Network Working Group                                          S. Venaas
Internet-Draft                                                   UNINETT
Intended status: Informational                              May 22, 2007
Expires: November 23, 2007

                            ssmping Protocol

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Copyright Notice

   Copyright (C) The IETF Trust (2007).


   ssmping is a tool that is used to check whether one can receive SSM,
   as well as obtaining some additional information. ssmping requires
   both a client and a server supporting the ssmping protocol to work.
   We here specify this protocol.

Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",

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   document are to be interpreted as described in RFC 2119 [1].

Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Protocol description . . . . . . . . . . . . . . . . . . . . .  3
   3.  Options  . . . . . . . . . . . . . . . . . . . . . . . . . . .  4
     3.1.  Option format  . . . . . . . . . . . . . . . . . . . . . .  4
     3.2.  Defined Options  . . . . . . . . . . . . . . . . . . . . .  5
   4.  Packet Format  . . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  8
   6.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . .  9
   Intellectual Property and Copyright Statements . . . . . . . . . . 10

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1.  Introduction

   ssmping is a tool that is used to check whether one can receive SSM,
   and it can also give other information like the time to establish the
   tree, number of router hops the packets have traveled, packet delay
   and loss.  The ssmping functionality resembles ICMP echo request/
   reply using UDP and a client and a server that supports the ssmping
   protocol.  It is used by a client to verify that it can receive
   multicast from the server, as well as some additional information.
   The protocol as specified here is based on an actual implementation
   of a tool [3] that has been found useful by many organisations.

2.  Protocol description

   Before going into the protocol details we will describe how it is
   used and what information it may provide.  The typical usage is as
   follows.  A server runs continuously in order to serve request from
   clients.  At some point a client application may try to verify
   multicast reception from such a server.  The client will need to know
   a unicast address of a server.  The client joins an SSM channel (S,G)
   where S is a unicast address of the server, and G is a standardised
   multicast group for use by ssmping.  After joining the channel, the
   client sends ssmping requests as UDP to a standardised ssmping port
   and the unicast address of the server.  The requests are sent
   periodically, e.g. once per second, to the server.  The requests
   contain a serial number, and typically a timestamp.  The requests are
   typically, but not necessarily always, simply echoed back by the
   server.  To each request, the server sends two replies.  One as
   unicast back to the port and address the request was sourced from,
   and also as multicast back to the port the request came from.  It is
   currently left open which port the request is sourced from, whether
   this port should be standardised or not.  The TTL or Hop Limit of the
   replies are set to 64.  The client should leave the SSM channel when
   it has finished its measurements.

   By use of this protocol, a client can obtain information on several
   aspects of the multicast quality.  First of all, 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 in receiving multicast is
   indeed caused by a multicast problem.  If it does receive multicast,
   it knows not only that it can receive, but it may get some
   information on how long it takes 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 unicast request is sent to when the
   reply is received.  For multicast we also talk about RTT, but then we

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   mean from the unicast request is sent to when the multicast reply is
   received.  Since the server sets TTL or Hop Limit to 64, it can also
   know the number of router hops it is away from the source.  By
   comparing with the unicast replies, it can see whether there are
   differences in RTT and number of hops etc for unicast and multicast.
   Provided that the server sends the unicast and multicast replies
   nearly simultaneously, it may also be able to measure difference in
   one way delay for unicast and multicast on the path from server to
   client, and also if there are differences in delay variation.
   Servers may optionally specify a timestamp.  This may be useful if
   the unicast and multicast replies can not be sent nearly
   simultaneously, or if the client and server have synchronised clocks.

   The ssmping requests and replies have 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.  Generally the client includes a number of options in the
   request, and a server may simply echo the content back (only changing
   the message type), without inspecting the options.  However, there
   are a number of options that a server implementation may support,
   where the client may ask for a certain information or behaviour from
   the server.  In some cases the server will need to add options in the
   response.  The response will then first contain the exact options
   from the request, and then right after those, options appended by the

3.  Options

   There are a number of different options.  Most of the options are
   only used by clients and simply echoed back by the server, where the
   server doesn't care about their contents.  There are however some
   client options that the server may care about.  There are also server
   options that may be requested by the client.  Generally a simple
   client will only include a few options, and get exactly the same
   options and values echoed back.  Strictly speaking the protocol could
   work without any options.  Without sending any options a client would
   still be able to tell whether multicast is working or not, however
   with the use of some of the basic options a client can obtain a lot
   more information.

3.1.  Option format

   All options are TLVs formatted as specified below.

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       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.  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

   Client Identifier, type 1.  Length MUST be non-zero.  Only used by
   clients.  A client SHOULD include this.  The client may use any value
   it likes to be able to detect whether a reply is a reply to this
   query or not.  A server should treat this as opaque data, and simply
   leave it unchanged in the reply.  The value might be a process ID,
   perhaps process ID combined with an IP address because it may receive
   multicasted responses to queries from other clients.  It is left to
   the client implementor how to make use of this.

   Sequence number, type 2.  Length MUST be 4.  Only used by clients.  A
   client SHOULD include this.  This contains a 32 bit sequence number.
   The values would typically start at 1 and increase by one for each
   request in a sequence.

   Timestamp, type 3.  Length MUST be 8 bytes.  A client SHOULD include
   this.  A server MAY support this.  If supported it SHOULD be included
   in the reply if requested by the client.  The timestamp specifies the
   time when the 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 is
   optional for clients and servers to support this.  It allows a client
   to specify which group the server should send to.  This is currently

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   used by a tool called "asmping" to test ASM connectivity.  The server
   may have restrictions on which groups can be used.  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
      |  Addr Family  | Multicast group address...                    |
      +-+-+-+-+-+-+-+-+            ....                               |

   The address family is a value 0-127 as assigned by IANA for Internet
   Address Families [2].  This is followed by the group address.  For
   IPv4 the option value length will be 5, for IPv6 17.

   Option Request Option, type 5.  Length MUST be greater than 1.  The
   option contains a list of option types of options that the client
   requests from the server.  Supporting this is optional for both
   clients and servers.  The length of this option will be a non-zero
   even number, since it contains option types that each are two octets.
   The format of the 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          |
      |                             .....                             |

   The value might contain an odd number of options, including just one.
   This option might be used by the client to ask the server to include
   options like timestamp or version.

   Version, type 6.  Length MUST be non-zero.  Supporting this option is
   optional.  A server supporting this option SHOULD add it if and only
   if requested by the client.  The value is just unformatted text that
   might contain vendor and version information for the server
   implementation.  It may also contain information on which options the
   server supports.

   Reply size, type 7.  Length MUST be 2 octets.  This option is
   optional for clients and servers.  It can be used to request the
   server response to be of a certain size.  The value specifies the
   desired response size in octets.  A server supporting this will if
   necessary use the pad option to increase the size of the response.  A
   server should however not try to make the response shorter due to
   this option.  That is, it should not omit or shorten any option

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   values to try to accommodate this.  The response should never be
   shorter than if this option were not included.  Also, the pad option
   requires at least 3 octets, so the server will not pad the response
   size if the requested size is not at least 3 octets longer than the
   normal response size.

   Pad, type 8.  Length can be anything, including 0.  This option is
   used by servers to increase the response size if the client asks for
   a reply that is larger than what the server normally would send.  The
   addition of this option consumes a minimum of 3 octets, so it should
   only be added if the requested size is at least 3 octets more than
   the size of the normal (non-padded) response.

4.  Packet Format

   The format of the ssmping messages is a one octet message type,
   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 two message types defined.  Type 81 (the character Q in
   ASCII) specifies a query.  Type 65 (the character A in ASCII)
   specifies a response (answer).

   The options follow right after the type octet and are not aligned in
   any way (no spacing or padding).  I.e., options might start at any

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   octet boundary.  The option format is specified below

5.  Acknowledgements

   The ssmping idea 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,
   Nick Lamb and Dave Thaler have contributed in different ways to my
   implementation of the ssmping tools [3].  Hugo Santos has made an
   independent implementation of an ssmping server.  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 Olav Kvittem, Kamil Sarac and Trond
   Skjesol for reviewing and providing feedback on this draft.

6.  IANA Considerations

   As currently specified, ssmping would need a well known port number
   which the servers listen to.  It might be desirable to use SRV
   records instead or in addition to this.  For IPv6 SSM ssmping should
   ideally have a reserved group ID.  For the optional ASM functionality
   it would be useful to have a reserved IPv6 group ID, this may be the
   same as the one used for SSM.  It may also be useful to have a
   dedicated group for the optional IPv4 ASM functionality.  This
   section needs further work.

7.  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 a
   random ssmping server on the Internet send packets to this other
   host.  This 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.  In this unlikely event there
   would be an amplification effect where the host receives twice as
   many replies as there are requests sent.  An ssmping server should
   perform rate limiting, to guard against this being used as an DoS
   attack.  A client should also use the client identifier option to be
   able to distinguish replies to its own requests from replies that
   might be to other requests.  How the protocol should be designed to
   cope with rate limiting at the server requires further study.  One
   possibility might be that the server can choose to send generic
   replies, e.g. a packet every second without the usual client options
   but including sequence number and server time stamp, and where

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   clients do not send requests as long as they receive generic replies.

8.  References

8.1.  Normative References

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

   [2]  "IANA, Address Family Numbers",

8.2.  Informative References

   [3]  "ssmping implementation",

Author's Address

   Stig Venaas
   Trondheim  NO-7465


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