|Title||Session Announcement Protocol
|Author||M. Handley, C. Perkins, E. Whelan
Network Working Group M. Handley
Request for Comments: 2974 ACIRI
Category: Experimental C. Perkins
Session Announcement Protocol
Status of this Memo
This memo defines an Experimental Protocol for the Internet
community. It does not specify an Internet standard of any kind.
Discussion and suggestions for improvement are requested.
Distribution of this memo is unlimited.
Copyright (C) The Internet Society (2000). All Rights Reserved.
This document describes version 2 of the multicast session directory
announcement protocol, Session Announcement Protocol (SAP), and the
related issues affecting security and scalability that should be
taken into account by implementors.
In order to assist the advertisement of multicast multimedia
conferences and other multicast sessions, and to communicate the
relevant session setup information to prospective participants, a
distributed session directory may be used. An instance of such a
session directory periodically multicasts packets containing a
description of the session, and these advertisements are received by
other session directories such that potential remote participants can
use the session description to start the tools required to
participate in the session.
This memo describes the issues involved in the multicast announcement
of session description information and defines an announcement
protocol to be used. Sessions are described using the session
description protocol which is described in a companion memo .
A SAP announcer periodically multicasts an announcement packet to a
well known multicast address and port. The announcement is multicast
with the same scope as the session it is announcing, ensuring that
the recipients of the announcement are within the scope of the
session the announcement describes (bandwidth and other such
constraints permitting). This is also important for the scalability
of the protocol, as it keeps local session announcements local.
A SAP listener learns of the multicast scopes it is within (for
example, using the Multicast-Scope Zone Announcement Protocol )
and listens on the well known SAP address and port for those scopes.
In this manner, it will eventually learn of all the sessions being
announced, allowing those sessions to be joined.
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 .
3 Session Announcement
As noted previously, a SAP announcer periodically sends an
announcement packet to a well known multicast address and port.
There is no rendezvous mechanism - the SAP announcer is not aware of
the presence or absence of any SAP listeners - and no additional
reliability is provided over the standard best-effort UDP/IP
That announcement contains a session description and SHOULD contain
an authentication header. The session description MAY be encrypted
although this is NOT RECOMMENDED (see section 7).
A SAP announcement is multicast with the same scope as the session it
is announcing, ensuring that the recipients of the announcement are
within the scope of the session the announcement describes. There are
a number of possibilities:
IPv4 global scope sessions use multicast addresses in the range
18.104.22.168 - 22.214.171.124 with SAP announcements being sent to
126.96.36.199 (note that 188.8.131.52 is used by the obsolete
SAPv0 and MUST NOT be used).
IPv4 administrative scope sessions using administratively scoped IP
multicast as defined in . The multicast address to be used for
announcements is the highest multicast address in the relevant
administrative scope zone. For example, if the scope range is
184.108.40.206 - 220.127.116.11, then 18.104.22.168 is used for SAP
IPv6 sessions are announced on the address FF0X:0:0:0:0:0:2:7FFE
where X is the 4-bit scope value. For example, an announcement
for a link-local session assigned the address
FF02:0:0:0:0:0:1234:5678, should be advertised on SAP address
Ensuring that a description is not used by a potential participant
outside the session scope is not addressed in this memo.
SAP announcements MUST be sent on port 9875 and SHOULD be sent with
an IP time-to-live of 255 (the use of TTL scoping for multicast is
If a session uses addresses in multiple administrative scope ranges,
it is necessary for the announcer to send identical copies of the
announcement to each administrative scope range. It is up to the
listeners to parse such multiple announcements as the same session
(as identified by the SDP origin field, for example). The
announcement rate for each administrative scope range MUST be
calculated separately, as if the multiple announcements were
Multiple announcers may announce a single session, as an aid to
robustness in the face of packet loss and failure of one or more
announcers. The rate at which each announcer repeats its
announcement MUST be scaled back such that the total announcement
rate is equal to that which a single server would choose.
Announcements made in this manner MUST be identical.
If multiple announcements are being made for a session, then each
announcement MUST carry an authentication header signed by the same
key, or be treated as a completely separate announcement by
An IPv4 SAP listener SHOULD listen on the IPv4 global scope SAP
address and on the SAP addresses for each IPv4 administrative scope
zone it is within. The discovery of administrative scope zones is
outside the scope of this memo, but it is assumed that each SAP
listener within a particular scope zone is aware of that scope zone.
A SAP listener which supports IPv6 SHOULD also listen to the IPv6 SAP
3.1 Announcement Interval
The time period between repetitions of an announcement is chosen such
that the total bandwidth used by all announcements on a single SAP
group remains below a preconfigured limit. If not otherwise
specified, the bandwidth limit SHOULD be assumed to be 4000 bits per
Each announcer is expected to listen to other announcements in order
to determine the total number of sessions being announced on a
particular group. Sessions are uniquely identified by the
combination of the message identifier hash and originating source
fields of the SAP header (note that SAP v0 announcers always set the
message identifier hash to zero, and if such an announcement is
received the entire message MUST be compared to determine
Announcements are made by periodic multicast to the group. The base
interval between announcements is derived from the number of
announcements being made in that group, the size of the announcement
and the configured bandwidth limit. The actual transmission time is
derived from this base interval as follows:
1. The announcer initializes the variable tp to be the last time a
particular announcement was transmitted (or the current time if
this is the first time this announcement is to be made).
2. Given a configured bandwidth limit in bits/second and an
announcement of ad_size bytes, the base announcement interval
in seconds is
interval =max(300; (8*no_of_ads*ad_size)/limit)
3. An offset is calculated based on the base announcement interval
offset= rand(interval* 2/3)-(interval/3)
4. The next transmission time for an announcement derived as
tn =tp+ interval+ offset
The announcer then sets a timer to expire at tn and waits. At time
tn the announcer SHOULD recalculate the next transmission time. If
the new value of tn is before the current time, the announcement is
sent immediately. Otherwise the transmission is rescheduled for the
new tn. This reconsideration prevents transient packet bursts on
startup and when a network partition heals.
4 Session Deletion
Sessions may be deleted in one of several ways:
Explicit Timeout The session description payload may contain
timestamp information specifying the start- and end-times of the
session. If the current time is later than the end-time of the
session, then the session SHOULD be deleted from the receiver's
Implicit Timeout A session announcement message should be received
periodically for each session description in a receiver's session
cache. The announcement period can be predicted by the receiver
from the set of sessions currently being announced. If a session
announcement message has not been received for ten times the
announcement period, or one hour, whichever is the greater, then
the session is deleted from the receiver's session cache. The one
hour minimum is to allow for transient network partitionings.
Explicit Deletion A session deletion packet is received specifying
the session to be deleted. Session deletion packets SHOULD have a
valid authentication header, matching that used to authenticate
previous announcement packets. If this authentication is missing,
the deletion message SHOULD be ignored.
5 Session Modification
A pre-announced session can be modified by simply announcing the
modified session description. In this case, the version hash in the
SAP header MUST be changed to indicate to receivers that the packet
contents should be parsed (or decrypted and parsed if it is
encrypted). The session itself, as distinct from the session
announcement, is uniquely identified by the payload and not by the
message identifier hash in the header.
The same rules apply for session modification as for session
o Either the modified announcement must contain an authentication
header signed by the same key as the cached session announcement
it is modifying, or:
o The cached session announcement must not contain an authentication
header, and the session modification announcement must originate
from the same host as the session it is modifying.
If an announcement is received containing an authentication header
and the cached announcement did not contain an authentication header,
or it contained a different authentication header, then the modified
announcement MUST be treated as a new and different announcement, and
displayed in addition to the un-authenticated announcement. The same
should happen if a modified packet without an authentication header
is received from a different source than the original announcement.
These rules prevent an announcement having an authentication header
added by a malicious user and then being deleted using that header,
and it also prevents a denial-of-service attack by someone putting
out a spoof announcement which, due to packet loss, reaches some
participants before the original announcement. Note that under such
circumstances, being able to authenticate the message originator is
the only way to discover which session is the correct session.
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
| V=1 |A|R|T|E|C| auth len | msg id hash |
: originating source (32 or 128 bits) :
| optional authentication data |
: .... :
| optional payload type |
+ +-+- - - - - - - - - -+
| |0| |
+ - - - - - - - - - - - - - - - - - - - - +-+ |
: payload :
Figure 1: Packet format
6 Packet Format
SAP data packets have the format described in figure 1.
V: Version Number. The version number field MUST be set to 1 (SAPv2
announcements which use only SAPv1 features are backwards
compatible, those which use new features can be detected by other
means, so the SAP version number doesn't need to change).
A: Address type. If the A bit is 0, the originating source field
contains a 32-bit IPv4 address. If the A bit is 1, the
originating source contains a 128-bit IPv6 address.
R: Reserved. SAP announcers MUST set this to 0, SAP listeners MUST
ignore the contents of this field.
T: Message Type. If the T field is set to 0 this is a session
announcement packet, if 1 this is a session deletion packet.
E: Encryption Bit. If the encryption bit is set to 1, the payload of
the SAP packet is encrypted. If this bit is 0 the packet is not
encrypted. See section 7 for details of the encryption process.
C: Compressed bit. If the compressed bit is set to 1, the payload is
compressed using the zlib compression algorithm . If the
payload is to be compressed and encrypted, the compression MUST be
Authentication Length. An 8 bit unsigned quantity giving the number
of 32 bit words following the main SAP header that contain
authentication data. If it is zero, no authentication header is
Authentication data containing a digital signature of the packet,
with length as specified by the authentication length header
field. See section 8 for details of the authentication process.
Message Identifier Hash. A 16 bit quantity that, used in combination
with the originating source, provides a globally unique identifier
indicating the precise version of this announcement. The choice
of value for this field is not specified here, except that it MUST
be unique for each session announced by a particular SAP announcer
and it MUST be changed if the session description is modified (and
a session deletion message SHOULD be sent for the old version of
Earlier versions of SAP used a value of zero to mean that the hash
should be ignored and the payload should always be parsed. This
had the unfortunate side-effect that SAP announcers had to study
the payload data to determine how many unique sessions were being
advertised, making the calculation of the announcement interval
more complex that necessary. In order to decouple the session
announcement process from the contents of those announcements, SAP
announcers SHOULD NOT set the message identifier hash to zero.
SAP listeners MAY silently discard messages if the message
identifier hash is set to zero.
Originating Source. This gives the IP address of the original source
of the message. This is an IPv4 address if the A field is set to
zero, else it is an IPv6 address. The address is stored in
network byte order.
SAPv0 permitted the originating source to be zero if the message
identifier hash was also zero. This practise is no longer legal,
and SAP announcers SHOULD NOT set the originating source to zero.
SAP listeners MAY silently discard packets with the originating
source set to zero.
The header is followed by an optional payload type field and the
payload data itself. If the E or C bits are set in the header both
the payload type and payload are encrypted and/or compressed.
The payload type field is a MIME content type specifier, describing
the format of the payload. This is a variable length ASCII text
string, followed by a single zero byte (ASCII NUL). The payload type
SHOULD be included in all packets. If the payload type is
`application/sdp' both the payload type and its terminating zero byte
MAY be omitted, although this is intended for backwards compatibility
with SAP v1 listeners only.
The absence of a payload type field may be noted since the payload
section of such a packet will start with an SDP `v=0' field, which is
not a legal MIME content type specifier.
All implementations MUST support payloads of type `application/sdp'
. Other formats MAY be supported although since there is no
negotiation in SAP an announcer which chooses to use a session
description format other than SDP cannot know that the listeners are
able to understand the announcement. A proliferation of payload
types in announcements has the potential to lead to severe
interoperability problems, and for this reason, the use of non-SDP
payloads is NOT RECOMMENDED.
If the packet is an announcement packet, the payload contains a
If the packet is a session deletion packet, the payload contains a
session deletion message. If the payload format is `application/sdp'
the deletion message is a single SDP line consisting of the origin
field of the announcement to be deleted.
It is desirable for the payload to be sufficiently small that SAP
packets do not get fragmented by the underlying network.
Fragmentation has a loss multiplier effect, which is known to
significantly affect the reliability of announcements. It is
RECOMMENDED that SAP packets are smaller than 1kByte in length,
although if it is known that announcements will use a network with a
smaller MTU than this, then that SHOULD be used as the maximum
recommended packet size.
7 Encrypted Announcements
An announcement is received by all listeners in the scope to which it
is sent. If an announcement is encrypted, and many of the receivers
do not have the encryption key, there is a considerable waste of
bandwidth since those receivers cannot use the announcement they have
received. For this reason, the use of encrypted SAP announcements is
NOT RECOMMENDED on the global scope SAP group or on administrative
scope groups which may have many receivers which cannot decrypt those
The opinion of the authors is that encrypted SAP is useful in special
cases only, and that the vast majority of scenarios where encrypted
SAP has been proposed may be better served by distributing session
details using another mechanism. There are, however, certain
scenarios where encrypted announcements may be useful. For this
reason, the encryption bit is included in the SAP header to allow
experimentation with encrypted announcements.
This memo does not specify details of the encryption algorithm to be
used or the means by which keys are generated and distributed. An
additional specification should define these, if it is desired to use
Note that if an encrypted announcement is being announced via a
proxy, then there may be no way for the proxy to discover that the
announcement has been superseded, and so it may continue to relay the
old announcement in addition to the new announcement. SAP provides
no mechanism to chain modified encrypted announcements, so it is
advisable to announce the unmodified session as deleted for a short
time after the modification has occurred. This does not guarantee
that all proxies have deleted the session, and so receivers of
encrypted sessions should be prepared to discard old versions of
session announcements that they may receive. In most cases however,
the only stateful proxy will be local to (and known to) the sender,
and an additional (local-area) protocol involving a handshake for
such session modifications can be used to avoid this problem.
Session announcements that are encrypted with a symmetric algorithm
may allow a degree of privacy in the announcement of a session, but
it should be recognized that a user in possession of such a key can
pass it on to other users who should not be in possession of such a
key. Thus announcements to such a group of key holders cannot be
assumed to have come from an authorized key holder unless there is an
appropriate authentication header signed by an authorized key holder.
In addition the recipients of such encrypted announcements cannot be
assumed to only be authorized key holders. Such encrypted
announcements do not provide any real security unless all of the
authorized key holders are trusted to maintain security of such
session directory keys. This property is shared by the multicast
session tools themselves, where it is possible for an un-trustworthy
member of the session to pass on encryption keys to un-authorized
users. However it is likely that keys used for the session tools
will be more short lived than those used for session directories.
Similar considerations should apply when session announcements are
encrypted with an asymmetric algorithm, but then it is possible to
restrict the possessor(s) of the private key, so that announcements
to a key-holder group can not be made, even if one of the untrusted
members of the group proves to be un-trustworthy.
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
| V=1 |P| Auth | |
| Format specific authentication subheader |
: .................. :
Figure 2: Format of the authentication data in the SAP header
8 Authenticated Announcements
The authentication header can be used for two purposes:
o Verification that changes to a session description or deletion of
a session are permitted.
o Authentication of the identity of the session creator.
In some circumstances only verification is possible because a
certificate signed by a mutually trusted person or authority is not
available. However, under such circumstances, the session originator
may still be authenticated to be the same as the session originator
of previous sessions claiming to be from the same person. This may
or may not be sufficient depending on the purpose of the session and
the people involved.
Clearly the key used for the authentication should not be trusted to
belong to the session originator unless it has been separately
authenticated by some other means, such as being certified by a
trusted third party. Such certificates are not normally included in
an SAP header because they take more space than can normally be
afforded in an SAP packet, and such verification must therefore take
place by some other mechanism. However, as certified public keys are
normally locally cached, authentication of a particular key only has
to take place once, rather than every time the session directory
retransmits the announcement.
SAP is not tied to any single authentication mechanism.
Authentication data in the header is self-describing, but the precise
format depends on the authentication mechanism in use. The generic
format of the authentication data is given in figure 2. The
structure of the format specific authentication subheader, using both
the PGP and the CMS formats, is discussed in sections 8.1 and 8.2
respectively. Additional formats may be added in future.
Version Number, V: The version number of the authentication format
specified by this memo is 1.
Padding Bit, P: If necessary the authentication data is padded to be
a multiple of 32 bits and the padding bit is set. In this case
the last byte of the authentication data contains the number of
padding bytes (including the last byte) that must be discarded.
Authentication Type, Auth: The authentication type is a 4 bit
encoded field that denotes the authentication infrastructure the
sender expects the recipients to use to check the authenticity and
integrity of the information. This defines the format of the
authentication subheader and can take the values: 0 = PGP format,
1 = CMS format. All other values are undefined and SHOULD be
If a SAP packet is to be compressed or encrypted, this MUST be done
before the authentication is added.
The digital signature in the authentication data MUST be calculated
over the entire packet, including the header. The authentication
length MUST be set to zero and the authentication data excluded when
calculating the digital signature.
It is to be expected that sessions may be announced by a number of
different mechanisms, not only SAP. For example, a session
description may placed on a web page, sent by email or conveyed in a
session initiation protocol. To ease interoperability with these
other mechanisms, application level security is employed, rather than
using IPsec authentication headers.
8.1 PGP Authentication
A full description of the PGP protocol can be found in . When
using PGP for SAP authentication the basic format specific
authentication subheader comprises a digital signature packet as
described in . The signature type MUST be 0x01 which means the
signature is that of a canonical text document.
8.2 CMS Authentication
A full description of the Cryptographic Message Syntax can be found
in . The format specific authentication subheader will, in the
CMS case, have an ASN.1 ContentInfo type with the ContentType being
Use is made of the option available in PKCS#7 to leave the content
itself blank as the content which is signed is already present in the
packet. Inclusion of it within the SignedData type would duplicate
this data and increase the packet length unnecessarily. In addition
this allows recipients with either no interest in the authentication,
or with no mechanism for checking it, to more easily skip the
There SHOULD be only one signerInfo and related fields corresponding
to the originator of the SAP announcement. The signingTime SHOULD be
present as a signedAttribute. However, due to the strict size
limitations on the size of SAP packets, certificates and CRLs SHOULD
NOT be included in the signedData structure. It is expected that
users of the protocol will have other methods for certificate and CRL
9 Scalability and caching
SAP is intended to announce the existence of long-lived wide-area
multicast sessions. It is not an especially timely protocol:
sessions are announced by periodic multicast with a repeat rate on
the order of tens of minutes, and no enhanced reliability over UDP.
This leads to a long startup delay before a complete set of
announcements is heard by a listener. This delay is clearly
undesirable for interactive browsing of announced sessions.
In order to reduce the delays inherent in SAP, it is recommended that
proxy caches are deployed. A SAP proxy cache is expected to listen
to all SAP groups in its scope, and to maintain an up-to-date list of
all announced sessions along with the time each announcement was last
received. When a new SAP listeners starts, it should contact its
local proxy to download this information, which is then sufficient
for it to process future announcements directly, as if it has been
The protocol by which a SAP listener contacts its local proxy cache
is not specified here.
10 Security Considerations
SAP contains mechanisms for ensuring integrity of session
announcements, for authenticating the origin of an announcement and
for encrypting such announcements (sections 7 and 8).
As stated in section 5, if a session modification announcement is
received that contains a valid authentication header, but which is
not signed by the original creator of the session, then the session
must be treated as a new session in addition to the original session
with the same SDP origin information unless the originator of one of
the session descriptions can be authenticated using a certificate
signed by a trusted third party. If this were not done, there would
be a possible denial of service attack whereby a party listens for
new announcements, strips off the original authentication header,
modifies the session description, adds a new authentication header
and re-announces the session. If a rule was imposed that such spoof
announcements were ignored, then if packet loss or late starting of a
session directory instance caused the original announcement to fail
to arrive at a site, but the spoof announcement did so, this would
then prevent the original announcement from being accepted at that
A similar denial-of-service attack is possible if a session
announcement receiver relies completely on the originating source and
hash fields to indicate change, and fails to parse the remainder of
announcements for which it has seen the origin/hash combination
A denial of service attack is possible from a malicious site close to
a legitimate site which is making a session announcement. This can
happen if the malicious site floods the legitimate site with huge
numbers of (illegal) low TTL announcements describing high TTL
sessions. This may reduce the session announcement rate of the
legitimate announcement to below a tenth of the rate expected at
remote sites and therefore cause the session to time out. Such an
attack is likely to be easily detectable, and we do not provide any
mechanism here to prevent it.
A. Summary of differences between SAPv0 and SAPv1
For this purpose SAPv0 is defined as the protocol in use by version
2.2 of the session directory tool, sdr. SAPv1 is the protocol
described in the 19 November 1996 version of this memo. The packet
headers of SAP messages are the same in V0 and V1 in that a V1 tool
can parse a V0 announcement header but not vice-versa. In SAPv0, the
fields have the following values:
o Version Number: 0
o Message Type: 0 (Announcement)
o Authentication Type: 0 (No Authentication)
o Encryption Bit: 0 (No Encryption)
o Compression Bit: 0 (No compression)
o Message Id Hash: 0 (No Hash Specified)
o Originating Source: 0 (No source specified, announcement has
not been relayed)
B. Summary of differences between SAPv1 and SAPv2
The packet headers of SAP messages are the same in V1 and V2 in that
a V2 tool can parse a V1 announcement header but not necessarily
o The A bit has been added to the SAP header, replacing one of the
bits of the SAPv1 message type field. If set to zero the
announcement is of an IPv4 session, and the packet is backwards
compatible with SAPv1. If set to one the announcement is of an
IPv6 session, and SAPv1 listeners (which do not support IPv6) will
see this as an illegal message type (MT) field.
o The second bit of the message type field in SAPv1 has been
replaced by a reserved, must-be-zero, bit. This bit was unused in
SAPv1, so this change just codifies existing usage.
o SAPv1 specified encryption of the payload. SAPv2 includes the E
bit in the SAP header to indicate that the payload is encrypted,
but does not specify any details of the encryption.
o SAPv1 allowed the message identifier hash and originating source
fields to be set to zero, for backwards compatibility. This is no
o SAPv1 specified gzip compression. SAPv2 uses zlib (the only known
implementation of SAP compression used zlib, and gzip compression
was a mistake).
o SAPv2 provides a more complete specification for authentication.
o SAPv2 allows for non-SDP payloads to be transported. SAPv1
required that the payload was SDP.
o SAPv1 included a timeout field for encrypted announcement, SAPv2
does not (and relies of explicit deletion messages or implicit
SAP and SDP were originally based on the protocol used by the sd
session directory from Van Jacobson at LBNL. Version 1 of SAP was
designed by Mark Handley as part of the European Commission MICE
(Esprit 7602) and MERCI (Telematics 1007) projects. Version 2
includes authentication features developed by Edmund Whelan, Goli
Montasser-Kohsari and Peter Kirstein as part of the European
Commission ICE-TEL project (Telematics 1005), and support for IPv6
developed by Maryann P. Maher and Colin Perkins.
D. Authors' Addresses
AT&T Center for Internet Research at ICSI,
International Computer Science Institute,
1947 Center Street, Suite 600,
Berkeley, CA 94704, USA
USC Information Sciences Institute
4350 N. Fairfax Drive, Suite 620
Arlington, VA 22203, USA
Department of Computer Science,
University College London,
London, WC1E 6BT, UK
 Bradner, S., "Key words for use in RFCs to indicate requirement
levels", BCP 14, RFC 2119, March 1997.
 Callas, J., Donnerhacke, L., Finney, H. and R. Thayer. "OpenPGP
message format", RFC 2440, November 1998.
 Deutsch, P. and J.-L. Gailly, "Zlib compressed data format
specification version 3.3", RFC 1950, May 1996.
 Handley, M. and V. Jacobson, "SDP: Session Description Protocol",
RFC 2327, April 1998.
 Handley, M., Thaler, D. and R. Kermode, "Multicast-scope zone
announcement protocol (MZAP)", RFC 2776, February 2000.
 Housley, R., "Cryptographic message syntax", RFC 2630, June 1999.
 Mayer, D., "Administratively scoped IP multicast", RFC 2365, July
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