Network Working Group                                          S. Venaas
Internet-Draft                                                   UNINETT
Intended status: Informational                                 H. Santos                         February 12, 2008
Expires: May 22, August 15, 2008                                    NEC Europe Ltd.
                                                       November 19, 2007

                            ssmping

                        Multicast Ping Protocol
                      draft-ietf-mboned-ssmping-02
                      draft-ietf-mboned-ssmping-03

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Copyright Notice

   Copyright (C) The IETF Trust (2007). (2008).

Abstract

   The ssmping protocol Multicast Ping Protocol specified in this document allows for
   checking whether one an endpoint can receive multicast, both Source-Specific Source-
   Specific Multicast (SSM) and Any-Source Multicast (ASM), as well as (ASM).  It can also
   be used to obtain additional multicast related information. 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  . . . . . . . . . . . . . . . . . . . . .  5  6
   4.  Packet Format  . . . . . . . . . . . . . . . . . . . . . . . .  8  9
   5.  Message types and options  . . . . . . . . . . . . . . . . . .  9 10
   6.  Client Behaviour . . . . . . . . . . . . . . . . . . . . . . . 10
   7.  Server Behaviour . . . . . . . . . . . . . . . . . . . . . . . 11 12
   8.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 12 13
   9.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 13
   10. Security Considerations  . . . . . . . . . . . . . . . . . . . 13
   11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 13 14
     11.1. Normative References . . . . . . . . . . . . . . . . . . . 13 14
     11.2. Informative References . . . . . . . . . . . . . . . . . . 13
   Authors' Addresses 14
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 13 . 15
   Intellectual Property and Copyright Statements . . . . . . . . . . 15 16

1.  Introduction

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

   The protocol here specified is based on the actual implementation of
   the ssmping and asmping tools [3] [5] 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 of an ssmping/asmping session is as follows. follows: A server runs continuously
   to serve requests from clients.  When a user
   decides to verify  A client can test the multicast
   reception from this server, provided it knows a specific server
   (knowing one of the server's unicast addresses is required), address of
   the
   client 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 SSM
   channel (S,G) where S is a unicast address of the target server, or
   an ASM group G, where 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 ssmping multicast ping
   requests to the server.  The requests are sent using UDP with
   destination port set to the standardised ssmping 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 back by the server, except the
   server may add a few options.  To  For each request, the server sends two
   replies.  One as reply is unicast back to the port and source IP address the request was sent from, and
   also one as source
   UDP port of the request, while another is multicast back to requested
   multicast group G and the source UDP port of the request.  Both the
   replies are sent from the same port from which the request
   originated with the requested group G as destination address. was
   received.  The server should specify the TTL used for both the
   unicast and multicast messages (we recommend at least 64) and
   includes a TTL option for 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,
   it 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), or whether the client and server have synchronised
   clocks. load).

3.  Protocol specification

   There are four different ssmping 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 ssmping 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
   content back (only changing the message type), without inspecting the
   options.  However, the server SHOULD add a TTL option, and there are some
   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.

   This document defines a
   A server MUST NOT process unknown options, but they MUST still be
   included in the Echo Reply.  A client MUST ignore any unknown
   options.

   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

   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 that
   only partly follow this specification.  They can be regarded as
   version 1 and do not use this option.

   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 this its Init/Request or not.  A server should treat
   this as opaque data, and simply leave it unchanged in the reply (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 implementor 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 Request message MUST copy it into
   the Reply.  In
   addition, a server supporting this option, SHOULD include it in Reply
   messages, if requested by the client.  Note that this means that the
   option may be present in the Reply message twice; both a client
   timestamp as part of the echoed Request, and another timestamp added
   by the server.  The timestamp specifies the time when the Request message
   (query or reply)
   is sent.  The first 4 bytes specify the number of seconds since the
   Epoch (beginning of the year 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 1.  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 [2]. [4].  This is followed by the group
   address.  For IPv4  Length of the option value length will be 6, 6 for IPv6 18. IPv4, and 18 for
   IPv6.

   Option Request Option, type 5.  Length MUST be greater than 1.  This
   option MAY be used in Init client messages (Init and Request messages.  It messages).  A
   server MUST NOT be used
   in any other messages, send this option, except that if it is present in a
   Request message, the server will, MUST include it in a Reply, echo
   back this option if present in reply (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.

   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.

   Type 7, Reserved.  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, and option.  Servers
   MUST ignore it.

   Pad, type 8.  Length can be anything, including 0.  This treat it as an unknown option is
   used by clients to increase the request size (not process it if received, but
   if received in order to have the
   server deliver responses of a particular size.  If the server adds
   any options when responding, Request message, it should, if possible make the response
   the same size as MUST be echoed back in the request Reply
   message).

   Type 8, Reserved.  This option code value was used by shrinking the pad early
   implementations for an option (i.e., that is now deprecated.  This option
   should no longer be used.  Clients MUST not
   simply including it, as is, like all other client options).  If the
   options added by the server consume as much space use this option.  Servers
   MUST treat it as the pad an unknown option
   does, or more, the server should remove (not process it if received, but
   if received in a Request message, it MUST be echoed back in the entire pad option. 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.  Although  A server SHOULD specify this option is not absolutely required, the
   server is expected to use it 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 [2]. [4].  This is followed by a prefix length
   (0-32 for IPv4, 0-128 for IPv6), and finally a group address.  The
   group address need only contain enough octets to cover the prefix
   length bits (e.g., there need be no group address if the prefix
   length is 0, the group address would have to be 3 octets long if the
   prefix length is 17-24).  Any bits past the prefix length MUST be
   ignored.  For IPv4 the option value length will be 3-7, while for
   IPv6 3-19.

   Session ID, type 11.  Length MUST be non-zero.  A server MAY 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 Reply 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 client state.  It is left to the server
   implementer to decide whether it is useful and how to make use of it.

   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 (beginning of the year 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       |          Option                               |
      +-+-+-+-+-+-+-+-+            .                                  |
      |                            .                                  |
      |                            .                                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Option                               |
      |                            .                                  |
      |                            .                                  |
      |                            .                                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
                                   .
                                   .
                                   .
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |                          Option                               |
      |                            .                                  |
      |                            .                                  |
      |                            .                                  |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   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 Response (Answer).  Type 73 (the
   character I in ASCII) is an Init message, and type 83 (the character
   S in ACII) 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

   For the readers 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 |      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/RQ|  | 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    |  NOT ECHO  |
      Pad
      Reserved (8)          | NOT  |      NOT        |  MAY  NOT    | ECHO* ECHO  |
      TTL (9)               | NOT  |      NOT        |  NOT    |SHOULD |
      Multicast prefix (10) | MAY  |      MAY        |  NOT    |  NOT  |
      Session ID (11)       | NOT  |      MAY        |  ECHO   |  MAY  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 back in the response, with the exact same option
   value.  The exception is ECHO* where the option value may be
   modified.  ECHO for a client means that it MUST echo the option it got
   in the last message from the server in the following 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.  The server is then free to decide which group it
   should return.  A client may also allow a user to specify a 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 specifiy the group option.  If the last message from the
   server contained a Session ID, then it MUST 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 may be changed by the server.  Hence a
   client implementation MUST 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 have an option for the user to obtain server
   information.  If the user asks for 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 and possibly pick a new group.

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 in the given
   range.  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 clients IP address or identifier, or just use a fixed group.
   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 may 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 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 (only exception is the pad option), 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 and 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 which prefixes the client can try to
   request addresses from.  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 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.  Acknowledgements

   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 [3]. [5].  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, 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

9.  IANA Considerations

   IANA is requested to provide a UDP port number for use by this
   protocol, and also to provide a registry registries for ssmping 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 RFC
   (it may be Informational), while types 192-255 are freely available
   for experimental, private or vendor specifc use.

   There should also be an option type registry.  Option types 0-49151
   can be assigned referencing an RFC (it may be Informational), while
   types 49152-65535 are freely available for experimental, private or
   vendor specifc use.

10.  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 ssmping multicast ping server on the Internet send packets to this
   other host.  This hehaviour behaviour is fairly harmless.  The worst case is if
   the host receiving the unicast replies also happen to be performing an
   ssmping test towards that particular server. joined to
   the multicast group used.  In this unlikely event, case, there would be an
   amplification effect where the host receives twice as many replies as
   there are requests sent.  An ssmping server

   Servers should perform rate limiting, to guard against this function
   being used as a DoS attack.  A  By default, clients should send at most
   one request per second, and servers should perform rate limiting if a
   client sends more frequent requests.  Server implementations should
   provide administrative control of which client IP addresses to serve,
   and may also allow certain clients to perform more rapid requests.
   Implementors of applications/tools using this protocol should
   consider the UDP guidelines [6], in particular if clients are to
   send, or servers are to accept, requests at rates exceeding one per
   second.

   A client should use the client ID option to distinguish replies to
   its own requests from replies that might be to other requests.

11.  References

11.1.  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>.

11.2.  Informative References

   [3]

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

Authors' Addresses

   [6]  Eggert, L. and G. Fairhurst, "UDP Usage Guidelines for
        Application Designers", draft-ietf-tsvwg-udp-guidelines-05 (work
        in progress), February 2008.

Author's Address

   Stig Venaas
   UNINETT
   Trondheim  NO-7465
   Norway

   Email: venaas@uninett.no

   Hugo Santos
   NEC Europe Ltd.
   Kurfuersten-Anlage 36
   Heidelberg  69115
   Germany

   Email: hugo.santos@nw.neclab.eu

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