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The ssmping protocol specified in this document allows for checking whether one can receive multicast, both Source-Specific Multicast (SSM) and Any-Source Multicast (ASM), as well as to obtain additional multicast related information. This protocol is based on an implementation of tools called ssmping and asmping.
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 (Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” March 1997.) [1].
1.
Introduction
2.
Architecture
3.
Protocol specification
3.1.
Option format
3.2.
Defined Options
4.
Packet Format
5.
Message types and options
6.
Client Behaviour
7.
Server Behaviour
8.
Acknowledgements
9.
IANA Considerations
10.
Security Considerations
11.
References
11.1.
Normative References
11.2.
Informative References
§
Authors' Addresses
§
Intellectual Property and Copyright Statements
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The ssmping protocol specified in this document allows for checking multicast connectivity. Not only reception of multicast (SSM or ASM), but can also provide other information like multicast tree setup time, the number of hops the packets have traveled, as well as the packet delay and loss. This functionality resembles, in part, the ICMP Echo Request/Reply infrastructure, but uses UDP and requires both a client and a server implementing this protocol.
The protocol here specified is based on the actual implementation of the ssmping and asmping tools [3] (, “ssmping implementation,” .) which are widely used by the Internet community to conduct multicast connectivity tests.
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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 usage of an ssmping/asmping session is as follows. A server runs continuously to serve requests from clients. When a user decides to verify the multicast reception from a specific server (knowing one of the server's unicast addresses is required), the client will 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, the client joins an SSM channel (S,G) where S is a unicast address of the target server, or an ASM group G, where G is the group specified by the server.
After joining the channel, the client unicasts ssmping requests to the server. The requests are sent using UDP with destination port set to the standardised ssmping port [TBD]. The requests are sent periodically, e.g., once per second, to the server. The requests contain a sequence number, and typically a timestamp. The requests are echoed back by the server, except the server may add a few options. To each request, the server sends two replies. One as unicast back to the port and address the request was sent from, and also one as multicast back to the port from which the request originated with the requested group G as destination address. The server should specify the TTL used for both the unicast and multicast messages (we recommend at least 64) and includes a TTL option for the client to compute the number of hops. The client should leave the channel/group when it has finished its measurements.
By use of this protocol, a client can obtain information about several multicast delivery characteristics. First, by receiving unicast replies, it can verify that the server is receiving the unicast requests, is operational and responding. Hence, provided that the client receives unicast replies, a failure to receive multicast indicates either a multicast problem or a multicast administrative restriction. If it does receive multicast, it knows not only that it can receive; it may also estimate the amount of time it took to establish the multicast tree (at least if it is in the range of seconds), whether there are packet drops, and the length and variation of Round Trip Times (RTT). For unicast, the RTT is the time from when the unicast request is sent to when the reply is received. The measured multicast RTT also references the client's unicast request. By use of the TTL option specifying the TTL of the replies when they are originated, the client can also determine the number of router hops it is from the source. Since similar information is obtained in the unicast replies, the host may compare its multicast and unicast results and is able to check for differences in the number of hops, RTT, etc. Provided that the server sends the unicast and multicast replies nearly simultaneously, it may also be able to measure the difference in one way delay for unicast and multicast on the path from server to client, and also differences in delay. Servers may optionally specify a timestamp. This may be useful since the unicast and multicast replies can not be sent simultaneously (the delay depending on the host's operating system and load), or whether the client and server have synchronised clocks.
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There are four different ssmping message types. There are Echo Request and Echo Reply messages used for the actual measurements; there is an Init message that SHOULD be used to initialise a ping session and negotiate which group to use; and finally a Server Response message that is mainly used in response to the Init message.
The ssmping messages share a common format: one octet specifying the message type, followed by a number of options in TLV (Type, Length and Value) format. This makes the protocol easily extendible. The Init message generally contains some prefix options asking the server for a group from one of the specified prefixes. The server responds with a Server Response message that contains the group address to use, or possibly prefix options describing what multicast groups the server may be able to provide. For an Echo Request the client generally includes a number of options, and a server may simply echo the content back (only changing the message type), without inspecting the options. However, the server SHOULD add a TTL option, and there are some other options that a server implementation MAY support, e.g., the client may ask for certain information or a specific behaviour from the server. The Echo Replies (one unicast and one multicast) MUST first contain the exact options from the request (in the same order), and then, immediately following, options appended by 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, 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.
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All options are TLVs formatted as specified below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type (2 octets) specifies the option. The different options are defined below.
Length (2 octets) specifies the length of the value field. Depending on the option type, it can be from 0 to 65535.
Value. The value must always be of the specified length. See the respective option definitions for possible values. If the length is 0, the value field is not included.
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Version, type 0. Length MUST be 1. This option MUST always be included in all messages, and the value MUST be set to 2 (in decimal). Note that there are older implementations of ssmping that only partly follow this specification. They can be regarded as version 1 and do not use this option.
Client ID, type 1. Length MUST be non-zero. A client SHOULD always include this option in all messages (both Init and Request). The client may use any value it likes to be able to detect whether a reply is a reply to this Init/Request or not. A server should treat this as opaque data, and simply leave it unchanged in the reply (both Server Response and Reply). The value might be a process ID, perhaps process ID combined with an IP address because it may receive multicast responses to queries from other clients. It is left to the client implementor how to make use of this option.
Sequence number, type 2. Length MUST be 4. A client MUST always include this in Request messages and MUST NOT include it in Init messages. A server replying to a Request message MUST copy it into the Reply (or Server Response message on error). This contains a 32 bit sequence number. The values would typically start at 1 and increase by one for each request in a sequence.
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 MUST copy it into the Reply. In addition, a server supporting this option, SHOULD include it in Reply messages, if requested by the client. Note that this means that the option may be present in the Reply message twice; both a client timestamp as part of the echoed Request, and another timestamp added by the server. The timestamp specifies the time when the message (query or reply) is sent. The first 4 bytes specify the number of seconds since the Epoch (beginning of the year 1970). The next 4 bytes specify the number of microseconds since the last second since the Epoch.
Multicast group, type 4. Length MUST be greater than 1. It MAY be used in Server Response messages to tell the client what group to use in subsequent Request messages. It MUST be used in Request messages to tell the server what group address to respond to (this group would typically be previously obtained in a Server Response message). It MUST be used in Reply messages (copied from the Request message). It MUST NOT be used in Init messages. The format of the option value is as below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Family | Multicast group address... |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ .... |
The address family is a value 0-65535 as assigned by IANA for Internet Address Families [2] (, “IANA, Address Family Numbers,” .). This is followed by the group address. For IPv4 the option value length will be 6, for IPv6 18.
Option Request Option, type 5. Length MUST be greater than 1. This option MAY be used in Init and Request messages. It MUST NOT be used in any other messages, except that a server will, in a Reply, echo back this option if present in the Request. This option contains a list of option types for options that the client is requesting from the server. Support for this option is optional for both clients and servers. The length of this option will be a non-zero even number, since it contains one or more option types that are two octets each. The format of the option value is as below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..... |
This option might be used by the client to ask the server to include options like Timestamp or Server Information. A client MAY request Server Information in Init messages; it MUST NOT request it in other messages. A client MAY request a Timestamp in Request messages; it MUST NOT request it in other messages.
Server Information, type 6. Length MUST be non-zero. It MAY be used in Server Response messages and MUST NOT be used in other messages. Support for this option is optional. A server supporting this option SHOULD add it in Server Response messages if and only if requested by the client. The value is a UTF-8 string that might contain vendor and version information for the server implementation. It may also contain information on which options the server supports. An interactive client MAY support this option, and SHOULD then allow a user to request this string and display it.
Type 7, Reserved. This option code value was used by early implementations for an option that is now deprecated. This option should no longer be used. Clients MUST not use this option, and Servers MUST ignore it.
Pad, type 8. Length can be anything, including 0. This option is used by clients to increase the request size in order to have the server deliver responses of a particular size. If the server adds any options when responding, it should, if possible make the response the same size as the request by shrinking the pad option (i.e., not simply including it, as is, like all other client options). If the options added by the server consume as much space as the pad option does, or more, the server should remove the entire pad option.
TTL, type 9. Length MUST be 1. This option contains a single octet specifying the TTL of a Reply message. Every time a server sends a unicast or multicast Reply message, it SHOULD include this option specifying the TTL. This is used by clients to determine the number of hops the messages have traversed. It MUST NOT be used in other messages. Although this option is not absolutely required, the server is expected to use it if it knows what the TTL of the Reply will be. In general the server can specify a specific TTL to the host stack.
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.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Address Family | Prefix Length |Partial address|
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ .... |
The address family is a value 0-65535 as assigned by IANA for Internet Address Families [2] (, “IANA, Address Family Numbers,” .). This is followed by a prefix length (0-32 for IPv4, 0-128 for IPv6), and finally a group address. The group address need only contain enough octets to cover the prefix length bits (e.g., there need be no group address if the prefix length is 0, the group address would have to be 3 octets long if the prefix length is 17-24). Any bits past the prefix length MUST be ignored. For IPv4 the option value length will be 3-7, while for IPv6 3-19.
Session ID, type 11. Length MUST be non-zero. A server MAY include this in Server Response and Reply messages. If a client receives this option in a message, the client MUST echo the Session ID option in Reply messages, with the exact same value, until the next message is received from the server. If the next message from the server has no Session ID or a new Session ID value, the client should do the same, either not use the Session ID, or use the new value.
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The format of all messages is a one octet message type, directly followed by a variable number of options.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Option |
+-+-+-+-+-+-+-+-+ . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option |
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.
.
.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option |
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
There are four message types defined. Type 81 (the character Q in ASCII) specifies an Echo Request (Query). Type 65 (the character A in ASCII) specifies an Echo Response (Answer). Type 73 (the character I in ASCII) is an Init message, and type 83 (the character S in ACII) is a Server Response message.
The options directly follow the type octet and are not aligned in any way (no spacing or padding), i.e., options might start at any octet boundary. The option format is specified above.
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For the readers convenience we provide the matrix below, showing what options can go in what messages.
Option / Message Type | Init | Server Response | Request | Reply | -----------------------------------------------------------------+ Version (0) | MUST | MUST | MUST | ECHO | Client ID (1) |SHOULD| ECHO | SHOULD | ECHO | Sequence number (2) | NOT | ECHO | MUST | ECHO | Timestamp (3) | NOT | NOT | SHOULD |ECHO/RQ| Multicast group (4) | NOT | MAY | MUST | ECHO | Option Request (5) | MAY | NOT | MAY | ECHO | Server Information (6)| NOT | RQ | NOT | NOT | Reserved (7) | NOT | NOT | NOT | NOT | Pad (8) | NOT | NOT | MAY | ECHO* | TTL (9) | NOT | NOT | NOT |SHOULD | Multicast prefix (10) | MAY | MAY | NOT | NOT | Session ID (11) | NOT | MAY | ECHO | MAY |
NOT means that the option MUST NOT be included. ECHO for a server means that if the option is specified by the client, then the server MUST echo the option back in the response, with the exact same option value. The exception is ECHO* where the option value may be modified. ECHO for a client means that it MUST echo the option it got in the last message from the server in the following 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.
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We will consider how a typical interactive client using the above protocol would behave. A client need only require a user to specify the unicast address of the server. It can then send an Init message with a prefix option containing the desired address family and zero prefix length. The server is then free to decide which group it should return. A client may also allow a user to specify a group address(es) or prefix(es) (for IPv6, the user may only be required to specify a scope or an RP address, from which the client can construct the desired prefix, possibly embedded-RP). From this the client can specify one or more prefix options in an Init message to tell the server which address it would prefer. If the user specifies a group address, that can be encoded as a prefix of maximal length (e.g. 32 for IPv4). The prefix options are in prioritised order, i.e., the client should put the most preferred prefix first.
If the client receives a Server Response message containing a group address it can start sending Request messages, see the next paragraph. If there is no group address option, it would typically exit with an error message. The server may have included some prefix options in the Server Response. The client may use this to provide the user some feedback on what prefixes or scopes are available.
Assuming the client got a group address in a Server Response it can start pinging. Before it does that it should let the user know which group is being used. When sending ping Requests the client must always specifiy the group option. If the last message from the server contained a Session ID, then it MUST 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 have an option for the user to obtain server information. If the user asks for server information, the client can send an Init message with no prefix options, but with an Option Request Option, requesting the server to return a Server Information option. The server will return server information if supported, and it may also return a list of prefixes it supports. It will however not return a group address. The client may also try to obtain only a list of prefixes by sending an Init message with no prefixes and not requesting any specific options.
Note that a client may pick a multicast group and send Request messages without first going through the Init - Server Response negotiation. If this is supported by the server and the server is okay with the group used, the server can then send Reply messages as usual. If the server is not okay, it will send a Server Response telling the client to stop and possibly pick a new group.
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We will consider how a typical server using the above protocol would behave. First we consider how to respond to Init messages. If the Init message contains prefix options, the server should look at them in order and see if it can assign a multicast address in the given range. The server would be configured, possibly have a default, specifying which groups it can offer. It may have a large pool just picking a group at random, possibly choose a group based on hashing of the clients IP address or identifier, or just use a fixed group. It is left to the server to decide whether it should allow the same address to be used simultaneously by multiple clients. If the server finds a suitable group address, it returns this in a group option in a Server Response message. The server may additionally include a Session ID. This may help the server if it is to keep some state, for instance for making sure the client uses the group it got assigned. A good Session ID would be a random byte string that is hard to predict. If the server cannot find a suitable group address, or if there were no prefixes in the Init message, it may send a Server Response message containing prefix options listing what prefixes may be available to the client. Finally, if the Init message requests the Server Information option, it should include that.
When the server receives a Request message, it may first check that the group address and Session ID (if provided) are valid. If the server is satisfied it will send a unicast Reply message back to the client, and also a multicast Reply message to the group address. The Reply messages contain the exact options and in the same order, as in the Request (only exception is the pad option), 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 group address and Session ID, it may send a Server Response message asking the client to stop. This Server Response must echo the sequence number from the Request. This Server Response may contain which prefixes the client can try to request addresses from. The unicast and multicast Reply messages have identical UDP payload apart from possibly TTL and timestamp option values.
Note that the server may receive Request messages with no prior Init message. This may happen when the server restarts or if a client sends a Request with no prior Init message. The server may go ahead and respond if it is okay with the group used. In the responses it may add a Session ID which will then be in later requests from the client. If the group is not okay, the server sends back a Server Response. The Response is just as if it got an Init message with no prefixes. If the server adds or modifies the SessionID in replies, it MUST use the exact same SessionID in the unicast and multicast replies.
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The ssmping concept was proposed by Pavan Namburi, Kamil Sarac and Kevin C. Almeroth in the paper SSM-Ping: A Ping Utility for Source Specific Multicast, and also the Internet Draft draft-sarac-mping-00.txt. Mickael Hoerdt has contributed with several ideas. Alexander Gall, Nicholas Humfrey, Nick Lamb and Dave Thaler have contributed in different ways to the implementation of the ssmping tools at [3] (, “ssmping implementation,” .). Many people in communities like TERENA, Internet2 and the M6Bone have used early implementations of ssmping and provided feedback that have influenced the current protocol. Thanks to Kevin Almeroth, Toerless Eckert, Gorry Fairhurst, Liu Hui, Bharat Joshi, Olav Kvittem, Kamil Sarac, Pekka Savola, Trond Skjesol and Cao Wei for reviewing and providing feedback on this draft.
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IANA is requested to provide a UDP port number for use by this protocol, and also provide a registry for ssmping option types.
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There are some security issues to consider. One is that a host may send a request with an IP source address of another host, and make an arbitrary ssmping server on the Internet send packets to this other host. This hehaviour is fairly harmless. The worst case is if the host receiving the unicast replies also happen to be performing an ssmping test towards that particular server. In this unlikely event, there would be an amplification effect where the host receives twice as many replies as there are requests sent. An ssmping server should perform rate limiting, to guard against this function being used as a DoS attack. A client should also use the client ID option to distinguish replies to its own requests from replies that might be to other requests.
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| [1] | Bradner, S., “Key words for use in RFCs to Indicate Requirement Levels,” BCP 14, RFC 2119, March 1997 (TXT, HTML, XML). |
| [2] | “IANA, Address Family Numbers.” |
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| [3] | “ssmping implementation.” |
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| Stig Venaas | |
| UNINETT | |
| Trondheim NO-7465 | |
| Norway | |
| Email: | venaas@uninett.no |
| Hugo Santos | |
| NEC Europe Ltd. | |
| Kurfuersten-Anlage 36 | |
| Heidelberg 69115 | |
| Germany | |
| Email: | hugo.santos@nw.neclab.eu |
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