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Internet Engineering Task Force (IETF) S. Venaas
Request for Comments: 6450 Cisco Systems
Category: Standards Track December 2011
ISSN: 2070-1721
Multicast Ping Protocol
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, such as
multicast tree setup time. This protocol is based on an
implementation of tools called "ssmping" and "asmping".
Status of This Memo
This is an Internet Standards Track document.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Further information on
Internet Standards is available in Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc6450.
Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Venaas Standards Track [Page 1]
RFC 6450 Multicast Ping Protocol December 2011
Table of Contents
1. Introduction ....................................................2
1.1. Requirements Language ......................................2
2. Architecture ....................................................3
3. Protocol Specification ..........................................6
3.1. Option Format ..............................................7
3.2. Defined Options ............................................7
3.3. Packet Format .............................................13
3.4. Message Types and Options .................................13
3.5. Rate Limiting .............................................15
3.5.1. Message Rate Variables .............................16
4. Client Behaviour ...............................................16
5. Server Behaviour ...............................................18
6. Recommendations for Implementers ...............................19
7. IANA Considerations ............................................20
8. Security Considerations ........................................21
9. Acknowledgments ................................................22
10. References ....................................................23
10.1. Normative References .....................................23
10.2. Informative References ...................................23
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, such as multicast tree setup time, the number of hops
the packets have traveled, and packet delay and loss. This
functionality resembles, in part, the ICMP Echo Request/Reply
mechanism [RFC0792], but uses UDP [RFC0768] and requires that both a
client and a server implement this protocol. Intermediate routers
are not required to support this protocol. They forward protocol
messages and data traffic as usual.
This protocol is based on the current implementation of the ssmping
and asmping tools [IMPL], which are widely used by the Internet
community to conduct multicast connectivity tests.
1.1. 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 [RFC2119].
Venaas Standards Track [Page 2]
RFC 6450 Multicast Ping Protocol December 2011
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 protocol is used between clients and servers to check multicast
connectivity. Servers are multicast sources, and clients are
multicast receivers. A server may be configured with a set of ranges
of multicast addresses that can be used for testing, or it may use
some implementation defaults. Depending on the server configuration
or the implementation, it may control which clients (which unicast
addresses) are allowed to use different group ranges, and also
whether clients can select a group address, or if the group address
is selected by the server. Whether several clients are allowed to
simultaneously use the same multicast address also depends on
configuration and/or implementation.
In addition to the above state, a server normally has runtime soft
state. The server must generally perform rate limiting to restrict
the number of client requests it handles. This rate limiting is
per-client IP address. This state need usually only be maintained
for a few seconds, depending on the limit used. If the server
provides unique multicast addresses to clients, it must also have
soft state for tracking which multicast addresses are used by which
client IP address. This state should expire if the server has not
received requests within a few minutes. The exact timeout should
ideally be configurable to cope with different environments. If a
client is expected to perform multicast ping checks continuously for
a long period of time, and to cope with requests not reaching the
client for several minutes, then this timeout needs to be extended.
In order to verify the client IP address, the server should perform a
return routability check by giving the client a non-predictable
session ID. This would then also be part of the server soft state
for that client.
Before it can perform a multicast ping test, a client must know the
unicast address of a server. In addition, it may be configured with
a multicast address or range to use. In that case, the client will
tell the server which group or range it wishes to use. If not, the
server is left to decide the group. Normally, a client sends
Default-Client-Request-Rate requests per second. It may, however, be
configured to use another rate. See the definition of
Default-Client-Request-Rate in Section 3.5.1. Note that the value
can be less than 1.
Venaas Standards Track [Page 3]
RFC 6450 Multicast Ping Protocol December 2011
At runtime, a client generates a client ID that is unique for the
ping test. This ID is included in all messages sent by the client.
Further, if not supplied with a specific group address, the client
will receive from the server a group address that is used for the
ping requests. It may also receive a Session ID from the server.
The client ID, group address, and Session ID (if received) will then
be fixed for all ping requests in this session. When a client
receives replies from a server, it will verify the client ID in the
reply, and ignore it if not matching what it used in the requests.
For each reply, it may print or record information like round trip
time, number of hops, etc. The client may, once a ping session is
ended, calculate and print or record statistics based on the entire
ping session.
The typical protocol usage is as follows:
A server runs continuously to serve requests from clients. A
client has somehow learned the unicast address of the server and
tests the multicast reception from the server.
The client application will then send a unicast message to the
server, asking for a group to use. Optionally, a user may request
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), where G is the multicast group address
specified by the server, and S is the unicast address used to
reach the server.
After joining the group/channel, the client unicasts multicast
ping requests to the server. The requests are sent using UDP with
the destination port set to the standardised multicast ping port
(9903). The requests are sent periodically to the server. The
rate is by default Default-Client-Request-Rate (Section 3.5.1)
requests per second, but the client may be configured to use
another rate. These requests contain a sequence number and,
typically, a timestamp. The requests are echoed by the server,
which may add a few options.
For each request, the server sends two replies. One reply is
unicast to the source IP address and source UDP port of the
requesting client. The other reply is multicast to the requested
multicast group G and the source UDP port of the requesting
client.
Venaas Standards Track [Page 4]
RFC 6450 Multicast Ping Protocol December 2011
Both replies are sent from the same port on which the request was
received. The server should specify the TTL (IPv4 time-to-live or
IPv6 hop-count) used for both the unicast and multicast messages
(TTL of at least 64 is recommended) by including a TTL option.
This allows the client to compute the number of hops. The client
should leave the group/channel when it has finished its
measurements.
By use of this protocol, a client (or a user of the client) can
obtain information about several multicast delivery characteristics.
First, by receiving unicast replies, the client can verify that the
server is receiving the unicast requests, and 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 multicast
traffic but that 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 (RTTs).
For unicast, the RTT is the time from when the unicast request is
sent to the time when the reply is received. The measured multicast
RTT also references the client's unicast request. By 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, such as the number of hops, and RTT.
The number of multicast hops and changes in the number of hops over
time may reveal details about the multicast tree and multicast tree
changes. 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.
Servers may optionally specify a timestamp. This may be useful,
since the unicast and multicast replies cannot be sent simultaneously
(the delay is dependent on the host's operating system and load).
Venaas Standards Track [Page 5]
RFC 6450 Multicast Ping Protocol December 2011
3. Protocol Specification
There are four different message types:
o Echo Request and Echo Reply messages, which are used for the
actual measurements.
o An Init message, which SHOULD be used to initialise a ping session
and negotiate which group to use.
o A Server Response message, which 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.
Message types in the range 0-253 are reserved and available for
allocation in an IANA registry. Message types 254 and 255 are freely
available for experimental use. See Section 7.
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 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, but
it severely limits what information a client can obtain and hence
makes the protocol less useful. However, the server SHOULD add a TTL
option (allowing the client to determine the number of router hops a
reply has traversed), 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 Reply
messages (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.
Venaas Standards Track [Page 6]
RFC 6450 Multicast Ping Protocol December 2011
The size of the protocol messages is generally smaller than the Path
MTU, and fragmentation is not a concern. There may, however, be
cases where the Path MTU is really small, or where a client sends
large requests in order to verify that it can receive fragmented
multicast datagrams. This document does not specify whether Path MTU
Discovery should be performed, etc. A possible extension could be an
option where a client requests Path MTU Discovery and receives the
current Path MTU from the server.
This document defines a number of different options. Some options do
not require processing by servers and are simply returned unmodified
in the reply. There are, however, other client options that the
server may care about, as well as 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 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.
Length (2 octets) specifies the length of the value field. Depending
on the option type, it can be from 0 to 65535.
Value must always be of the specified length. See the respective
option definitions for possible values. If the length is 0, the
value field is not included.
3.2. Defined Options
This document defines the following options: Version (0), Client ID
(1), Sequence Number (2), Client Timestamp (3), Multicast Group (4),
Option Request (5), Server Information (6), TTL (9), Multicast Prefix
(10), Session ID (11), and Server Timestamp (12). Option values 7
and 8 are deprecated and must not be allocated by any future
document. The options are defined below.
Venaas Standards Track [Page 7]
RFC 6450 Multicast Ping Protocol December 2011
Option types in the range 0-65531 are reserved and available for
allocation in an IANA registry. Option types in the range
65532-65535 are not registered and are freely available for
experimental use. See Section 7.
Version, type 0
Length MUST be 1. This option MUST always be included in all
messages, and for the current specified protocol this value
MUST be set to 2 (in decimal). Note that there are
implementations of older revisions of this protocol that only
partly follow this specification. They can be regarded as
version 1 and do not use this option. If a server receives a
message with a version other than 2 (or missing), the server
SHOULD (unless it supports the particular version) send a
Server Response message back with version set to 2. This tells
the client that the server expects version 2 messages. Client
ID and Sequence Number options MUST be echoed if present, so
that a client can be certain it is a response to one of its
messages, and to exactly which message. The server SHOULD NOT
include any other options. A client receiving a response with
a version other than 2 MUST stop sending requests to the server
(unless it supports the particular version).
Client ID, type 1
Length MUST be non-zero. A client SHOULD always include this
option in all messages (both Init and Echo Request). The
client may use any value it likes to detect whether a reply is
a reply to its Init/Echo 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 Echo Reply). The
value might be a pseudo-random byte string that is likely to be
unique, possibly combined with the client IP address.
Predictability is not a big concern here. This is used by the
client to ensure that server messages are in response to its
requests. In some cases, a client may receive multicast
responses to queries from other clients. It is left to the
client implementer how to use this option.
Sequence Number, type 2
Length MUST be 4. A client MUST always include this in Echo
Request messages and MUST NOT include it in Init messages. A
server replying to an Echo Request message MUST copy it into
the Echo Reply (or Server Response message on error). The
sequence number is a 32-bit integer. Values typically start at
1 and increase by one for each Echo Request in a sequence.
Venaas Standards Track [Page 8]
RFC 6450 Multicast Ping Protocol December 2011
Client Timestamp, type 3
Length MUST be 8. A client SHOULD include this in Echo Request
messages and MUST NOT include it in Init messages. A server
replying to an Echo Request message MUST copy it into the Echo
Reply. The timestamp specifies the time when the Echo 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 second
specified in the first 4 bytes. This option would typically be
used by a client to compute round trip times.
Note that while this protocol uses the above 32-bit format, it
would have been better to use another format, such as the one
defined in NTPv4 [RFC5905]. This should be considered for
future extensions of the protocol.
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 Echo Request messages. It MUST be used in Echo
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 Echo Reply
messages (copied from the Echo 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 [ADDRFAMILY]. This is followed by
the group address. Length of the option value will be 6 for
IPv4, and 18 for IPv6.
Option Request, type 5
Length MUST be greater than 1. This option MAY be used in
client messages (Init and Echo Request messages). A server
MUST NOT send this option, except that if it is present in an
Echo Request message, the server MUST echo it in replies (Echo
Reply message) to the Echo Request. This option contains a
list of option types for options that the client is requesting
Venaas Standards Track [Page 9]
RFC 6450 Multicast Ping Protocol December 2011
from the server. Support for this option is OPTIONAL for both
clients and servers. The length of this option will be a
non-zero even number, since it contains one or more option
types that are two octets each. The format of the option value
is as below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..... |
This option might be used by the client to ask the server to
include options like Server 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
Server Timestamp in Echo 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 (Echo Reply messages) to the
Echo 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 [RFC3629] 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. Although support for this is
OPTIONAL, we say that a server SHOULD return it if requested,
since this may be helpful to a user running the client. It is,
however, purely informational; it is not needed for the
protocol to function.
Venaas Standards Track [Page 10]
RFC 6450 Multicast Ping Protocol December 2011
Deprecated, type 7
This option code value was used by implementations of version 1
of this protocol, and is not used in this version. Servers
MUST treat it as an unknown option (not process it if received,
but if received in an Echo Request message, it MUST be echoed
in the Echo Reply message).
Deprecated, type 8
This option code value was used by implementations of version 1
of this protocol, and is not used in this version. Servers
MUST treat it as an unknown option (not process it if received,
but if received in an Echo Request message, it MUST be echoed
in the Echo Reply message).
TTL, type 9
Length MUST be 1. This option contains a single octet
specifying the TTL of an Echo Reply message. Every time a
server sends a unicast or multicast Echo 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 Echo
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. Also note that this option
SHOULD be included even when not requested by the client. The
protocol will work even if this option is not included, but it
limits what information a client can obtain.
If the server did not include this TTL option, there is no
reliable way for the client to know the initial TTL of the Echo
Reply, and therefore the client SHOULD NOT attempt to calculate
the number of hops the message has traversed.
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), and it MAY be used in
Server Response messages to tell the client from what
prefix(es) it may try to obtain a group. It MUST NOT be used
in Echo Request/Reply messages. Note that this option MAY be
included multiple times to specify multiple prefixes.
Venaas Standards Track [Page 11]
RFC 6450 Multicast Ping Protocol December 2011
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 [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.
Session ID, type 11
Length MUST be 4 or larger. A server SHOULD include this in
Server Response messages. If a client receives this option in
a message, the client MUST echo the Session ID option in
subsequent Echo Request messages, with the exact same 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 a pseudo-random byte string that is hard
to predict; see [RFC4086]. The string MUST be at least 4 bytes
long. The Session ID can be used to mitigate spoofing of the
source address of Echo Request messages. We say that this
option SHOULD be used, because it is important for security
reasons. There may, however, be environments where this is not
required. See Section 8, "Security Considerations", for
details.
Server Timestamp, type 12
Length MUST be 8 bytes. A server supporting this option SHOULD
include it in Echo Reply messages, if requested by the client.
The timestamp specifies the time when the Echo 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 second specified in the first
4 bytes. If this option is not included, the protocol will
still work, but it makes it impossible for a client to compute
one-way delay.
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RFC 6450 Multicast Ping Protocol December 2011
Note that while this protocol uses the above 32-bit format, it
would have been better to use another format, such as the one
defined in NTPv4 [RFC5905]. This should be considered for
future extensions of the protocol.
3.3. Packet Format
The format of all 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 | 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. Type 83 (the character S in ASCII) is
a Server Response message.
The options immediately 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.
3.4. Message Types and Options
We will now describe each of the four 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. That is,
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.
Venaas Standards Track [Page 13]
RFC 6450 Multicast Ping Protocol December 2011
Server Response, type 83
This message is sent by a server, either as a response to an
Init, or in response to an Echo Request. When responding to an
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 an Echo Request, the message tells
the client to stop sending Echo Request messages. 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. It SHOULD also include the Session ID
option.
Echo Request, type 81
This message is sent by a client, asking the server to send
unicast and multicast Echo Reply messages. It MUST include
Version, Sequence Number, and Multicast Group options. If a
Session ID was received in a Server Response message, then the
Session ID MUST be included. It SHOULD include Client ID and
Client Timestamp options. It MAY include an Option Request
option.
Echo Reply, type 65
This message is sent by a server in response to an Echo Request
message. This message is always sent in pairs, one as unicast
and one as multicast. The contents of the messages are mostly
the same. The server always echoes all of the options (but
never the Session ID) from the Echo Request. Any options in
the Echo Request that are unsupported by the server are also to
be echoed. The two Echo Reply messages SHOULD both always
contain a TTL option (not necessarily equal). When requested,
both Echo Reply messages SHOULD also contain Server Timestamps
(not necessarily equal).
Venaas Standards Track [Page 14]
RFC 6450 Multicast Ping Protocol December 2011
The matrix below summarises what options can go in what messages.
\ Message Type | Init | Server | Echo | Echo |
Option \ | | Response | Request | Reply |
---------------------- -+--------+----------+---------+--------+
Version (0) | MUST | MUST | 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 |
Deprecated (7) | NOT | NOT | NOT | ECHO |
Deprecated (8) | NOT | NOT | NOT | ECHO |
TTL (9) | NOT | NOT | NOT | SHOULD |
Multicast Prefix (10) | MAY | MAY | NOT | NOT |
Session ID (11) | NOT | SHOULD | ECHO | NOT |
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 only applies to the Session ID
option. If it is present in the Server Response, then it MUST be
present with the exact same option value in the following Echo
Request messages. "RQ" means that the server SHOULD include the
option in the response, when requested by the client using the Option
Request option.
3.5. Rate Limiting
Clients MUST by default send at most Default-Client-Request-Rate
(Section 3.5.1) Echo Request messages per second. Note that the
value can be less than 1. Servers MUST by default perform rate
limiting, to guard against this protocol being used for denial-of-
service (DoS) attacks. A server MUST by default limit the number of
clients that can be served at the same time, and for a given client,
a server MUST also by default respond to, on average, at most
Default-Server-Rate-Limit (see Section 3.5.1) Echo Request messages
per second. Note that the value can be less than 1. Server
implementations should provide configuration options allowing certain
clients to perform more rapid rates of Echo Request messages. If
higher rates are allowed for specific client IP addresses, then Init
messages and the Session ID option MUST be used to help mitigate
spoofing.
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Implementers of applications/tools using this protocol SHOULD
consider the UDP guidelines [RFC5405], in particular if clients are
to send, or servers are to accept, Echo Request messages at rates
exceeding the defaults given in this document. See Section 8,
"Security Considerations", for additional discussion.
3.5.1. Message Rate Variables
There are two variables that control message rates. They are defined
as follows.
Default-Client-Request-Rate
This variable defines the default client echo request rate,
specifying the number of requests per second. Note that the
value may be less than one. For example, a value of 0.1 means
one packet per 10 seconds. The value 1 is RECOMMENDED, but the
value might be too small or large, depending on the type of
network in which the client is deployed. The value 1 is chosen
because it should be safe in most deployments, and it is
similar to what is typically used for the common tool "ping"
for ICMP Echo Request messages.
Default-Server-Rate-Limit
This variable defines the default per-client rate limit
that a server uses for responding to Echo Request messages.
The average rate of replies MUST NOT exceed
Default-Server-Rate-Limit per second. Note that the value may
be less than one. For example, a value of 0.1 means an average
of one packet per 10 seconds. The value 1 is RECOMMENDED, but
the value might be too small or large, depending on the type of
network in which the client is deployed. The value 1 is chosen
because it should be safe in most deployments. This value
SHOULD be high enough to accept the value chosen for the
Default-Client-Request-Rate.
4. Client Behaviour
We will consider how a typical interactive client using the above
protocol would behave.
A client only requires 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 can then decide which group, from the
specified family, it should return. A client may also allow the user
to specify group address(es) or prefix(es) (for IPv6, the user may
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only be required to specify a scope or a Rendezvous Point (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 Echo Request messages; see the next
paragraph. If there is no group address option, the client 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 the multicast pings, after letting the user know which
group is being used. Normally, a client should send at most
Default-Client-Request-Rate (Section 3.5.1) Echo Request messages per
second.
When sending the Echo Request messages, the client must always
include the group option. If the Server Response contained a Session
ID option, then it must also include a Session ID option, with the
exact same value, in the Echo Request messages. If a client receives
a Server Response message in response to an Echo Request (that is, a
Server Response message containing a sequence number), this means
there is an error, and it should stop sending Echo Request messages.
This could 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
that the server 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 not, however, 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.
Although this technique is not recommended, a client may pick a
multicast group and send Echo Request messages without first going
through the Init - Server Response negotiation. If this is supported
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by the server and the server is okay with the group used, the server
can then send Echo Reply messages as usual. If the server is not
okay with the group used, it will send a Server Response telling the
client to stop.
5. Server Behaviour
We will consider how a typical server using the above protocol would
behave, first looking at 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 with a (possibly
default) set of groups it can offer. It may have a large pool and
pick a group at random, or possibly choose a group based on hashing
of the client's IP address or identifier, or simply 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 address
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, to make sure the client uses the
group assigned to it. A good Session ID would be a pseudo-random
byte string that is hard to predict; see [RFC4086]. 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,
the server should include that option.
When the server receives an Echo Request message, it must first check
that the group address and Session ID (if provided) are valid. If
the server is satisfied, it will send a unicast Echo Reply message
back to the client, and also a multicast Echo Reply message to the
group address. The Echo Reply messages contain the exact options
(but no Session ID), and in the same order as in the Echo Request;
after that, the server adds a TTL option and additional options if
needed. For example, it may add a timestamp if requested by the
client. If the server is not happy with the Echo Request (such as
bad group address or Session ID, or request is too large), it may
send a Server Response message asking the client to stop. This
Server Response must echo the sequence number from the Echo Request.
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This Server Response may contain group prefixes from which a client
can try to request a group address. The unicast and multicast Echo
Reply messages have identical UDP payload, apart from possibly TTL
and timestamp option values.
Note that the server may receive Echo Request messages with no prior
Init message. This may happen when the server restarts or if a
client sends an Echo Request with no prior Init message. The server
may go ahead and respond if it is okay with the group and Session ID
(if included) used. If it is not okay with this information, the
server sends back a Server Response.
6. 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 group prefix 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 that 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 group prefix or multiple group prefixes to specific clients,
e.g., site-scoped addresses for clients that are inside the site.
As specified in Section 3.5, for a given client, a server must
by default respond to at most an average rate of
Default-Server-Rate-Limit Echo Request messages 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
Default-Server-Rate-Limit messages per client per second. Server
implementations should provide administrative control of which client
IP addresses to serve, and may also allow certain clients to perform
more rapid rates of Echo Request messages.
If a server uses different policies for different IP addresses, it
should require clients to send Init messages and return an
unpredictable Session ID to help mitigate spoofing. This is an
absolute requirement if exceeding the default rate limit. See the
specification in Section 3.5.
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7. IANA Considerations
IANA has assigned UDP user port 9903 (multicast-ping) for use by this
protocol. IANA also provides registries for message and option
types.
IANA has created a message types registry. Message types are in the
range 0-255. Message types 0-253 are registered following the
procedures for Specification Required from RFC 5226 [RFC5226], while
types 254 and 255 are for experimental use. The registry includes
the messages defined in Section 3.4. A message specification MUST
describe the behaviour with known option types as well as the default
behaviour with unknown option types.
IANA has created an option type registry. Option types 0-65531 are
registered following the procedures for Specification Required from
RFC 5226 [RFC5226], while types 65532-65535 are for experimental use.
The registry should include the options defined in Section 3.2. An
option specification must describe how the option may be used with
the known message types. This includes which message types the
option may be used with.
The initial registry definitions are as follows:
Multicast Ping Protocol Parameters:
Registry Name: Multicast Ping Protocol Message Types
Reference: RFC 6450
Registration Procedures: Specification Required
Registry:
Type Name Reference
----------- ------------------------------------ ----------
65 Echo Reply RFC 6450
73 Init RFC 6450
81 Echo Request RFC 6450
83 Server Response RFC 6450
254-255 Experimental
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RFC 6450 Multicast Ping Protocol December 2011
Registry Name: Multicast Ping Protocol Option Types
Reference: RFC 6450
Registration Procedures: Specification Required
Registry:
Type Name Reference
----------- ------------------------------------ ----------
0 Version RFC 6450
1 Client ID RFC 6450
2 Sequence Number RFC 6450
3 Client Timestamp RFC 6450
4 Multicast Group RFC 6450
5 Option Request RFC 6450
6 Server Information RFC 6450
7 Deprecated RFC 6450
8 Deprecated RFC 6450
9 TTL RFC 6450
10 Multicast Prefix RFC 6450
11 Session ID RFC 6450
12 Server Timestamp RFC 6450
65532-65535 Experimental
8. Security Considerations
There are some security issues to consider. One is that a host may
send an Echo 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 Echo Reply messages also
happens to be joined to the multicast group used. This is less of a
problem for SSM, where also the source address of the server must
match the address joined. In this case, there would be an
amplification effect, where the host receives twice as many replies
as there are requests sent. See below for how spoofing can be
mitigated.
For ASM (Any-Source Multicast), a host could also make a multicast
ping server send multicast packets to a group that is used for
something else, possibly disturbing other uses of that group.
However, server implementations should allow administrators to
restrict which groups a server responds to. The administrator should
then try to configure a set of groups that are not used for other
purposes. Another concern is bandwidth. To limit the bandwidth
used, a server MUST by default limit the number of clients that can
be served at the same time, and a server MUST also by default perform
per-client rate limiting.
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In order to help mitigate spoofing, a server SHOULD require that the
client 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 Echo
Request messages that have a valid Session ID associated with the
source IP address of the Echo Request. Note, however, that a server
is replying with a Server Response message if the Session ID is
invalid. This is used to tell the client that something is wrong and
that it should stop sending requests, and start over if necessary.
This means, however, that someone may spoof a client request, and
have the server send a message back to the client address. One
solution here would be for the server to have a very low rate limit
for the Server Response messages.
Note that the use of a Session ID only to some degree helps mitigate
spoofing. An attacker that is on the path between a client and a
server may eavesdrop the traffic, learn a valid Session ID, and
generate Echo Request messages using this ID. The server will
respond as long as the Session ID remains valid.
This protocol may be used to establish a covert channel between a
multicast ping client and other hosts listening to a multicast group.
A client can, for instance, send an Echo Request containing an
undefined option with arbitrary data. The server would echo this
back in an Echo Reply that may reach other hosts listening to that
group. One solution that should be considered for future protocol
versions is to reply with a hash of the data, rather than simply a
copy of the same data.
9. Acknowledgments
The ssmping concept was proposed by Pavan Namburi, Kamil Sarac, and
Kevin C. Almeroth in the paper "SSM-Ping: A Ping Utility for Source
Specific Multicast" (2004) and also "MPing: A Ping Utility for IP
Multicast" (Sarac and Almeroth, 2004). Mickael Hoerdt contributed
several ideas. Alexander Gall, Nicholas Humfrey, Nick Lamb, and Dave
Thaler have contributed in different ways to the implementation of
the ssmping tools [IMPL]. Many people in communities like the
Trans-European Research and Education Networking Association
(TERENA), Internet2, and the M6Bone (IPv6 multicast network) have
used early implementations of ssmping and provided feedback that
influenced the current protocol. Thanks to Kevin Almeroth, Tony
Ballardie, Bill Cerveny, Toerless Eckert, Marshall Eubanks, 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 document. In particular,
Hugo, Gorry, and Bharat provided lots of input on several revisions
of the document.
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RFC 6450 Multicast Ping Protocol December 2011
10. References
10.1. Normative References
[ADDRFAMILY]
IANA, "Address Family Numbers",
<http://www.iana.org/assignments/address-family-numbers>.
[RFC0768] Postel, J., "User Datagram Protocol", STD 6, RFC 768,
August 1980.
[RFC0792] Postel, J., "Internet Control Message Protocol", STD 5,
RFC 792, September 1981.
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3629] Yergeau, F., "UTF-8, a transformation format of ISO
10646", STD 63, RFC 3629, November 2003.
[RFC4086] Eastlake 3rd, D., Schiller, J., and S. Crocker,
"Randomness Requirements for Security", BCP 106, RFC 4086,
June 2005.
[RFC5226] Narten, T. and H. Alvestrand, "Guidelines for Writing an
IANA Considerations Section in RFCs", BCP 26, RFC 5226,
May 2008.
10.2. Informative References
[IMPL] Venaas, S., "ssmping implementation",
<http://software.uninett.no/ssmping/>.
[RFC5405] Eggert, L. and G. Fairhurst, "Unicast UDP Usage Guidelines
for Application Designers", BCP 145, RFC 5405,
November 2008.
[RFC5905] Mills, D., Martin, J., Ed., Burbank, J., and W. Kasch,
"Network Time Protocol Version 4: Protocol and Algorithms
Specification", RFC 5905, June 2010.
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RFC 6450 Multicast Ping Protocol December 2011
Author's Address
Stig Venaas
Cisco Systems
Tasman Drive
San Jose, CA 95134
USA
EMail: stig@cisco.com
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