Network Working Group S. Venaas
Internet-Draft UNINETT
Intended status: Informational H. Santos
Expires: January 10, 2008 July 9, 2007
ssmping Protocol
draft-ietf-mboned-ssmping-01
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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. Architecture . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Protocol specification . . . . . . . . . . . . . . . . . . . . 4
3.1. Option format . . . . . . . . . . . . . . . . . . . . . . 5
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
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 9
Intellectual Property and Copyright Statements . . . . . . . . . . 10
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1. Introduction
ssmping is a tool that is designed to allow a local host to check
whether it is able to receive a multicast flow (SSM by default, or
ASM when specific options are used) originated by a remote host.
Additionally it is able to report other information such as the
amount of time used to establish the multicast tree, the number of
hops the flow's packets have traveled as well as the packet delay and
loss. This functionality resembles in part the ICMP Echo Request/
Reply infrastructure but over UDP and implemented by both the ssmping
client and server. The protocol here specified is based on the
actual implementation of the ssmping tool [3] which is widely used by
the Internet community to conduct multicast connectivity tests.
2. Architecture
Before going into the protocol details we will describe how it is
used and what information it may provide. The typical usage of an
ssmping session is as follows. A server runs continuously in order
to serve request from clients. When a host decides to verify the
multicast reception from a specific server (knowing one of the
server's unicast addresses is required), the ssmping client joins an
SSM channel (S,G) where S is a unicast address of the target server
and G is the standard multicast group defined for use by ssmping.
After joining the channel, the client sends ssmping requests
encapsulated in UDP to the 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
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 estimate the amount of time it took to establish
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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 the unicast
request is sent to when the reply is received. The measured
multicast RTT also references the client's unicast request. 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 obtaining the same
values by 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 difference in one way delay for unicast and multicast on the
path from server to client, and also differences in delay variation.
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 when the
client and server have synchronised clocks.
3. Protocol specification
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.
This document defines a number of different options. Some options
don't 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. 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. The
protocol here defined does not require the use of any options, and a
client may operate without specifying any. However some of the
options allow the client to obtain additional information.
Unless otherwise specified, an option MUST NOT be used multiple times
in a request. Also unless otherwise specified, an option MUST NOT be
appended by the server multiple times. Note that some options, like
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timestamp, may be added by both the client and the server. In that
case the timestamp option would be in the response twice. But as
said above, it is not used multiple times in the request, and not
appended multiple times by the server.
3.1. Option format
All options are TLVs formatted as specified below.
0 1 2 3
0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Type | Length |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
| Value |
| . |
| . |
| . |
+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
Type (2 octets) specifies the option. The different options are
defined below.
Length (2 octets) specifies the length of the value field. Depending
on the option type it can be from 0 to 65535.
Value. The value must always be of the specified length. See the
respective option definitions for possible values. If the length is
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
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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
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
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server supports.
Type 7, Reserved. This option code value was used by early
implementations for an option that now is deprecated. This 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 sizes in order to get
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 like for 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.
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).
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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
octet boundary. The option format is specified above.
5. Acknowledgements
The ssmping concept was proposed by Pavan Namburi, Kamil Sarac and
Kevin C. Almeroth in the paper SSM-Ping: A Ping Utility for Source
Specific Multicast, and also the Internet Draft
draft-sarac-mping-00.txt. Mickael Hoerdt has contributed with
several ideas. Alexander Gall, Nicholas Humfrey, Nick Lamb and Dave
Thaler have contributed in different ways to my implementation of the
ssmping tools [3]. 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, Olav
Kvittem, Kamil Sarac, Pekka Savola, Trond Skjesol and Cao Wei 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.
There may also be a need for an ssmping option registry. The exact
IANA considerations need to be clarified before this document can go
to working group last call.
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 a DoS
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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
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/>.
Authors' Addresses
Stig Venaas
UNINETT
Trondheim NO-7465
Norway
Email: venaas@uninett.no
Hugo Santos
Urb Glicinias, Smart Residence, 211
Aveiro 3810
Portugal
Email: hugo@fivebits.net
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