet community to conduct multicast connectivity tests.

2.  Architecture

   Before describing the protocol in detail, we provide a brief overview
   of how the protocol may be used and what information it may provide.
   The typical protocol usage is as follows: A server runs continuously
   to serve requests from clients.  A client can test the multicast
   reception from this server, provided it knows a unicast address of
   the server.  It will then send a unicast message to the server asking
   for a group to use.  Optionally the user may have requested a
   specific group or scope, in which case the client will ask for a
   group matching the user's request.  The server will respond with a
   group to use, or an error if no group is available.  Next, for ASM,
   the client joins an ASM group G, while for SSM it joins a channel
   (S,G).  Here G is the group specified by the server, and S is the
   unicast address used to reach the server.

   After joining the channel, the client unicasts multicast ping
   requests to the server.  The requests are sent using UDP with
   destination port set to the standardised multicast ping port [TBD].
   The requests are sent periodically, e.g., once per second, to the
   server.  The requests contain a sequence number, and typically a
   timestamp.  The requests are echoed by the server, except the server
   may add a few options.  For each request, the server sends two
   replies.  One reply is unicast back to the source IP address and
   source UDP port of the request, while another is multicast to the
   requested multicast group G and the source UDP port of the request.
   Both replies are sent from the same port on which the request was
   received.  The server should specify the TTL used for both the
   unicast and multicast messages (we recommend at least 64) by
   including a TTL option; allowing the client to compute the number of
   hops.  The client should leave the channel/group when it has finished
   its measurements.

   By use of this protocol, a client can obtain information about
   several multicast delivery characteristics.  First, by receiving
   unicast replies, it can verify that the server is receiving the
   unicast requests, is operational and responding.  Hence, provided
   that the client receives unicast replies, a failure to receive
   multicast indicates either a multicast problem or a multicast
   administrative restriction.  If it does receive multicast, it knows
   not only that it can receive; it may also estimate the amount of time
   it took to establish the multicast tree (at least if it is in the
   range of seconds), whether there are packet drops, and the length and
   variation of Round Trip Times (RTT).  For unicast, the RTT is the
   time from when the unicast request is sent to when the reply is
   received.  The measured multicast RTT also references the client's
   unicast request.  By use of the TTL option specifying the TTL of the
   replies when they are originated, the client can also determine the
   number of router hops it is from the source.  Since similar
   information is obtained in the unicast replies, the host may compare
   its multicast and unicast results and is able to check for
   differences in the number of hops, RTT, etc.  The number of multicast
   hops and changes in the number of hops over time, may also reveal
   details about the multicast tree and multicast tree changes.  E.g.,
   with PIM-SM one may be able to tell whether the forwarding is on a
   shared or source-specific tree and when an eventual switch occurs.
   Provided that the server sends the unicast and multicast replies
   nearly simultaneously, the client may also be able to measure the
   difference in one way delay for unicast and multicast on the path
   from server to client, and also differences in delay.  Servers may
   optionally specify a timestamp.  This may be useful since the unicast
   and multicast replies can not be sent simultaneously (the delay
   depending on the host's operating system and load).

3.  Protocol specification

   There are four different message types.  There are Echo Request and
   Echo Reply messages used for the actual measurements; there is an
   Init message that SHOULD be used to initialise a ping session and
   negotiate which group to use; and finally a Server Response message
   that is mainly used in response to the Init message.  The messages
   MUST always be in network byte order.  UDP checksums MUST always be
   used.

   The messages share a common format: one octet specifying the message
   type, followed by a number of options in TLV (Type, Length and Value)
   format.  This makes the protocol easily extendible.  The Init message
   generally contains some prefix options asking the server for a group
   from one of the specified prefixes.  The server responds with a
   Server Response message that contains the group address to use, or
   possibly prefix options describing what multicast groups the server
   may be able to provide.  For an Echo Request the client generally
   includes a number of options, and a server MAY simply echo the
   contents (only changing the message type) without inspecting the
   options if it does not support any options.  This might be true for a
   simple Multicast Ping Protocol server.  However, the server SHOULD
   add a TTL option, and there are other options that a server
   implementation MAY support, e.g., the client may ask for certain
   information or a specific behaviour from the server.  The Echo
   Replies (one unicast and one multicast) MUST first contain the exact
   options from the request (in the same order), and then, immediately
   following, any options appended by the server.  A server MUST NOT
   process unknown options, but they MUST still be included in the Echo
   Reply.  A client MUST ignore any unknown options.

   The size of the protocol messages is generally smaller than the Path
   MTU and fragmentation is not a concern.  There may however be cases
   where the Path MTU is really small, or that a client sends large
   requests in order to verify that it can receive fragmented multicast
   datagrams.  This document does not specify whether Path MTU Discovery
   should be performed, etc.  A possible extension could be an option
   where a client requests Path MTU Discovery and receives the current
   Path MTU from the server.

   This document defines a number of different options.  Some options do
   not require processing by servers and are simply returned unmodified
   in the reply.  There are, however, other client options that the
   server may care about, and also server options that may be requested
   by a client.  Unless otherwise specified, an option MUST NOT be used
   multiple times in the same message.

3.1.  Option format

   All options are TLVs formatted as specified below.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |             Type              |           Length              |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                             Value                             |
      |                               .                               |
      |                               .                               |
      |                               .                               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Type (2 octets) specifies the option.  The different options are
   defined below.

   Length (2 octets) specifies the length of the value field.  Depending
   on the option type, it can be from 0 to 65535.

   Value.  The value must always be of the specified length.  See the
   respective option definitions for possible values.  If the length is
   0, the value field is not included.

3.2.  Defined Options

   This document defines the following options: Version (0), Client ID
   (1), Sequence Number (2), Client Timestamp (3), Multicast Group (4),
   Option Request Option (5), Server Information (6), TTL (9), Multicast
   Prefix (10), Session ID (11) and Server Timestamp (12).  Values 7 and
   8 are reserved.  The options are defined below.

      Version, type 0

         Length MUST be 1.  This option MUST always be included in all
         messages, and the value MUST be set to 2 (in decimal).  Note
         that there are older implementations of ssmping older revisions of this
         protocol that only partly follow this specification.  They can
         be regarded as version 1 and do not use this option.  If a
         server receives a message with a version other than 2 (or
         missing), the server SHOULD (unless it supports the particular
         version) send a Response message back with version set to 2.
         Client ID and Sequence Number options SHOULD be echoed if
         present.  It SHOULD not include any other options.  A client
         receiving a response with a version other than 2, MUST (unless
         it supports the particular version), stop sending requests to
         the server.

      Client ID, type 1

         Length MUST be non-zero.  A client SHOULD always include this
         option in all messages (both Init and Request).  The client may
         use any value it likes to be able to detect whether a reply is
         a reply to its Init/Request or not.  A server should treat this
         as opaque data, and MUST echo this option back in the reply if
         present (both Server Response and Reply).  The value might be a
         process ID, perhaps process ID combined with an IP address
         because it may receive multicast responses to queries from
         other clients.  It is left to the client implementer how to
         make use of this option.

      Sequence Number, type 2

         Length MUST be 4.  A client MUST always include this in Request
         messages and MUST NOT include it in Init messages.  A server
         replying to a Request message MUST copy it into the Reply (or
         Server Response message on error).  This contains a 32 bit
         sequence number.  The values would typically start at 1 and
         increase by one for each request in a sequence.

      Client Timestamp, type 3

         Length MUST be 8 bytes.  A client SHOULD include this in
         Request messages and MUST NOT include it in Init messages.  A
         server replying to a Request message MUST copy it into the
         Reply.  The timestamp specifies the time when the Request
         message is sent.  The first 4 bytes specify the number of
         seconds since the Epoch (0000 UTC Jan 1, 1970).  The next 4
         bytes specify the number of microseconds since the last second
         since the Epoch.

      Multicast Group, type 4

         Length MUST be greater than 2.  It MAY be used in Server
         Response messages to tell the client what group to use in
         subsequent Request messages.  It MUST be used in Request
         messages to tell the server what group address to respond to
         (this group would typically be previously obtained in a Server
         Response message).  It MUST be used in Reply messages (copied
         from the Request message).  It MUST NOT be used in Init
         messages.  The format of the option value is as below.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |        Address Family         |  Multicast group address...   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  ....                         |

         The address family is a value 0-65535 as assigned by IANA for
         Internet Address Families [4]. [3].  This is followed by the group
         address.  Length of the option value will be 6 for IPv4, and 18
         for IPv6.

      Option Request Option, type 5

         Length MUST be greater than 1.  This option MAY be used in
         client messages (Init and Request messages).  A server MUST NOT
         send this option, except that if it is present in a Request
         message, the server MUST echo it in replies (Reply message) to
         the Request.  This option contains a list of option types for
         options that the client is requesting from the server.  Support
         for this option is optional for both clients and servers.  The
         length of this option will be a non-zero even number, since it
         contains one or more option types that are two octets each.
         The format of the option value is as below.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |          Option Type          |          Option Type          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                             .....                             |

         This option might be used by the client to ask the server to
         include options like Timestamp or Server Information.  A client
         MAY request Server Information in Init messages; it MUST NOT
         request it in other messages.  A client MAY request a Timestamp
         in Request messages; it MUST NOT request it in other messages.
         Subject to enforcing the above restrictions, a server
         supporting this option SHOULD include the requested options in
         responses (Reply messages) to the Request containing the Option
         Request Option.  The server may according to implementation or
         local configuration, not necessarily include all the requested
         options, or possibly none.  Any options included are appended
         to the echoed options, similar to other options included by the
         server.

      Server Information, type 6

         Length MUST be non-zero.  It MAY be used in Server Response
         messages and MUST NOT be used in other messages.  Support for
         this option is optional.  A server supporting this option
         SHOULD add it in Server Response messages if and only if
         requested by the client.  The value is a UTF-8 string that
         might contain vendor and version information for the server
         implementation.  It may also contain information on which
         options the server supports.  An interactive client MAY support
         this option, and SHOULD then allow a user to request this
         string and display it.

      Reserved, type 7

         This option code value was used by early implementations for an
         option that is now deprecated.  This option should no longer be
         used.  Clients MUST NOT use this option.  Servers MUST treat it
         as an unknown option (not process it if received, but if
         received in a Request message, it MUST be echoed in the Reply
         message).

      Reserved, type 8

         This option code value was used by early implementations for an
         option that is now deprecated.  This option should no longer be
         used.  Clients MUST NOT use this option.  Servers MUST treat it
         as an unknown option (not process it if received, but if
         received in a Request message, it MUST be echoed in the Reply
         message).

      TTL, type 9

         Length MUST be 1.  This option contains a single octet
         specifying the TTL of a Reply message.  Every time a server
         sends a unicast or multicast Reply message, it SHOULD include
         this option specifying the TTL.  This is used by clients to
         determine the number of hops the messages have traversed.  It
         MUST NOT be used in other messages.  A server SHOULD specify
         this option if it knows what the TTL of the Reply will be.  In
         general the server can specify a specific TTL to the host
         stack.  Note that the TTL is not necessarily the same for
         unicast and multicast.

      Multicast Prefix, type 10

         Length MUST be greater than 2.  It MAY be used in Init messages
         to request a group within the prefix(es), it MAY be used in
         Server Response messages to tell the client what prefix(es) it
         may try to obtain a group from.  It MUST NOT be used in
         Request/Reply messages.  Note that this option MAY be included
         multiple times to specify multiple prefixes.

        0                   1                   2                   3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |        Address Family         | Prefix Length |Partial address|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     ....      |

         The address family is a value 0-65535 as assigned by IANA for
         Internet Address Families [4]. [3].  This is followed by a prefix
         length (4-32 for IPv4, 8-128 for IPv6, or 0 for the special
         'wildcard' use discussed below), and finally a group address.
         For any family, prefix length 0 means that any multicast
         address from that family is acceptable.  This is what we call
         'wildcard'.  The group address need only contain enough octets
         to cover the prefix length bits (i.e., the group address would
         have to be 3 octets long if the prefix length is 17-24, and
         there need be no group address for the wildcard with prefix
         length 0).  Any bits past the prefix length MUST be ignored.
         For IPv4, the option value length will be 4-7, while for IPv6,
         it will be 4-19, and for the wildcard, it will be 3.

      Session ID, type 11

         Length MUST be non-zero.  A server SHOULD include this in
         Server Response and Reply messages.  If a client receives this
         option in a message, the client MUST echo the Session ID option
         in subsequent Request messages, with the exact same value,
         until the next message is received from the server.  If the
         next message from the server has no Session ID or a new Session
         ID value, the client should do the same, either not use the
         Session ID, or use the new value.  The Session ID may help the
         server in keeping track of clients and possibly manage per
         client state.  The value of a new Session ID should be chosen
         pseudo randomly so that it is hard to predict.  This can be
         used to prevent spoofing of the source address of Request
         messages, see the Security Considerations for details.

      Server Timestamp, type 12

         Length MUST be 8 bytes.  A server supporting this option,
         SHOULD include it in Reply messages, if requested by the
         client.  The timestamp specifies the time when the Reply
         message is sent.  The first 4 bytes specify the number of
         seconds since the Epoch (0000 UTC Jan 1, 1970).  The next 4
         bytes specify the number of microseconds since the last second
         since the Epoch.

4.  Packet Format

   The format of all messages is a one octet message type, directly
   followed by a variable number of options.

       0                   1                   2                   3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |    Type       |          Options ...                          |
      +-+-+-+-+-+-+-+-+            .                                  |
      |                            .                                  |
      |                            .                                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-      .....

   There are four message types defined.  Type 81 (the character Q in
   ASCII) specifies an Echo Request (Query).  Type 65 (the character A
   in ASCII) specifies an Echo Reply (Answer).  Type 73 (the character I
   in ASCII) is an Init message, and type 83 (the character S in ASCII)
   is a Server Response message.

   The options directly follow the type octet and are not aligned in any
   way (no spacing or padding), i.e., options might start at any octet
   boundary.  The option format is specified above.

5.  Message types and options

   There are four message types defined.  We will now describe each of
   the message types and which options they may contain.

      Init, type 73

         This message is sent by a client to request information from a
         server.  It is mainly used for requesting a group address, but
         it may also be used to check which group prefixes the server
         may provide groups from, or other server information.  It MUST
         include a Version option, and SHOULD include a Client ID.  It
         MAY include Option Request and Multicast Prefix Options.  This
         message is a request for a group address if and only if it
         contains Multicast Prefix options.  If multiple Prefix options
         are included, they should be in prioritised order.  I.e., the
         server will consider the prefixes in the order they are
         specified, and if it finds a group for a prefix, it will only
         return that one group, not considering the remaining prefixes.

      Server Response, type 83

         This message is sent by a server.  Either as a response to an
         Init, or in response to a Request.  When responding to Init, it
         may provide the client with a multicast group (if requested by
         the client), or it may provide other server information.  In
         response to a Request, the message tells the client to stop
         sending Requests.  The Version option MUST always be included.
         Client ID and Sequence Number options are echoed if present in
         the client message.  When providing a group to the client, it
         includes a Multicast Group option.  It SHOULD include Server
         Information and Prefix options if requested.

      Echo Request, type 81

         This message is sent by a client, asking the server to send
         unicast and multicast replies.  It MUST include Version,
         Sequence Number and Multicast Group options.  If the last
         message (if any) received from the server contained a Session
         ID, then this MUST also be included.  It SHOULD include Client
         ID and Client Timestamp options.  It MAY include an Option
         Request option.

      Echo Reply, type 65

         This message is sent by a server in response to an Echo Request
         message.  This message is always sent in pairs, one as unicast
         and one as multicast.  The contents of the messages are mostly
         the same.  The server echoes most of the options from the Echo
         Request (any options in the Request that are unsupported by the
         server, are always echoed).  The only option that may be
         present in the Request which is not always echoed, is the
         Session ID option.  In most cases the server would echo it, but
         the server may also change or omit it.  The two Reply messages
         SHOULD both contain a TTL option (not necessarily equal), and
         both SHOULD also contain Server Timestamps (not necessarily
         equal) when requested.

   For the reader's convenience we provide the matrix below, showing
   what options can go in what messages.

      Option / Message Type | Init | Server Response | Request | Reply |
      -----------------------------------------------------------------+
      Version (0)           | MUST |      ECHO       |  MUST   | ECHO  |
      Client ID (1)         |SHOULD|      ECHO       | SHOULD  | ECHO  |
      Sequence Number (2)   | NOT  |      ECHO       |  MUST   | ECHO  |
      Client Timestamp (3)  | NOT  |      NOT        | SHOULD  | ECHO  |
      Multicast Group (4)   | NOT  |      MAY        |  MUST   | ECHO  |
      Option Request (5)    | MAY  |      NOT        |  MAY    | ECHO  |
      Server Information (6)| NOT  |       RQ        |  NOT    |  NOT  |
      Reserved (7)          | NOT  |      NOT        |  NOT    | ECHO  |
      Reserved (8)          | NOT  |      NOT        |  NOT    | ECHO  |
      TTL (9)               | NOT  |      NOT        |  NOT    |SHOULD |
      Multicast Prefix (10) | MAY  |      MAY        |  NOT    |  NOT  |
      Session ID (11)       | NOT  |      MAY        |  ECHO   |  MAY  |
      Server Timestamp (12) | NOT  |      NOT        |  NOT    |  RQ   |

   NOT means that the option MUST NOT be included.  ECHO for a server
   means that if the option is specified by the client, then the server
   MUST echo the option in the response, with the exact same option
   value.  ECHO for a client means that it MUST echo the option it got
   in the last message from the server in any subsequent messages it
   sends.  RQ means that the server SHOULD include the option in the
   response, when requested by the client using the Option Request
   option.

6.  Client Behaviour

   We will consider how a typical interactive client using the above
   protocol would behave.  A client need only require a user to specify
   the unicast address of the server.  It can then send an Init message
   with a prefix option containing the desired address family and zero
   prefix length (wildcard entry).  The server is then free to decide
   which group, from the specified family, it should return.  A client
   may also allow a user to specify group address(es) or prefix(es) (for
   IPv6, the user may only be required to specify a scope or an RP
   address, from which the client can construct the desired prefix,
   possibly embedded-RP).  From this the client can specify one or more
   prefix options in an Init message to tell the server which address it
   would prefer.  If the user specifies a group address, that can be
   encoded as a prefix of maximal length (e.g., 32 for IPv4).  The
   prefix options are in prioritised order, i.e., the client should put
   the most preferred prefix first.

   If the client receives a Server Response message containing a group
   address it can start sending Request messages, see the next
   paragraph.  If there is no group address option, it would typically
   exit with an error message.  The server may have included some prefix
   options in the Server Response.  The client may use this to provide
   the user some feedback on what prefixes or scopes are available.

   Assuming the client got a group address in a Server Response, it can
   start pinging.  Before it does that, it should let the user know
   which group is being used.  Normally, a client should send at most
   one ping request per second.  When sending ping Requests, the client
   must always include the group option.  If the last message from the
   server contained a Session ID, then it must also include that with
   the same value.  Typically it would receive a Session ID in a Server
   Response together with the group address, and then the ID would stay
   the same during the entire ping sequence.  However, if for instance
   the server process is restarted, it may still be possible to continue
   pinging but the Session ID may be changed by the server.  Hence a
   client implementation must always use the last Session ID it
   received, and not necessarily the one from the Server Response
   message.  If a client receives a Server Response message in response
   to a Request message (that is, a Server Response message containing a
   sequence number), this means there is an error and it should stop
   sending Requests.  This may for instance happen after server restart.

   The client may allow the user to request server information.  If the
   user requests server information, the client can send an Init message
   with no prefix options, but with an Option Request Option, requesting
   the server to return a Server Information option.  The server will
   return server information if supported, and it may also return a list
   of prefixes it supports.  It will however not return a group address.
   The client may also try to obtain only a list of prefixes by sending
   an Init message with no prefixes and not requesting any specific
   options.

   Note that a client may pick a multicast group and send Request
   messages without first going through the Init - Server Response
   negotiation.  If this is supported by the server and the server is
   okay with the group used, the server can then send Reply messages as
   usual.  If the server is not okay, it will send a Server Response
   telling the client to stop.

7.  Server Behaviour

   We will consider how a typical server using the above protocol would
   behave.  First we consider how to respond to Init messages.  If the
   Init message contains prefix options, the server should look at them
   in order and see if it can assign a multicast address from the given
   prefix.  The server would be configured, possibly have a default,
   specifying which groups it can offer.  It may have a large pool just
   picking a group at random, possibly choose a group based on hashing
   of the client's IP address or identifier, or just use a fixed group.
   A server could possibly decide whether to include site scoped group
   ranges based on the client's IP address.  It is left to the server to
   decide whether it should allow the same address to be used
   simultaneously by multiple clients.  If the server finds a suitable
   group address, it returns this in a group option in a Server Response
   message.  The server should additionally include a Session ID.  This
   may help the server if it is to keep some state, for instance for
   making sure the client uses the group it got assigned.  A good
   Session ID would be a pseudo random byte string that is hard to
   predict.  If the server cannot find a suitable group address, or if
   there were no prefixes in the Init message, it may send a Server
   Response message containing prefix options listing what prefixes may
   be available to the client.  Finally, if the Init message requests
   the Server Information option, it should include that.

   When the server receives a Request message, it may first check that
   the group address and Session ID (if provided) are valid.  If the
   server is satisfied, it will send a unicast Reply message back to the
   client, and also a multicast Reply message to the group address.  The
   Reply messages contain the exact options and in the same order, as in
   the Request, and after that the server adds a TTL option and
   additional options if needed.  E.g., it may add a timestamp if
   requested by the client.  If the server is not happy with the Request
   (bad group address or Session ID, request is too large etc), it may
   send a Server Response message asking the client to stop.  This
   Server Response must echo the sequence number from the Request.  This
   Server Response may contain group prefixes from which a client can
   try to request a group address.  The unicast and multicast Reply
   messages have identical UDP payload apart from possibly TTL and
   timestamp option values.

   Note that the server may receive Request messages with no prior Init
   message.  This may happen when the server restarts or if a client
   sends a Request with no prior Init message.  The server may go ahead
   and respond if it is okay with the group used.  In the responses it
   may add a Session ID which will then be in later requests from the
   client.  If the group is not okay, the server sends back a Server
   Response.  The Response is just as if it got an Init message with no
   prefixes.  If the server adds or modifies the SessionID in replies,
   it must use the exact same SessionID in the unicast and multicast
   replies.

   By default, a server should perform rate limiting and for a given
   client, respond to at most one Request message per second.  A leaky
   bucket algorithm is suggested, where the rate can be higher for a few
   seconds, but the average rate should by default be limited to a
   message per client per second.

8.  Recommendations for Implementers

   The protocol as specified is fairly flexible and leaves a lot of
   freedom for implementers.  In this section we present some
   recommendations.

   Server administrators should be able to configure one or multiple
   group prefixes in a server implementation.  When deploying servers on
   the Internet and in other environments, the server administrator
   should be able to restrict the server to respond to only a few
   multicast groups which should not be currently used by multicast
   applications.  A server implementation should also provide
   flexibility for an administrator to apply various policies to provide
   one or multiple group prefixes to specific clients, e.g., site scoped
   addresses for clients that are inside the site.  Clients could be
   identified by their IP address provided that clients are required to
   send Init messages, and they receive an unpredictable Session ID.
   See also Section 11.

   Clients should by default send at most one request per second.
   Servers must should perform rate limiting, to guard against this protocol
   being used for DoS attacks.  By default, clients  The server should send for a given client,
   respond to at most one request Request message per second, and servers should perform second.  A leaky bucket
   algorithm is suggested, where the rate limiting if can be higher for a few
   seconds, but the average rate should by default be limited to a
   message per client sends more frequent requests. per second.  Server implementations should provide
   administrative control of which client IP addresses to serve, and may
   also allow certain clients to perform more rapid requests.
   Implementers of applications/tools using this protocol should
   consider the UDP guidelines [6], [5], in particular if clients are to
   send, or servers are to accept, requests at rates exceeding one per
   second.  If higher rates are allowed for specific IP addresses, then
   Init messages and the Session ID option should be used to help
   prevent spoofing.  See Section 11.

9.  Acknowledgements  Acknowledgments

   The ssmping concept was proposed by Pavan Namburi, Kamil Sarac and
   Kevin C. Almeroth in the paper SSM-Ping: A Ping Utility for Source
   Specific Multicast, and also the Internet Draft
   draft-sarac-mping-00.txt.  Mickael Hoerdt has contributed with
   several ideas.  Alexander Gall, Nicholas Humfrey, Nick Lamb and Dave
   Thaler have contributed in different ways to the implementation of
   the ssmping tools at [5]. [4].  Many people in communities like TERENA,
   Internet2 and the M6Bone have used early implementations of ssmping
   and provided feedback that have influenced the current protocol.
   Thanks to Kevin Almeroth, Toerless Eckert, Gorry Fairhurst, Alfred
   Hoenes, Liu Hui, Bharat Joshi, Olav Kvittem, Hugo Santos, Kamil
   Sarac, Pekka Savola, Trond Skjesol and Cao Wei for reviewing and
   providing feedback on this draft.  In particular Hugo, Gorry and
   Bharat have provided lots of input on several revisions of the draft draft.

10.  IANA Considerations

   IANA is requested to provide a well-known UDP port number for use by
   this protocol, and also to provide registries for message and option
   types.

   There should be a message types registry.  Message types are in the
   range 0-255.  Message types 0-191 can be assigned referencing an require specification (an RFC
   (it may be Informational), or
   other permanent and readily available reference), while types 192-255
   are freely available for experimental, private or vendor specifc use. experimental use and are not registered.  The registry should
   include the messages defined in Section 5 5.  A message specification
   must describe the behaviour with known option types as well as the
   default behaviour with unknown ones.

   There should also be an option type registry.  Option types 0-49151
   can be assigned referencing an
   require specification (an RFC (it may be Informational), or other permanent and readily
   available reference), while types 49152-65535 are freely available for experimental, private or
   vendor specifc use. experimental
   use and are not registered.  The registry should include the options
   defined in Section 3.2.  An option specification must describe how
   the option may be used with the known message types.  This includes
   which message types the option may be used with.

   The initial registry definitions could be as follows:

   Multicast Ping Protocol Parameters:

   Registry Name: Multicast Ping Protocol Message Types
   Reference: [this doc]
   Registration Procedures: Specification Required

   Registry:
   Type         Name                                  Reference
   -----------  ------------------------------------  ----------
   65           Echo Reply                            [this doc]
   73           Init                                  [this doc]
   81           Echo Request                          [this doc]
   83           Server Response                       [this doc]
   192-255      Experimental

   Registry Name: Multicast Ping Protocol Option Types
   Reference: [this doc]
   Registration Procedures: Specification Required

   Registry:
   Type         Name                                  Reference
   -----------  ------------------------------------  ----------
   0            Version                               [this doc]
   1            Client ID                             [this doc]
   2            Sequence Number                       [this doc]
   3            Client Timestamp                      [this doc]
   4            Multicast Group                       [this doc]
   5            Option Request Option                 [this doc]
   6            Server Information                    [this doc]
   7            Reserved                              [this doc]
   8            Reserved                              [this doc]
   9            TTL                                   [this doc]
   10           Multicast Prefix                      [this doc]
   11           Session ID                            [this doc]
   12           Server Timestamp                      [this doc]
   49152-65535  Experimental

11.  Security Considerations

   There are some security issues to consider.  One is that a host may
   send a request with an IP source address of another host, and make an
   arbitrary multicast ping server on the Internet send packets to this
   other host.  This behaviour is fairly harmless.  The worst case is if
   the host receiving the unicast replies also happen to be joined to
   the multicast group used.  In this case, there would be an
   amplification effect where the host receives twice as many replies as
   there are requests sent.  See below for how spoofing can be
   prevented.

   For ASM (Any-Source Multicast) a host could also make a multicast
   ping server send multicast packets to a group that is used for
   something else, possibly disturbing other uses of that group.  The
   main concern is bandwidth.  Since there is a well-known port, it
   should not be received by other applications.  Due to this, a server
   on the Internet SHOULD perform rate limiting.

   In order to help prevent spoofing, a server SHOULD require the client
   to send an Init message, and return an unpredictable Session ID in
   the response.  The ID should be associated with the IP address and
   have a limited lifetime.  The server SHOULD then only respond to
   Request messages that have a valid Session ID associated with the
   source IP address of the Request.

   Server implementations should allow administrators to restrict which
   groups a server responds to, and also perform rate limiting.  This is
   discussed in Section 8). 8.

12.  References

12.1.  Normative References

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

   [2]  Postel, J., "User Datagram Protocol", STD 6, RFC 768,
        August 1980.

   [3]  Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
        Specification", RFC 2460, December 1998.

   [4]  "IANA, Address Family Numbers",
        <http://www.iana.org/assignments/address-family-numbers>.

12.2.  Informative References

   [5]

   [4]  "ssmping implementation",
        <http://www.venaas.no/multicast/ssmping/>.

   [6]

   [5]  Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines for
        Application Designers", draft-ietf-tsvwg-udp-guidelines-10 draft-ietf-tsvwg-udp-guidelines-11 (work
        in progress), August October 2008.

Author's Address

   Stig Venaas
   UNINETT
   Trondheim  NO-7465
   Norway

   Email: venaas@uninett.no

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