This documents describes a mechanism for communicating limits, such as stanza size limits that is in effect on a particular stream, in order to allow the sending party to avoid reaching those limits.
Where stanza size limits have been deployed, very often this leads to problems with large stanzas causing connection outages, most often User Avatar (XEP-0084) [1] and XMPP Registrar Function (XEP-0053) [2] result stanzas, which can be very large due to embedded images.
If stanza size limit violations are met with stream errors then this may lead to temporary connection outage, which may a few seconds to recover from.
These requirements will enable XMPP clients and servers to adapt data they generate, such that it will fit within the limits required by the recipient, or reject overly large stanzas early, rather than following a trial-and-error approach.
For any XMPP stream, there is an "initiating entity" (a client or server) and a "responding entity" that they are connecting to. The responding entity advertises its limits in the <stream:features/> element that it sends at the start of the stream.
The limits are enclosed in a <limits/> element qualified by the 'urn:xmpp:stream-limits:0' namespace. This element SHOULD contain the following child elements:
<stream:features>
<mechanisms xmlns="urn:ietf:params:xml:ns:xmpp-sasl">
<mechanism>SCRAM-SHA-1</mechanism>
<mechanism>PLAIN</mechanism>
</mechanisms>
<limits xmlns="urn:xmpp:stream-limits:0">
<max-bytes>10000</max-bytes>
<idle-seconds>1800</idle-seconds>
</limits>
</stream:features>
Servers using Bidirectional Server-to-Server Connections (XEP-0288) [6] to establish a bidirectional stream with another server do not get an opportunity to send <stream:features/> to the responding entity. For a server to advertise the limits about what it is willing to accept on such a stream, the <limits/> element can be included in the <bidi/> element.
<bidi xmlns='urn:xmpp:bidi'>
<limits xmlns="urn:xmpp:stream-limits:0">
<max-bytes>10000</max-bytes>
<idle-seconds>1800</idle-seconds>
</limits>
</bidi>
If, after serialization, a stanza exceeds the limits that have been advertised on a stream, it SHOULD NOT be sent on that stream. Instead, a server SHOULD return an error to the sender. Such an error SHOULD contain the <policy-violation/> error condition, and SHOULD NOT contain a 'by' attribute (as the policy being violated is not the current entity's). A <text/> may also be included, explaining the limit that would be exceeded.
It is acceptable for the limits on a stream to change whenever new stream features are announced - such as before and after authentication of the connecting entity.
OPTIONAL.
OPTIONAL.
OPTIONAL.
REQUIRED.
Very large stanzas may incur memory and processing costs on the receiving entity. Advertising the actual limits could inform an attacker of how large a stanza to construct in order to maximize e.g. DoS effectiveness. Best combined with network level rate limits on raw bytes.
None.
This specification defines the following namespace:
Also, the following stream feature:
<limits xmlns='urn:xmpp:stream-limits:0'/>
The ability for a client to announce limits on what it will receive on a client-to-server stream is deliberately not provided by this specification. This vastly simplifies discovery of the maximum limits between any two JIDs, and it avoids situations where the server is unable to deliver incoming stanzas to some or all of an account's connected clients. Clients will already be protected from denial-of-service through excessive stanza sizes due to the server's own limits.
TBD.
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The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 6120) and XMPP IM (RFC 6121) specifications contributed by the XMPP Standards Foundation to the Internet Standards Process, which is managed by the Internet Engineering Task Force in accordance with RFC 2026. Any protocol defined in this document has been developed outside the Internet Standards Process and is to be understood as an extension to XMPP rather than as an evolution, development, or modification of XMPP itself.
The primary venue for discussion of XMPP Extension Protocols is the <standards@xmpp.org> discussion list.
Discussion on other xmpp.org discussion lists might also be appropriate; see <https://xmpp.org/community/> for a complete list.
Errata can be sent to <editor@xmpp.org>.
The following requirements keywords as used in this document are to be interpreted as described in RFC 2119: "MUST", "SHALL", "REQUIRED"; "MUST NOT", "SHALL NOT"; "SHOULD", "RECOMMENDED"; "SHOULD NOT", "NOT RECOMMENDED"; "MAY", "OPTIONAL".
1. XEP-0084: User Avatar <https://xmpp.org/extensions/xep-0084.html>.
2. XEP-0053: XMPP Registrar Function <https://xmpp.org/extensions/xep-0053.html>.
3. RFC 6120: Extensible Messaging and Presence Protocol (XMPP): Core <http://tools.ietf.org/html/rfc6120>.
4. XEP-0198: Stream Management <https://xmpp.org/extensions/xep-0198.html>.
5. XEP-0199: XMPP Ping <https://xmpp.org/extensions/xep-0199.html>.
6. XEP-0288: Bidirectional Server-to-Server Connections <https://xmpp.org/extensions/xep-0288.html>.
Note: Older versions of this specification might be available at https://xmpp.org/extensions/attic/
First draft.
@report{alvefur2022sla,
title = {Stream Limits Advertisement},
author = {Alvefur, Kim and Wild, Matthew},
type = {XEP},
number = {xxxx},
version = {0.0.1},
institution = {XMPP Standards Foundation},
url = {https://xmpp.org/extensions/xep-xxxx.html},
date = {2022-10-20/2022-10-20},
}END