Network Working Group S. Venaas
Internet-Draft UNINETT
Intended status: Informational May 22, 2007
Expires: November 23, 2007
ssmping Protocol
draft-ietf-mboned-ssmping-00
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Copyright (C) The IETF Trust (2007).
Abstract
ssmping is a tool that is used to check whether one can receive SSM,
as well as obtaining some additional information. ssmping requires
both a client and a server supporting the ssmping protocol to work.
We here specify this protocol.
Requirements Language
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
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"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in RFC 2119 [1].
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Protocol description . . . . . . . . . . . . . . . . . . . . . 3
3. Options . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Option format . . . . . . . . . . . . . . . . . . . . . . 4
3.2. Defined Options . . . . . . . . . . . . . . . . . . . . . 5
4. Packet Format . . . . . . . . . . . . . . . . . . . . . . . . 7
5. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 8
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
7. Security Considerations . . . . . . . . . . . . . . . . . . . 8
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 9
8.1. Normative References . . . . . . . . . . . . . . . . . . . 9
8.2. Informative References . . . . . . . . . . . . . . . . . . 9
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 10
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1. Introduction
ssmping is a tool that is used to check whether one can receive SSM,
and it can also give other information like the time to establish the
tree, number of router hops the packets have traveled, packet delay
and loss. The ssmping functionality resembles ICMP echo request/
reply using UDP and a client and a server that supports the ssmping
protocol. It is used by a client to verify that it can receive
multicast from the server, as well as some additional information.
The protocol as specified here is based on an actual implementation
of a tool [3] that has been found useful by many organisations.
2. Protocol description
Before going into the protocol details we will describe how it is
used and what information it may provide. The typical usage is as
follows. A server runs continuously in order to serve request from
clients. At some point a client application may try to verify
multicast reception from such a server. The client will need to know
a unicast address of a server. The client joins an SSM channel (S,G)
where S is a unicast address of the server, and G is a standardised
multicast group for use by ssmping. After joining the channel, the
client sends ssmping requests as UDP to a standardised ssmping port
and the unicast address of the server. The requests are sent
periodically, e.g. once per second, to the server. The requests
contain a serial number, and typically a timestamp. The requests are
typically, but not necessarily always, simply echoed back by the
server. To each request, the server sends two replies. One as
unicast back to the port and address the request was sourced from,
and also as multicast back to the port the request came from. It is
currently left open which port the request is sourced from, whether
this port should be standardised or not. The TTL or Hop Limit of the
replies are set to 64. The client should leave the SSM channel when
it has finished its measurements.
By use of this protocol, a client can obtain information on several
aspects of the multicast quality. First of all, by receiving unicast
replies, it can verify that the server is receiving the unicast
requests, is operational and responding. Hence provided that the
client receives unicast replies, a failure in receiving multicast is
indeed caused by a multicast problem. If it does receive multicast,
it knows not only that it can receive, but it may get some
information on how long it takes to establish the multicast tree (at
least if it is in the range of seconds), whether there are packet
drops, and the length and variation of round trip times (RTT). For
unicast the RTT is the time from unicast request is sent to when the
reply is received. For multicast we also talk about RTT, but then we
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mean from the unicast request is sent to when the multicast reply is
received. Since the server sets TTL or Hop Limit to 64, it can also
know the number of router hops it is away from the source. By
comparing with the unicast replies, it can see whether there are
differences in RTT and number of hops etc for unicast and multicast.
Provided that the server sends the unicast and multicast replies
nearly simultaneously, it may also be able to measure difference in
one way delay for unicast and multicast on the path from server to
client, and also if there are differences in delay variation.
Servers may optionally specify a timestamp. This may be useful if
the unicast and multicast replies can not be sent nearly
simultaneously, or if the client and server have synchronised clocks.
The ssmping requests and replies have a common format, one octet
specifying the message type, followed by a number of options in TLV
(Type, Length and Value) format. This makes the protocol easily
extendible. Generally the client includes a number of options in the
request, and a server may simply echo the content back (only changing
the message type), without inspecting the options. However, there
are a number of options that a server implementation may support,
where the client may ask for a certain information or behaviour from
the server. In some cases the server will need to add options in the
response. The response will then first contain the exact options
from the request, and then right after those, options appended by the
server.
3. Options
There are a number of different options. Most of the options are
only used by clients and simply echoed back by the server, where the
server doesn't care about their contents. There are however some
client options that the server may care about. There are also server
options that may be requested by the client. Generally a simple
client will only include a few options, and get exactly the same
options and values echoed back. Strictly speaking the protocol could
work without any options. Without sending any options a client would
still be able to tell whether multicast is working or not, however
with the use of some of the basic options a client can obtain a lot
more information.
3.1. Option format
All options are TLVs formatted as specified below.
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0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type (2 octets) specifies the option. The different options are
defined below.
Length (2 octets) specifies the length of the value. Depending on
the option type it can be from 0 to 65535.
Value. The value must always be of the specified length. See the
respective option definitions for possible values. If the length is
0, the value field is not included.
3.2. Defined Options
Client Identifier, type 1. Length MUST be non-zero. Only used by
clients. A client SHOULD include this. The client may use any value
it likes to be able to detect whether a reply is a reply to this
query or not. A server should treat this as opaque data, and simply
leave it unchanged in the reply. The value might be a process ID,
perhaps process ID combined with an IP address because it may receive
multicasted responses to queries from other clients. It is left to
the client implementor how to make use of this.
Sequence number, type 2. Length MUST be 4. Only used by clients. A
client SHOULD include this. This contains a 32 bit sequence number.
The values would typically start at 1 and increase by one for each
request in a sequence.
Timestamp, type 3. Length MUST be 8 bytes. A client SHOULD include
this. A server MAY support this. If supported it SHOULD be included
in the reply if requested by the client. The timestamp specifies the
time when the message (query or reply) is sent. The first 4 bytes
specify the number of seconds since the Epoch (beginning of the year
1970). The next 4 bytes specify the number of microseconds since the
last second since the Epoch.
Multicast group, type 4. Length MUST be greater than 1. It is
optional for clients and servers to support this. It allows a client
to specify which group the server should send to. This is currently
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used by a tool called "asmping" to test ASM connectivity. The server
may have restrictions on which groups can be used. The format of the
option value is as below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Addr Family | Multicast group address... |
+-+-+-+-+-+-+-+-+ .... |
The address family is a value 0-127 as assigned by IANA for Internet
Address Families [2]. This is followed by the group address. For
IPv4 the option value length will be 5, for IPv6 17.
Option Request Option, type 5. Length MUST be greater than 1. The
option contains a list of option types of options that the client
requests from the server. Supporting this is optional for both
clients and servers. The length of this option will be a non-zero
even number, since it contains option types that each are two octets.
The format of the value is as below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option Type | Option Type |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| ..... |
The value might contain an odd number of options, including just one.
This option might be used by the client to ask the server to include
options like timestamp or version.
Version, type 6. Length MUST be non-zero. Supporting this option is
optional. A server supporting this option SHOULD add it if and only
if requested by the client. The value is just unformatted text that
might contain vendor and version information for the server
implementation. It may also contain information on which options the
server supports.
Reply size, type 7. Length MUST be 2 octets. This option is
optional for clients and servers. It can be used to request the
server response to be of a certain size. The value specifies the
desired response size in octets. A server supporting this will if
necessary use the pad option to increase the size of the response. A
server should however not try to make the response shorter due to
this option. That is, it should not omit or shorten any option
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values to try to accommodate this. The response should never be
shorter than if this option were not included. Also, the pad option
requires at least 3 octets, so the server will not pad the response
size if the requested size is not at least 3 octets longer than the
normal response size.
Pad, type 8. Length can be anything, including 0. This option is
used by servers to increase the response size if the client asks for
a reply that is larger than what the server normally would send. The
addition of this option consumes a minimum of 3 octets, so it should
only be added if the requested size is at least 3 octets more than
the size of the normal (non-padded) response.
4. Packet Format
The format of the ssmping messages is a one octet message type,
followed by a variable number of options.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Option |
+-+-+-+-+-+-+-+-+ . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option |
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
.
.
.
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Option |
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
There are two message types defined. Type 81 (the character Q in
ASCII) specifies a query. Type 65 (the character A in ASCII)
specifies a response (answer).
The options follow right after the type octet and are not aligned in
any way (no spacing or padding). I.e., options might start at any
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octet boundary. The option format is specified below
5. Acknowledgements
The ssmping idea was proposed by Pavan Namburi, Kamil Sarac and Kevin
C. Almeroth in the paper SSM-Ping: A Ping Utility for Source Specific
Multicast, and also the Internet Draft draft-sarac-mping-00.txt.
Mickael Hoerdt has contributed with several ideas. Alexander Gall,
Nick Lamb and Dave Thaler have contributed in different ways to my
implementation of the ssmping tools [3]. Hugo Santos has made an
independent implementation of an ssmping server. Many people in
communities like TERENA, Internet2 and the M6Bone have used early
implementations of ssmping and provided feedback that have influenced
the current protocol. Thanks to Olav Kvittem, Kamil Sarac and Trond
Skjesol for reviewing and providing feedback on this draft.
6. IANA Considerations
As currently specified, ssmping would need a well known port number
which the servers listen to. It might be desirable to use SRV
records instead or in addition to this. For IPv6 SSM ssmping should
ideally have a reserved group ID. For the optional ASM functionality
it would be useful to have a reserved IPv6 group ID, this may be the
same as the one used for SSM. It may also be useful to have a
dedicated group for the optional IPv4 ASM functionality. This
section needs further work.
7. Security Considerations
There are some security issues to consider. One is that a host may
send a request with an IP source address of another host, and make a
random ssmping server on the Internet send packets to this other
host. This is fairly harmless. The worst case is if the host
receiving the unicast replies also happen to be performing an ssmping
test towards that particular server. In this unlikely event there
would be an amplification effect where the host receives twice as
many replies as there are requests sent. An ssmping server should
perform rate limiting, to guard against this being used as an DoS
attack. A client should also use the client identifier option to be
able to distinguish replies to its own requests from replies that
might be to other requests. How the protocol should be designed to
cope with rate limiting at the server requires further study. One
possibility might be that the server can choose to send generic
replies, e.g. a packet every second without the usual client options
but including sequence number and server time stamp, and where
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clients do not send requests as long as they receive generic replies.
8. References
8.1. Normative References
[1] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[2] "IANA, Address Family Numbers",
<http://www.iana.org/assignments/address-family-numbers>.
8.2. Informative References
[3] "ssmping implementation",
<http://www.venaas.no/multicast/ssmping/>.
Author's Address
Stig Venaas
UNINETT
Trondheim NO-7465
Norway
Email: venaas@uninett.no
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