Network Working Group S. Venaas
Internet-Draft UNINETT
Intended status: Informational September 18, 2008
Expires: March 22, 2009
Multicast Ping Protocol
draft-ietf-mboned-ssmping-05
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Abstract
The Multicast Ping Protocol specified in this document allows for
checking whether an endpoint can receive multicast, both Source-
Specific Multicast (SSM) and Any-Source Multicast (ASM). It can also
be used to obtain additional multicast related information like
multicast tree setup time etc. This protocol is based on an
implementation of tools called ssmping and asmping.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol specification . . . . . . . . . . . . . . . . . . . . 4
3.1. Option format . . . . . . . . . . . . . . . . . . . . . . 5
3.2. Defined Options . . . . . . . . . . . . . . . . . . . . . 6
4. Packet Format . . . . . . . . . . . . . . . . . . . . . . . . 10
5. Message types and options . . . . . . . . . . . . . . . . . . 11
6. Client Behaviour . . . . . . . . . . . . . . . . . . . . . . . 12
7. Server Behaviour . . . . . . . . . . . . . . . . . . . . . . . 14
8. Recommendations for Implementers . . . . . . . . . . . . . . . 15
9. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 16
10. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 16
11. Security Considerations . . . . . . . . . . . . . . . . . . . 16
12. References . . . . . . . . . . . . . . . . . . . . . . . . . . 17
12.1. Normative References . . . . . . . . . . . . . . . . . . . 17
12.2. Informative References . . . . . . . . . . . . . . . . . . 17
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 17
Intellectual Property and Copyright Statements . . . . . . . . . . 18
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1. Introduction
The Multicast Ping Protocol specified in this document allows for
checking multicast connectivity. In addition to checking reception
of multicast (SSM or ASM), the protocol can provide related
information like multicast tree setup time, the number of hops the
packets have traveled, as well as packet delay and loss. This
functionality resembles, in part, the ICMP Echo Request/Reply
mechanism, but uses UDP (RFC 768 [2] and RFC 2460 [3]) and requires
both a client and a server implementing this protocol. Intermediate
routers are not required to support this protocol. They forward
Protocol Messages and data traffic as usual.
The protocol here specified is based on the actual implementation of
the ssmping and asmping tools [5] which are widely used by the
Internet community to conduct multicast connectivity tests.
2. Architecture
Before describing the protocol in detail, we provide a brief overview
of how the protocol may be used and what information it may provide.
The typical protocol usage is as follows: A server runs continuously
to serve requests from clients. A client can test the multicast
reception from this server, provided it knows a unicast address of
the server. It will then send a unicast message to the server asking
for a group to use. Optionally the user may have requested a
specific group or scope, in which case the client will ask for a
group matching the user's request. The server will respond with a
group to use, or an error if no group is available. Next, for ASM,
the client joins an ASM group G, while for SSM it joins a channel
(S,G). Here G is the group specified by the server, and S is the
unicast address used to reach the server.
After joining the channel, the client unicasts multicast ping
requests to the server. The requests are sent using UDP with
destination port set to the standardised multicast ping port [TBD].
The requests are sent periodically, e.g., once per second, to the
server. The requests contain a sequence number, and typically a
timestamp. The requests are echoed by the server, except the server
may add a few options. For each request, the server sends two
replies. One reply is unicast back to the source IP address and
source UDP port of the request, while another is multicast to the
requested multicast group G and the source UDP port of the request.
Both replies are sent from the same port on which the request was
received. The server should specify the TTL used for both the
unicast and multicast messages (we recommend at least 64) by
including a TTL option; allowing the client to compute the number of
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hops. The client should leave the channel/group when it has finished
its measurements.
By use of this protocol, a client can obtain information about
several multicast delivery characteristics. First, by receiving
unicast replies, it can verify that the server is receiving the
unicast requests, is operational and responding. Hence, provided
that the client receives unicast replies, a failure to receive
multicast indicates either a multicast problem or a multicast
administrative restriction. If it does receive multicast, it knows
not only that it can receive; it may also estimate the amount of time
it took to establish the multicast tree (at least if it is in the
range of seconds), whether there are packet drops, and the length and
variation of Round Trip Times (RTT). For unicast, the RTT is the
time from when the unicast request is sent to when the reply is
received. The measured multicast RTT also references the client's
unicast request. By use of the TTL option specifying the TTL of the
replies when they are originated, the client can also determine the
number of router hops it is from the source. Since similar
information is obtained in the unicast replies, the host may compare
its multicast and unicast results and is able to check for
differences in the number of hops, RTT, etc. The number of multicast
hops and changes in the number of hops over time, may also reveal
details about the multicast tree and multicast tree changes. E.g.,
with PIM-SM one may be able to tell whether the forwarding is on a
shared or source-specific tree and when an eventual switch occurs.
Provided that the server sends the unicast and multicast replies
nearly simultaneously, the client may also be able to measure the
difference in one way delay for unicast and multicast on the path
from server to client, and also differences in delay. Servers may
optionally specify a timestamp. This may be useful since the unicast
and multicast replies can not be sent simultaneously (the delay
depending on the host's operating system and load).
3. Protocol specification
There are four different message types. There are Echo Request and
Echo Reply messages used for the actual measurements; there is an
Init message that SHOULD be used to initialise a ping session and
negotiate which group to use; and finally a Server Response message
that is mainly used in response to the Init message. The messages
MUST always be in network byte order. UDP checksums MUST always be
used.
The messages share a common format: one octet specifying the message
type, followed by a number of options in TLV (Type, Length and Value)
format. This makes the protocol easily extendible. The Init message
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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.
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
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0, the value field is not included.
3.2. Defined Options
This document defines the following options: Version (0), Client ID
(1), Sequence Number (2), Client Timestamp (3), Multicast Group (4),
Option Request Option (5), Server Information (6), TTL (9), Multicast
Prefix (10), Session ID (11) and Server Timestamp (12). Values 7 and
8 are reserved. The options are defined below.
Version, type 0
Length MUST be 1. This option MUST always be included in all
messages, and the value MUST be set to 2 (in decimal). Note
that there are older implementations of ssmping that only
partly follow this specification. They can be regarded as
version 1 and do not use this option. If a server receives a
message with a version other than 2 (or missing), the server
SHOULD (unless it supports the particular version) send a
Response message back with version set to 2. Client ID and
Sequence Number options SHOULD be echoed if present. It SHOULD
not include any other options. A client receiving a response
with a version other than 2, MUST (unless it supports the
particular version), stop sending requests to the server.
Client ID, type 1
Length MUST be non-zero. A client SHOULD always include this
option in all messages (both Init and Request). The client may
use any value it likes to be able to detect whether a reply is
a reply to its Init/Request or not. A server should treat this
as opaque data, and MUST echo this option back in the reply if
present (both Server Response and Reply). The value might be a
process ID, perhaps process ID combined with an IP address
because it may receive multicast responses to queries from
other clients. It is left to the client implementer how to
make use of this option.
Sequence Number, type 2
Length MUST be 4. A client MUST always include this in Request
messages and MUST NOT include it in Init messages. A server
replying to a Request message MUST copy it into the Reply (or
Server Response message on error). This contains a 32 bit
sequence number. The values would typically start at 1 and
increase by one for each request in a sequence.
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Client Timestamp, type 3
Length MUST be 8 bytes. A client SHOULD include this in
Request messages and MUST NOT include it in Init messages. A
server replying to a Request message MUST copy it into the
Reply. The timestamp specifies the time when the Request
message is sent. The first 4 bytes specify the number of
seconds since the Epoch (0000 UTC Jan 1, 1970). The next 4
bytes specify the number of microseconds since the last second
since the Epoch.
Multicast Group, type 4
Length MUST be greater than 2. It MAY be used in Server
Response messages to tell the client what group to use in
subsequent Request messages. It MUST be used in Request
messages to tell the server what group address to respond to
(this group would typically be previously obtained in a Server
Response message). It MUST be used in Reply messages (copied
from the Request message). It MUST NOT be used in Init
messages. The format of the option value is as below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Family | Multicast group address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ .... |
The address family is a value 0-65535 as assigned by IANA for
Internet Address Families [4]. This is followed by the group
address. Length of the option value will be 6 for IPv4, and 18
for IPv6.
Option Request Option, type 5
Length MUST be greater than 1. This option MAY be used in
client messages (Init and Request messages). A server MUST NOT
send this option, except that if it is present in a Request
message, the server MUST echo it in replies (Reply message) to
the Request. This option contains a list of option types for
options that the client is requesting from the server. Support
for this option is optional for both clients and servers. The
length of this option will be a non-zero even number, since it
contains one or more option types that are two octets each.
The format of the option value is as below.
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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
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..... |
This option might be used by the client to ask the server to
include options like Timestamp or Server Information. A client
MAY request Server Information in Init messages; it MUST NOT
request it in other messages. A client MAY request a Timestamp
in Request messages; it MUST NOT request it in other messages.
Subject to enforcing the above restrictions, a server
supporting this option SHOULD include the requested options in
responses (Reply messages) to the Request containing the Option
Request Option. The server may according to implementation or
local configuration, not necessarily include all the requested
options, or possibly none. Any options included are appended
to the echoed options, similar to other options included by the
server.
Server Information, type 6
Length MUST be non-zero. It MAY be used in Server Response
messages and MUST NOT be used in other messages. Support for
this option is optional. A server supporting this option
SHOULD add it in Server Response messages if and only if
requested by the client. The value is a UTF-8 string that
might contain vendor and version information for the server
implementation. It may also contain information on which
options the server supports. An interactive client MAY support
this option, and SHOULD then allow a user to request this
string and display it.
Reserved, type 7
This option code value was used by early implementations for an
option that is now deprecated. This option should no longer be
used. Clients MUST NOT use this option. Servers MUST treat it
as an unknown option (not process it if received, but if
received in a Request message, it MUST be echoed in the Reply
message).
Reserved, type 8
This option code value was used by early implementations for an
option that is now deprecated. This option should no longer be
used. Clients MUST NOT use this option. Servers MUST treat it
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as an unknown option (not process it if received, but if
received in a Request message, it MUST be echoed in the Reply
message).
TTL, type 9
Length MUST be 1. This option contains a single octet
specifying the TTL of a Reply message. Every time a server
sends a unicast or multicast Reply message, it SHOULD include
this option specifying the TTL. This is used by clients to
determine the number of hops the messages have traversed. It
MUST NOT be used in other messages. A server SHOULD specify
this option if it knows what the TTL of the Reply will be. In
general the server can specify a specific TTL to the host
stack. Note that the TTL is not necessarily the same for
unicast and multicast.
Multicast Prefix, type 10
Length MUST be greater than 2. It MAY be used in Init messages
to request a group within the prefix(es), it MAY be used in
Server Response messages to tell the client what prefix(es) it
may try to obtain a group from. It MUST NOT be used in
Request/Reply messages. Note that this option MAY be included
multiple times to specify multiple prefixes.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Family | Prefix Length |Partial address|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ .... |
The address family is a value 0-65535 as assigned by IANA for
Internet Address Families [4]. 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.
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Session ID, type 11
Length MUST be non-zero. A server SHOULD include this in
Server Response and Reply messages. If a client receives this
option in a message, the client MUST echo the Session ID option
in subsequent Request messages, with the exact same value,
until the next message is received from the server. If the
next message from the server has no Session ID or a new Session
ID value, the client should do the same, either not use the
Session ID, or use the new value. The Session ID may help the
server in keeping track of clients and possibly manage per
client state. The value of a new Session ID should be chosen
pseudo randomly so that it is hard to predict. This can be
used to prevent spoofing of the source address of Request
messages, see the Security Considerations for details.
Server Timestamp, type 12
Length MUST be 8 bytes. A server supporting this option,
SHOULD include it in Reply messages, if requested by the
client. The timestamp specifies the time when the Reply
message is sent. The first 4 bytes specify the number of
seconds since the Epoch (0000 UTC Jan 1, 1970). The next 4
bytes specify the number of microseconds since the last second
since the Epoch.
4. Packet Format
The format of all messages is a one octet message type, directly
followed by a variable number of options.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Options ... |
+-+-+-+-+-+-+-+-+ . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+- .....
There are four message types defined. Type 81 (the character Q in
ASCII) specifies an Echo Request (Query). Type 65 (the character A
in ASCII) specifies an Echo Reply (Answer). Type 73 (the character I
in ASCII) is an Init message, and type 83 (the character S in ASCII)
is a Server Response message.
The options directly follow the type octet and are not aligned in any
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way (no spacing or padding), i.e., options might start at any octet
boundary. The option format is specified above.
5. Message types and options
There are four message types defined. We will now describe each of
the message types and which options they may contain.
Init, type 73
This message is sent by a client to request information from a
server. It is mainly used for requesting a group address, but
it may also be used to check which group prefixes the server
may provide groups from, or other server information. It MUST
include a Version option, and SHOULD include a Client ID. It
MAY include Option Request and Multicast Prefix Options. This
message is a request for a group address if and only if it
contains Multicast Prefix options. If multiple Prefix options
are included, they should be in prioritised order. I.e., the
server will consider the prefixes in the order they are
specified, and if it finds a group for a prefix, it will only
return that one group, not considering the remaining prefixes.
Server Response, type 83
This message is sent by a server. Either as a response to an
Init, or in response to a Request. When responding to Init, it
may provide the client with a multicast group (if requested by
the client), or it may provide other server information. In
response to a Request, the message tells the client to stop
sending Requests. The Version option MUST always be included.
Client ID and Sequence Number options are echoed if present in
the client message. When providing a group to the client, it
includes a Multicast Group option. It SHOULD include Server
Information and Prefix options if requested.
Echo Request, type 81
This message is sent by a client, asking the server to send
unicast and multicast replies. It MUST include Version,
Sequence Number and Multicast Group options. If the last
message (if any) received from the server contained a Session
ID, then this MUST also be included. It SHOULD include Client
ID and Client Timestamp options. It MAY include an Option
Request option.
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Echo Reply, type 65
This message is sent by a server in response to an Echo Request
message. This message is always sent in pairs, one as unicast
and one as multicast. The contents of the messages are mostly
the same. The server echoes most of the options from the Echo
Request (any options in the Request that are unsupported by the
server, are always echoed). The only option that may be
present in the Request which is not always echoed, is the
Session ID option. In most cases the server would echo it, but
the server may also change or omit it. The two Reply messages
SHOULD both contain a TTL option (not necessarily equal), and
both SHOULD also contain Server Timestamps (not necessarily
equal) when requested.
For the reader's convenience we provide the matrix below, showing
what options can go in what messages.
Option / Message Type | Init | Server Response | Request | Reply |
-----------------------------------------------------------------+
Version (0) | MUST | ECHO | MUST | ECHO |
Client ID (1) |SHOULD| ECHO | SHOULD | ECHO |
Sequence Number (2) | NOT | ECHO | MUST | ECHO |
Client Timestamp (3) | NOT | NOT | SHOULD | ECHO |
Multicast Group (4) | NOT | MAY | MUST | ECHO |
Option Request (5) | MAY | NOT | MAY | ECHO |
Server Information (6)| NOT | RQ | NOT | NOT |
Reserved (7) | NOT | NOT | NOT | ECHO |
Reserved (8) | NOT | NOT | NOT | ECHO |
TTL (9) | NOT | NOT | NOT |SHOULD |
Multicast Prefix (10) | MAY | MAY | NOT | NOT |
Session ID (11) | NOT | MAY | ECHO | MAY |
Server Timestamp (12) | NOT | NOT | NOT | RQ |
NOT means that the option MUST NOT be included. ECHO for a server
means that if the option is specified by the client, then the server
MUST echo the option in the response, with the exact same option
value. ECHO for a client means that it MUST echo the option it got
in the last message from the server in any subsequent messages it
sends. RQ means that the server SHOULD include the option in the
response, when requested by the client using the Option Request
option.
6. Client Behaviour
We will consider how a typical interactive client using the above
protocol would behave. A client need only require a user to specify
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the unicast address of the server. It can then send an Init message
with a prefix option containing the desired address family and zero
prefix length (wildcard entry). The server is then free to decide
which group, from the specified family, it should return. A client
may also allow a user to specify group address(es) or prefix(es) (for
IPv6, the user may only be required to specify a scope or an RP
address, from which the client can construct the desired prefix,
possibly embedded-RP). From this the client can specify one or more
prefix options in an Init message to tell the server which address it
would prefer. If the user specifies a group address, that can be
encoded as a prefix of maximal length (e.g., 32 for IPv4). The
prefix options are in prioritised order, i.e., the client should put
the most preferred prefix first.
If the client receives a Server Response message containing a group
address it can start sending Request messages, see the next
paragraph. If there is no group address option, it would typically
exit with an error message. The server may have included some prefix
options in the Server Response. The client may use this to provide
the user some feedback on what prefixes or scopes are available.
Assuming the client got a group address in a Server Response, it can
start pinging. Before it does that, it should let the user know
which group is being used. Normally, a client should send at most
one ping request per second. When sending ping Requests, the client
must always include the group option. If the last message from the
server contained a Session ID, then it must also include that with
the same value. Typically it would receive a Session ID in a Server
Response together with the group address, and then the ID would stay
the same during the entire ping sequence. However, if for instance
the server process is restarted, it may still be possible to continue
pinging but the Session ID may be changed by the server. Hence a
client implementation must always use the last Session ID it
received, and not necessarily the one from the Server Response
message. If a client receives a Server Response message in response
to a Request message (that is, a Server Response message containing a
sequence number), this means there is an error and it should stop
sending Requests. This may for instance happen after server restart.
The client may allow the user to request server information. If the
user requests server information, the client can send an Init message
with no prefix options, but with an Option Request Option, requesting
the server to return a Server Information option. The server will
return server information if supported, and it may also return a list
of prefixes it supports. It will however not return a group address.
The client may also try to obtain only a list of prefixes by sending
an Init message with no prefixes and not requesting any specific
options.
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Note that a client may pick a multicast group and send Request
messages without first going through the Init - Server Response
negotiation. If this is supported by the server and the server is
okay with the group used, the server can then send Reply messages as
usual. If the server is not okay, it will send a Server Response
telling the client to stop.
7. Server Behaviour
We will consider how a typical server using the above protocol would
behave. First we consider how to respond to Init messages. If the
Init message contains prefix options, the server should look at them
in order and see if it can assign a multicast address from the given
prefix. The server would be configured, possibly have a default,
specifying which groups it can offer. It may have a large pool just
picking a group at random, possibly choose a group based on hashing
of the client's IP address or identifier, or just use a fixed group.
A server could possibly decide whether to include site scoped group
ranges based on the client's IP address. It is left to the server to
decide whether it should allow the same address to be used
simultaneously by multiple clients. If the server finds a suitable
group address, it returns this in a group option in a Server Response
message. The server should additionally include a Session ID. This
may help the server if it is to keep some state, for instance for
making sure the client uses the group it got assigned. A good
Session ID would be a pseudo random byte string that is hard to
predict. If the server cannot find a suitable group address, or if
there were no prefixes in the Init message, it may send a Server
Response message containing prefix options listing what prefixes may
be available to the client. Finally, if the Init message requests
the Server Information option, it should include that.
When the server receives a Request message, it may first check that
the group address and Session ID (if provided) are valid. If the
server is satisfied, it will send a unicast Reply message back to the
client, and also a multicast Reply message to the group address. The
Reply messages contain the exact options and in the same order, as in
the Request, and after that the server adds a TTL option and
additional options if needed. E.g., it may add a timestamp if
requested by the client. If the server is not happy with the Request
(bad group address or Session ID, request is too large etc), it may
send a Server Response message asking the client to stop. This
Server Response must echo the sequence number from the Request. This
Server Response may contain group prefixes from which a client can
try to request a group address. The unicast and multicast Reply
messages have identical UDP payload apart from possibly TTL and
timestamp option values.
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Note that the server may receive Request messages with no prior Init
message. This may happen when the server restarts or if a client
sends a Request with no prior Init message. The server may go ahead
and respond if it is okay with the group used. In the responses it
may add a Session ID which will then be in later requests from the
client. If the group is not okay, the server sends back a Server
Response. The Response is just as if it got an Init message with no
prefixes. If the server adds or modifies the SessionID in replies,
it must use the exact same SessionID in the unicast and multicast
replies.
By default, a server should perform rate limiting and for a given
client, respond to at most one Request message per second. A leaky
bucket algorithm is suggested, where the rate can be higher for a few
seconds, but the average rate should by default be limited to a
message per client per second.
8. Recommendations for Implementers
The protocol as specified is fairly flexible and leaves a lot of
freedom for implementers. In this section we present some
recommendations.
Server administrators should be able to configure one or multiple
group prefixes in a server implementation. When deploying servers on
the Internet and in other environments, the server administrator
should be able to restrict the server to respond to only a few
multicast groups which should not be currently used by multicast
applications. A server implementation should also provide
flexibility for an administrator to apply various policies to provide
one or multiple group prefixes to specific clients, e.g., site scoped
addresses for clients that are inside the site. Clients could be
identified by their IP address provided that clients are required to
send Init messages, and they receive an unpredictable Session ID.
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 UDP guidelines [6], in particular if clients are to
send, or servers are to accept, requests at rates exceeding one per
second.
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9. 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 [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, 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
10. IANA Considerations
IANA is requested to provide a UDP port number for use by this
protocol, and also to provide registries for message and option
types.
There should be a message types registry. Message types are in the
range 0-255. Message types 0-191 can be assigned referencing an 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 Section 5
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 Section 3.2.
11. Security Considerations
There are some security issues to consider. One is that a host may
send a request with an IP source address of another host, and make an
arbitrary multicast ping server on the Internet send packets to this
other host. This behaviour is fairly harmless. The worst case is if
the host receiving the unicast replies also happen to be joined to
the multicast group used. In this case, there would be an
amplification effect where the host receives twice as many replies as
there are requests sent.
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In order to help prevent spoofing, a server SHOULD require the client
to send an Init message, and return an unpredictable Session ID in
the response. The ID should be associated with the IP address and
have a limited lifetime. The server SHOULD then only respond to
Request messages that have a valid Session ID associated with the
source IP address of the Request.
Server implementations should allow administrators to restrict which
groups a server responds to, and also perform rate limiting. This is
discussed in Section 8).
12. References
12.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[3] Deering, S. and R. Hinden, "Internet Protocol, Version 6 (IPv6)
Specification", RFC 2460, December 1998.
[4] "IANA, Address Family Numbers",
<http://www.iana.org/assignments/address-family-numbers>.
12.2. Informative References
[5] "ssmping implementation",
<http://www.venaas.no/multicast/ssmping/>.
[6] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines for
Application Designers", draft-ietf-tsvwg-udp-guidelines-10 (work
in progress), August 2008.
Author's Address
Stig Venaas
UNINETT
Trondheim NO-7465
Norway
Email: venaas@uninett.no
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