One document matched: draft-ietf-mboned-ssmping-05.xml

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<rfc category="info" ipr="full3978" docName="draft-ietf-mboned-ssmping-05">
        <front>
                <title>Multicast Ping Protocol</title>
		<author initials='S.' surname='Venaas' fullname='Stig Venaas'>
			<organization>UNINETT</organization>
			<address>
				<postal>
					<city>Trondheim</city>
					<code>NO-7465</code>
					<country>Norway</country>
				</postal>
				<email>venaas@uninett.no</email>
			</address>
		</author>
                <date month="September" year="2008" />
                <abstract><t>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.
</t></abstract>
<note title="Requirements Language">
                        <t>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
<xref target="RFC2119">RFC 2119</xref>.</t>
                </note>
        </front>
        <middle>
                <section title="Introduction" anchor="intro">
		        <t>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 (<xref target="RFC0768">RFC 768</xref> and
<xref target="RFC2460">RFC 2460</xref>) 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.
</t><t>
The protocol here specified is based on the actual implementation of
the ssmping and asmping tools <xref target="impl"/> which are widely
used by the Internet community to conduct multicast connectivity tests.
</t>
                </section>
                <section title="Architecture" anchor="arch">
<t>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.
</t><t>
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.
</t>
<t>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).</t>
                </section>
                <section title="Protocol specification" anchor="protocol">
<t>
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.
</t>
<t>
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.
</t>
<t>
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.
</t>
			<section title="Option format" anchor="optformat">
<t>All options are TLVs formatted as specified below.
				<figure><artwork>
    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                             |
   |                               .                               |
   |                               .                               |
   |                               .                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
				</artwork></figure></t>
<t>Type (2 octets) specifies the option. The different options
are defined below.</t>
<t>Length (2 octets) specifies the length of the value field. Depending on
the option type, it can be from 0 to 65535.</t>
<t>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.</t>
			</section>
			<section title="Defined Options" anchor="opts">
<t>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.
</t>
<list style="empty">
<t>Version, type 0
<list style="empty"><t>
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. 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.
</t></list></t>
<t>Client ID, type 1
<list style="empty"><t>
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.
</t></list></t>
<t>Sequence Number, type 2
<list style="empty"><t>
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.
</t></list></t>
<t>Client Timestamp, type 3
<list style="empty"><t>
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.
</t></list></t>
<t>Multicast Group, type 4
<list style="empty"><t>
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.
				<figure><artwork>
	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...   |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+  ....                         |
				</artwork></figure>
The address family is a value 0-65535 as assigned by IANA for Internet
Address Families <xref target="addrfamily"/>.
This is followed by the group address.
Length of the option value will be 6 for IPv4, and 18 for IPv6.
</t></list></t>
<t>Option Request Option, type 5
<list style="empty"><t>
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.
				<figure><artwork>
	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          |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |                             .....                             |
				</artwork></figure>
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.
</t></list></t>
<t>Server Information, type 6
<list style="empty"><t>
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.
</t></list></t>
<t>Reserved, type 7
<list style="empty"><t>
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).
</t></list></t>
<t>Reserved, type 8
<list style="empty"><t>
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).
</t></list></t>
<t>TTL, type 9
<list style="empty"><t>
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.
</t></list></t>
<t>Multicast Prefix, type 10
<list style="empty"><t>
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.
				<figure><artwork>
	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|
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+     ....      |
				</artwork></figure>
The address family is a value 0-65535 as assigned by IANA for Internet
Address Families <xref target="addrfamily"/>.
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.
</t></list></t>
<t>Session ID, type 11
<list style="empty"><t>
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.
</t></list></t>
<t>Server Timestamp, type 12
<list style="empty"><t>
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.
</t></list></t>
</list>
			</section>
                </section>
                <section title="Packet Format" anchor="format">
<t>The format of all messages is a one octet message type,
directly followed by a variable number of options.
			<figure><artwork>
    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 ...                          |
   +-+-+-+-+-+-+-+-+            .                                  |
   |                            .                                  |
   |                            .                                  |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-      .....
      			</artwork></figure></t>
<t>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.</t>
<t>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.</t>
		</section>
		<section title="Message types and options" anchor="msgs">
<t>There are four message types defined. We will now describe
each of the message types and which options they may contain.
</t>
<list style="empty">
<t>Init, type 73
<list style="empty"><t>
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.
</t></list></t>
<t>Server Response, type 83
<list style="empty"><t>
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.
</t></list></t>
<t>Echo Request, type 81
<list style="empty"><t>
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.
</t></list></t>
<t>Echo Reply, type 65
<list style="empty"><t>
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.
</t></list></t>
</list>
<t>For the reader's convenience we provide the matrix below, showing
what options can go in what messages.
			<figure><artwork>
   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   |
      			</artwork></figure>
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.
</t>
		</section>
		<section title="Client Behaviour" anchor="clbeh">
<t>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.
</t><t>
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.
</t><t>
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.
</t><t>
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.
</t><t>
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.
</t>
		</section>
		<section title="Server Behaviour">
<t>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.
</t><t>
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.
<t></t>
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.
</t><t>
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.
</t>
		</section>
		<section title="Recommendations for Implementers" anchor="imprec">
<t>
The protocol as specified is fairly flexible and leaves a lot of
freedom for implementers. In this section we present some
recommendations.
</t>
<t>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.
</t><t>
Servers must perform rate limiting, to guard against this protocol
being used for DoS attacks. 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. Implementers of
applications/tools using this protocol should consider the
<xref target='I-D.ietf-tsvwg-udp-guidelines'>UDP guidelines</xref>,
in particular if clients are to send, or servers are to accept,
requests at rates exceeding one per second.
</t>
		</section>
		<section title="Acknowledgements">
<t>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 <xref target="impl"/>.
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</t>
		</section>
		<section title="IANA Considerations">
<t>IANA is requested to provide a UDP port number for use by this
protocol, and also to provide registries for message and option
types.
</t><t>
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.
The registry should include the messages defined in <xref target="msgs"/>
</t><t>
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.
The registry should include the options defined in <xref target="opts"/>.
</t>
		</section>
		<section title="Security Considerations">
<t>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.
</t><t>
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.
</t><t>
Server implementations should allow administrators to restrict
which groups a server responds to, and also perform rate
limiting. This is discussed in <xref target='imprec'/>).
</t>
		</section>
        </middle>
        <back>
                <references title="Normative References">
			<?rfc include="reference.RFC.0768" ?>
			<?rfc include="reference.RFC.2119" ?>
			<?rfc include="reference.RFC.2460" ?>
			<reference target="http://www.iana.org/assignments/address-family-numbers" anchor="addrfamily">
				<front>
					<title>IANA, Address Family Numbers</title>
				</front>
			</reference>
		</references>
                <references title="Informative References">
			<?rfc include="reference.I-D.ietf-tsvwg-udp-guidelines" ?>
			<reference target="http://www.venaas.no/multicast/ssmping/" anchor="impl">
				<front>
					<title>ssmping implementation</title>
				</front>
			</reference>
		</references>
        </back>
</rfc>