XEP-0238: XMPP Protocol Flows for Inter-Domain Federation

This specification provides detailed protocol flows for the establishment of communication between domains that provide XMPP services, including permutations for a wide variety of possible federation policies.

WARNING: This Informational document is Experimental. Publication as an XMPP Extension Protocol does not imply approval of this proposal by the XMPP Standards Foundation. Implementation of the best practice or protocol profile described herein is encouraged in exploratory implementations, although production systems should not deploy implementations of this protocol until it advances to a status of Draft.

Document Information

Series: XEP
Number: 0238
Publisher: XMPP Standards Foundation
Status: Experimental
Type: Informational
Version: 0.1
Last Updated: 2008-03-31
Approving Body: XMPP Council
Dependencies: XMPP Core, XEP-0220
Supersedes: None
Superseded By: None
Short Name: N/A
Wiki Page: <http://wiki.jabber.org/index.php/XMPP Protocol Flows for Inter-Domain Federation (XEP-0238)>

Author Information

Peter Saint-Andre

JabberID: stpeter@jabber.org
URI: https://stpeter.im/

Legal Notices

Copyright

This XMPP Extension Protocol is copyright (c) 1999 - 2008 by the XMPP Standards Foundation (XSF).

Permissions

Permission is hereby granted, free of charge, to any person obtaining a copy of this specification (the "Specification"), to make use of the Specification without restriction, including without limitation the rights to implement the Specification in a software program, deploy the Specification in a network service, and copy, modify, merge, publish, translate, distribute, sublicense, or sell copies of the Specification, and to permit persons to whom the Specification is furnished to do so, subject to the condition that the foregoing copyright notice and this permission notice shall be included in all copies or substantial portions of the Specification. Unless separate permission is granted, modified works that are redistributed shall not contain misleading information regarding the authors, title, number, or publisher of the Specification, and shall not claim endorsement of the modified works by the authors, any organization or project to which the authors belong, or the XMPP Standards Foundation.

Disclaimer of Warranty

## NOTE WELL: This Specification is provided on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, express or implied, including, without limitation, any warranties or conditions of TITLE, NON-INFRINGEMENT, MERCHANTABILITY, or FITNESS FOR A PARTICULAR PURPOSE. In no event shall the XMPP Standards Foundation or the authors of this Specification be liable for any claim, damages, or other liability, whether in an action of contract, tort, or otherwise, arising from, out of, or in connection with the Specification or the implementation, deployment, or other use of the Specification. ##

Limitation of Liability

In no event and under no legal theory, whether in tort (including negligence), contract, or otherwise, unless required by applicable law (such as deliberate and grossly negligent acts) or agreed to in writing, shall the XMPP Standards Foundation or any author of this Specification be liable for damages, including any direct, indirect, special, incidental, or consequential damages of any character arising out of the use or inability to use the Specification (including but not limited to damages for loss of goodwill, work stoppage, computer failure or malfunction, or any and all other commercial damages or losses), even if the XMPP Standards Foundation or such author has been advised of the possibility of such damages.

IPR Conformance

This XMPP Extension Protocol has been contributed in full conformance with the XSF's Intellectual Property Rights Policy (a copy of which may be found at <http://www.xmpp.org/extensions/ipr-policy.shtml> or obtained by writing to XSF, P.O. Box 1641, Denver, CO 80201 USA).

Discussion Venue

The preferred venue for discussion of this document is the Standards discussion list: <http://mail.jabber.org/mailman/listinfo/standards>.

Errata may be sent to <editor@xmpp.org>.

Relation to XMPP

The Extensible Messaging and Presence Protocol (XMPP) is defined in the XMPP Core (RFC 3920) and XMPP IM (RFC 3921) 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.

Conformance Terms

The following 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".

Table of Contents


1. Introduction
2. Terminology
3. Connection Success
4. Connections from Type 1 Services
    4.1. Type 1 to Type 1
    4.2. Type 1 to Type 2
    4.3. Type 1 to Type 3
    4.4. Type 1 to Type 4
    4.5. Type 1 to Type 5
    4.6. Type 1 to Type 6
5. Connections from Type 2 Services
    5.1. Type 2 to Type 1
    5.2. Type 2 to Type 2
    5.3. Type 2 to Type 3
    5.4. Type 2 to Type 4
    5.5. Type 2 to Type 5
    5.6. Type 2 to Type 6
6. Connections from Type 3 Services
    6.1. Type 3 to Type 1
    6.2. Type 3 to Type 2
    6.3. Type 3 to Type 3
    6.4. Type 3 to Type 4
    6.5. Type 3 to Type 5
    6.6. Type 3 to Type 6
7. Connections from Type 4 Services
    7.1. Type 4 to Type 1
    7.2. Type 4 to Type 2
    7.3. Type 4 to Type 3
    7.4. Type 4 to Type 4
    7.5. Type 4 to Type 5
    7.6. Type 4 to Type 6
8. Connections from Type 5 Services
    8.1. Type 5 to Type 1
    8.2. Type 5 to Type 2
    8.3. Type 5 to Type 3
    8.4. Type 5 to Type 4
    8.5. Type 5 to Type 5
    8.6. Type 5 to Type 6
9. Connections from Type 6 Services
    9.1. Type 6 to Type 1
    9.2. Type 6 to Type 2
    9.3. Type 6 to Type 3
    9.4. Type 6 to Type 4
    9.5. Type 6 to Type 5
    9.6. Type 6 to Type 6
10. Security Considerations
11. IANA Considerations
12. XMPP Registrar Considerations
13. Acknowledgements
Notes
Revision History

1. Introduction

XMPP Core [1] describes the client-server architecture upon which Jabber/XMPP communication is based. One aspect of such communication is "federation", i.e., the ability for two XMPP servers in different domains to exchange XML stanzas. There are at least four levels of federation:

  1. Permissive Federation -- a server accepts a connection from any other peer on the network, even without verifiying the identity of the peer based on DNS lookups. The lack of peer verification or authentication means that domains can be spoofed. Permissive federation was effectively outlawed on the Jabber network in October 2000 with the release of the jabberd 1.2 server, which included support for the newly-developed Server Dialback [2] protocol.

  2. Verified Federation -- a server accepts a connection from a peer only after the identity of the peer has been weakly verified via Server Dialback, based on information obtained via the Domain Name System (DNS) and verification keys exchanged in-band over XMPP. However, the connection is not encrypted. The use of identity verification effectively prevents domain spoofing, but federation requires proper DNS setup and is still subject to DNS poisoning attacks. Verified federation has been the default service policy followed by servers on the open XMPP network from October 2000 until now.

  3. Encrypted Federation -- a server accepts a connection from a peer only if the peer supports Transport Layer Security (TLS) as defined for XMPP in RFC 3920 [3] and the peer presents a digital certificate. However, the certificate may be self-signed, in which case mutual authentication is typically not possible. Therefore, after STARTTLS negotiation the parties proceed to weakly verify identity using Server Dialback. This combination results in an encrypted connection with weak identity verification.

  4. Trusted Federation -- a server accepts a connection from a peer only if the peer supports Transport Layer Security (TLS) and the peer presents a digital certificate issued by a trusted root certification authority (CA). The list of trusted root CAs is determined by local service policy, as is the level of trust accorded to various types of certificates (i.e., Class 1, Class 2, or Class 3). The use of trusted domain certificates effectively prevents DNS poisoning attacks but makes federation more difficult since typically such certificates are not easy to obtain.

The remainder of this document describes in more detail the protocol flows that make it possible to deploy verified federation, encrypted federation, and trusted federation. Protocol flows are shown for federation attempts between various combinations to illustrate the interaction between different federation policies.

2. Terminology

To simplify the text, this document uses the following terminology. For each service type, the domain "example.lit" is used to illustrate connections to that same service type.

Table 1: Example Servers

Service Type Federation Policy Certificate Protocols Supported Example Domain Example User
Type 1 Verified Only None XMPP 0.9 [4] and Server Dialback type1.lit citizen@type1.lit
Type 2 Verified Acceptable Self-signed XMPP 1.0 [5] and Server Dialback type2.lit juliet@type2.lit
Type 3 Verified Acceptable CA-issued XMPP 1.0 and Server Dialback type3.lit romeo@type3.lit
Type 4 Encrypted Required Self-signed XMPP 1.0 and Server Dialback type4.lit hamlet@type4.lit
Type 5 Encrypted Required CA-issued XMPP 1.0 and Server Dialback type5.lit bill@type5.lit
Type 6 Trusted Required CA-issued XMPP 1.0 type6.lit chris@type6.lit

3. Connection Success

The following table summarizes the results of connection attempts between the various services, where "U" stands for "Unsuccessful", "V" stands for "Verified", "E" stands for "Encrypted", and "T" stands for "Trusted". The rows indicate the initiating service and the columns indicate the receiving service.

Table 2: Connection Success

Type 1 Type 2 Type 3 Type 4 Type 5 Type 6
Type 1 V V V U U U
Type 2 V V E E U U
Type 3 V V E E E T
Type 4 U E E E E U
Type 5 U E T E T T
Type 6 U U T U T T

4. Connections from Type 1 Services

4.1 Type 1 to Type 1

In this scenario, an XMPP user citizen@type1.lit attempts to send an XML stanza to user@example.lit.

Example 1. Test Stanza

<iq from='citizen@type1.lit/foo'
    id='t1_t1'
    to='user@example.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type1.lit service attempts to initiate a server-to-server connection with example.lit (both of which support verified connections only and neither of which has a certificate).

First, the type1.lit service sends an initial stream header to example.lit.

Example 2. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type1.lit'
        to='example.lit'>

Next the example.lit service sends a response stream header to type1.lit.

Example 3. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='example.lit'
        id='idt1_t1o'
        to='type1.lit'>

Because neither service supports XMPP 1.0, the type1.lit service attempts to complete a server dialback negotiation with the example.lit service. Therefore it sends a dialback key to example.lit over the existing connection.

Example 4. Dialback Key

<db:result
    from='type1.lit'
    to='example.lit'>
  some-long-dialback-key
</db:result>

The example.lit service then performs a DNS lookup on the type1.lit domain, opens a TCP connection at the discovered IP address and port, and establishes a stream with the authoritative server for the type1.lit service.

Example 5. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='example.lit'
        to='type1.lit'>

The authoritative server for the type1.lit service then returns a response stream header.

Example 6. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type1.lit'
        id='idt1_t1r'
        to='example.lit'>

The example.lit service then sends a dialback verification request to the authoritative server for the type1.lit domain.

Example 7. Verification Request

<db:verify
    from='example.lit'
    id='idt1_t1o'
    to='type1.lit'>
  some-long-dialback-key
</db:verify>

Here we assume that the authoritative server for the type1.lit domain notifies the example.lit service that the key is valid.

Example 8. Key is Valid

<db:verify
    from='type1.lit'
    id='idt1_t1o'
    to='example.lit'
    type='valid'>
  some-long-dialback-key
</db:verify>

The example.lit service then returns a positive server dialback result to the originating server.

Example 9. Server Dialback Result

<db:result
    from='example.lit'
    to='type1.lit'
    type='valid'>
  some-long-dialback-key
</db:result>

Because the connection is successful, the type1.lit service routes the XML stanza from citizen@type1.lit to the example.lit service.

4.2 Type 1 to Type 2

In this scenario, an XMPP user citizen@type1.lit attempts to send an XML stanza to juliet@type2.lit:

Example 10. Test Stanza

<iq from='citizen@type1.lit/foo'
    id='t1_t2'
    to='juliet@type2.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type1.lit service (which supports verified connections only and does not have a certificate) attempts to initiate a server-to-server connection with the type2.lit service (which accepts verified connections and has a self-signed certificate).

First, the type1.lit service sends an initial stream header to type2.lit.

Example 11. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type1.lit'
        to='type2.lit'>

Next the type2.lit service sends a response stream header to type1.lit.

Example 12. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        id='idt1_t2o'
        to='type1.lit'
        version='1.0'>

The type2.lit service also sends stream features.

Example 13. Stream Features

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
  <dialback xmlns='urn:xmpp:features:dialback'/>
</stream:features>

Because the type1.lit service does not support XMPP 1.0, it ignores the stream features and attempts to complete a server dialback negotiation with the type2.lit service. Therefore it sends a dialback key to type2.lit over the existing connection.

Example 14. Dialback Key

<db:result
    from='type1.lit'
    to='type2.lit'>
  some-long-dialback-key
</db:result>

The type2.lit service then performs a DNS lookup on the type1.lit domain, opens a TCP connection at the discovered IP address and port, and establishes a stream with the authoritative server for the type1.lit service.

Example 15. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        to='type1.lit'
        version='1.0'>

The authoritative server for the type1.lit service then returns a response stream header.

Example 16. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type1.lit'
        id='idt1_t2r'
        to='type2.lit'>

The type2.lit service then sends a dialback verification request to the authoritative server for the type1.lit domain.

Example 17. Verification Request

<db:verify
    from='type2.lit'
    id='idt1_t2o'
    to='type1.lit'>
  some-long-dialback-key
</db:verify>

Here we assume that the authoritative server for the type1.lit domain notifies the type2.lit service that the key is valid.

Example 18. Key is Valid

<db:verify
    from='type1.lit'
    id='idt1_t2o'
    to='type2.lit'
    type='valid'>
  some-long-dialback-key
</db:verify>

The type2.lit service then returns a positive server dialback result to the originating server.

Example 19. Server Dialback Result

<db:result
    from='type2.lit'
    to='type1.lit'
    type='valid'>
  some-long-dialback-key
</db:result>

Because the connection is successful, the type1.lit service routes the XML stanza from citizen@type1.lit to the type2.lit service.

4.3 Type 1 to Type 3

In this scenario, an XMPP user citizen@type1.lit attempts to send an XML stanza to romeo@type3.lit.

Example 20. Test Stanza

<iq from='citizen@type1.lit/foo'
    id='t1_t3'
    to='romeo@type3.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type1.lit service (which supports verified connections only and does not have a certificate) attempts to initiate a server-to-server connection with the type3.lit service (which accepts verified connections and has a CA-issued certificate).

First, the type1.lit service sends an initial stream header to type3.lit.

Example 21. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type1.lit'
        to='type3.lit'>

Next the type3.lit service sends a response stream header to type1.lit.

Example 22. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        id='idt1_t3o'
        to='type1.lit'
        version='1.0'>

The type3.lit service also sends stream features.

Example 23. Stream Features

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
  <dialback xmlns='urn:xmpp:features:dialback'/>
</stream:features>

Because the type1.lit service does not support XMPP 1.0, it ignores the stream features and attempts to complete a server dialback negotiation with the type3.lit service. Therefore it sends a dialback key to the authoritative server for the type3.lit service.

Example 24. Dialback Key

<db:result
    from='type1.lit'
    to='type3.lit'>
  some-long-dialback-key
</db:result>

The type3.lit service then performs a DNS lookup on the type1.lit domain, opens a TCP connection at the discovered IP address and port, and establishes a stream with the authoritative server.

Example 25. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        to='type1.lit'
        version='1.0'>

The authoritative server for the type1.lit service then returns a response stream header.

Example 26. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type1.lit'
        id='idt1_t3r'
        to='type3.lit'>

The type3.lit service then sends a dialback verification request to the authoritative server for the type1.lit domain.

Example 27. Verification Request

<db:verify
    from='type3.lit'
    id='idt1_t3o'
    to='type1.lit'>
  some-long-dialback-key
</db:verify>

Here we assume that the authoritative server for the type1.lit domain notifies the type3.lit service that the key is valid.

Example 28. Key is Valid

<db:verify
    from='type1.lit'
    id='idt1_t3o'
    to='type2.lit'
    type='valid'>
  some-long-dialback-key
</db:verify>

The type3.lit service then returns a positive server dialback result to the originating server.

Example 29. Server Dialback Result

<db:result
    from='type3.lit'
    to='type1.lit'
    type='valid'>
  some-long-dialback-key
</db:result>

Because the connection is successful, the type1.lit service routes the XML stanza from citizen@type1.lit to the type3.lit service.

4.4 Type 1 to Type 4

In this scenario, an XMPP user citizen@type1.lit attempts to send an XML stanza to hamlet@type4.lit.

Example 30. Test Stanza

<iq from='citizen@type1.lit/foo'
    id='t1_t4'
    to='hamlet@type4.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type1.lit service (which supports verified connections only and does not have a certificate) attempts to initiate a server-to-server connection with type4.lit (which does not accept verified connections and has a self-signed certificate).

First, the type1.lit service sends an initial stream header to type4.lit.

Example 31. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type1.lit'
        to='type4.lit'>

Next the type4.lit service sends a response stream header to type1.lit.

Example 32. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type4.lit'
        id='idt1_t4o'
        to='type1.lit'
        version='1.0'>

The type4.lit service also sends stream features. Because the type4.lit service does not accept verified connections, it returns stream features with a notation that STARTTLS is required.

Example 33. Stream Features

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
    <required/>
  </starttls>
</stream:features>

Because the type1.lit service does not support XMPP 1.0, it ignores the stream features and attempts to complete a server dialback negotiation with the type4.lit service. Therefore it sends a dialback key to the authoritative server for the type4.lit service.

Example 34. Dialback Key

<db:result
    from='type1.lit'
    to='type4.lit'>
  some-long-dialback-key
</db:result>

The type4.lit service understands the server dialback protocol but since it requires STARTTLS at this point in the stream negotiation it returns a stream error to the type1.lit service, which should be <not-authorized/>.

Example 35. Stream Error

<stream:error>
  <not-authorized 
      xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>

Because the connection is unsuccessful, the type1.lit service returns a stanza error to citizen@type1.lit, which should be <remote-server-timeout/>.

Example 36. Error Stanza

<iq from='romeo@type4.lit'
    id='t1_t4'
    to='citizen@type1.lit/foo'
    type='error'>
  <error type='cancel'>
    <remote-server-timeout
        xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>

4.5 Type 1 to Type 5

In this scenario, an XMPP user citizen@type1.lit attempts to send an XML stanza to bill@type5.lit.

Example 37. Test Stanza

<iq from='citizen@type1.lit/foo'
    id='t1_t5'
    to='bill@type5.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type1.lit service (which supports verified connections only and does not have a certificate) attempts to initiate a server-to-server connection with type5.lit (which does not accept verified connections and has a CA-issued signed certificate).

First, the type1.lit service sends an initial stream header to type5.lit.

Example 38. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type1.lit'
        to='type5.lit'>

Next the type5.lit service sends a response stream header to type1.lit.

Example 39. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type5.lit'
        id='idt1_t5o'
        to='type1.lit'
        version='1.0'>

The type5.lit service also sends stream features. Because the type5.lit service does not accept verified connections, it returns stream features with a notation that STARTTLS is required.

Example 40. Stream Features

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
    <required/>
  </starttls>
</stream:features>

Because the type1.lit service does not support XMPP 1.0, it ignores the stream features and attempts to complete a server dialback negotiation with the type5.lit service. Therefore it sends a dialback key to the authoritative server for the type5.lit service.

Example 41. Dialback Key

<db:result
    from='type1.lit'
    to='type5.lit'>
  some-long-dialback-key
</db:result>

The type5.lit service understands the server dialback protocol but since it requires STARTTLS at this point in the stream negotiation it returns a stream error to the type1.lit service, which should be <not-authorized/>.

Example 42. Stream Error

<stream:error>
  <not-authorized 
      xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>

Because the connection is unsuccessful, the type1.lit service returns a stanza error to citizen@type1.lit, which should be <remote-server-timeout/>.

Example 43. Error Stanza

<iq from='bill@type5.lit'
    id='t1_t5'
    to='citizen@type1.lit/foo'
    type='error'>
  <error type='cancel'>
    <remote-server-timeout
        xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>

4.6 Type 1 to Type 6

In this scenario, an XMPP user citizen@type1.lit attempts to send an XML stanza to chris@type6.lit.

Example 44. Test Stanza

<iq from='citizen@type1.lit/foo'
    id='t1_t6'
    to='chris@type6.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type1.lit service (which supports verified connections only and does not have a certificate) attempts to initiate a server-to-server connection with the type6.lit service (which accepts only trusted connections, has a CA-issued certificate, and does not support Server Dialback).

First, the type1.lit service sends an initial stream header to type6.lit.

Example 45. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type1.lit'
        to='type6.lit'>

Next the type6.lit service sends a response stream header to type1.lit. Notice that the response stream header does not include the dialback namespace, since the type6.lit service does not support Server Dialback.

Example 46. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type6.lit'
        id='idt1_t6o'
        to='type1.lit'
        version='1.0'>

The type6.lit service also sends stream features. Because the type6.lit service does not accept untrusted connections, it returns stream features with a notation that STARTTLS is required.

Example 47. Stream Features

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
    <required/>
  </starttls>
</stream:features>

The type1.lit service does not detect support for server dialback by the type6.lit service but in any case attempts to complete server dialback.

Example 48. Dialback Key

<db:result
    from='type1.lit'
    to='type6.lit'>
  some-long-dialback-key
</db:result>

The type6.lit service does not accept dialback negotiations so it returns a <not-authorized/> stream error and closes the stream.

Example 49. Stream Error

<stream:error>
  <not-authorized 
      xmlns='urn:ietf:params:xml:ns:xmpp-streams'/>
</stream:error>
</stream:stream>

The type1.lit service closes the stream as well.

Example 50. Stream Close

</stream:stream>

Because the connection is unsuccessful, the type1.lit service returns a stanza error to citizen@type1.lit, which should be <remote-server-timeout/>.

Example 51. Error Stanza

<iq from='chris@type6.lit'
    id='t1_t6'
    to='citizen@type1.lit/foo'
    type='error'>
  <error type='cancel'>
    <remote-server-timeout
        xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>

5. Connections from Type 2 Services

5.1 Type 2 to Type 1

In this scenario, an XMPP user juliet@type2.lit attempts to send an XML stanza to citizen@type1.lit:

Example 52. Test Stanza

<iq from='juliet@type2.lit/foo'
    id='t2_t1'
    to='citizen@type1.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type2.lit service (which accepts verified connections and has a self-signed certificate) attempts to initiate a server-to-server connection with the type1.lit service (which supports verified connections only and does not have a certificate).

First, the type2.lit service sends an initial stream header to type1.lit.

Example 53. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        to='type1.lit'
        version='1.0'>

Next the type1.lit service sends a response stream header to type2.lit.

Example 54. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type1.lit'
        id='idt1_t2o'
        to='type2.lit'>

Because the type1.lit service does not support XMPP 1.0, it does not send stream features. Because the type2.lit service accepts verified connections, it attempts to verify the identity of type1.lit using server dialback. Therefore it sends a dialback key to type1.lit over the existing connection.

Example 55. Dialback Key

<db:result
    from='type2.lit'
    to='type1.lit'>
  some-long-dialback-key
</db:result>

The type1.lit service then performs a DNS lookup on the type2.lit domain, opens a TCP connection at the discovered IP address and port, and establishes a stream with the authoritative server for the type2.lit service.

Example 56. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type1.lit'
        to='type2.lit'>

The authoritative server for the type2.lit service then returns a response stream header.

Example 57. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        id='idt1_t2r'
        to='type1.lit'
        version='1.0'>

The type1.lit service then sends a dialback verification request to the authoritative server for the type2.lit domain.

Example 58. Verification Request

<db:verify
    from='type1.lit'
    id='idt2_t1o'
    to='type2.lit'>
  some-long-dialback-key
</db:verify>

Here we assume that the authoritative server for the type2.lit domain notifies the type1.lit service that the key is valid.

Example 59. Key is Valid

<db:verify
    from='type2.lit'
    id='idt1_t1o'
    to='type1.lit'
    type='valid'>
  some-long-dialback-key
</db:verify>

The type1.lit service then returns a positive server dialback result to the originating server (i.e., type2.lit).

Example 60. Server Dialback Result

<db:result
    from='type1.lit'
    to='type2.lit'
    type='valid'>
  some-long-dialback-key
</db:result>

Because the connection is successful, the type2.lit service routes the XML stanza from juliet@type2.lit to the type1.lit service.

5.2 Type 2 to Type 2

In this scenario, an XMPP user juliet@type2.lit attempts to send an XML stanza to user@example.lit:

Example 61. Test Stanza

<iq from='juliet@type2.lit/foo'
    id='t2_t2'
    to='user@example.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type2.lit service (which accepts verified connections and has a self-signed certificate) attempts to initiate a server-to-server connection with the example.lit service (which also supports verified connections and has a self-signed certificate).

First, the type2.lit service sends an initial stream header to example.lit.

Example 62. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        to='example.lit'
        version='1.0'>

Next the example.lit service sends a response stream header to type2.lit.

Example 63. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='example.lit'
        id='idt2_t2o'
        to='type2.lit'>

Because the example.lit service supports XMPP 1.0, it also sends stream features.

Example 64. Stream Features

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
  <dialback xmlns='urn:xmpp:features:dialback'/>
</stream:features>

We assume that type2.lit does not attempt STARTTLS negotiation but instead attempts server dialback for weak identity verification.

Example 65. Dialback Key

<db:result
    from='type2.lit'
    to='example.lit'>
  some-long-dialback-key
</db:result>

The example.lit service then performs a DNS lookup on the type2.lit domain, opens a TCP connection at the discovered IP address and port, and establishes a stream with the authoritative server for the type2.lit service.

Example 66. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='example.lit'
        to='type2.lit'>

The authoritative server for the type2.lit service then returns a response stream header.

Example 67. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        id='idt2_t2r'
        to='example.lit'
        version='1.0'>

The example.lit service then sends a dialback verification request to the authoritative server for the type2.lit domain.

Example 68. Verification Request

<db:verify
    from='example.lit'
    id='idt2_t2o'
    to='type2.lit'>
  some-long-dialback-key
</db:verify>

Here we assume that the authoritative server for the type2.lit domain notifies the example.lit service that the key is valid.

Example 69. Key is Valid

<db:verify
    from='type2.lit'
    id='idt2_t2o'
    to='example.lit'
    type='valid'>
  some-long-dialback-key
</db:verify>

The example.lit service then returns a positive server dialback result to the originating server (i.e., type2.lit).

Example 70. Server Dialback Result

<db:result
    from='example.lit'
    to='type2.lit'
    type='valid'>
  some-long-dialback-key
</db:result>

Because the connection is successful, the type2.lit service routes the XML stanza from juliet@type2.lit to the example.lit service.

5.3 Type 2 to Type 3

In this scenario, an XMPP user juliet@type2.lit attempts to send an XML stanza to romeo@type3.lit:

Example 71. Test Stanza

<iq from='juliet@type2.lit/foo'
    id='t2_t3'
    to='romeo@type3.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type2.lit service (which accepts verified connections and has a self-signed certificate) attempts to initiate a server-to-server connection with the type3.lit service (which also supports verified connections and has a CA-issued certificate).

First, the type2.lit service sends an initial stream header to type3.lit.

Example 72. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        to='type3.lit'
        version='1.0'>

Next the type3.lit service sends a response stream header to type2.lit.

Example 73. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        id='idt2_t3o'
        to='type2.lit'>

Because the type3.lit service supports XMPP 1.0, it also sends stream features.

Example 74. Stream Features

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
  <dialback xmlns='urn:xmpp:features:dialback'/>
</stream:features>

We assume that type2.lit does not attempt STARTTLS negotiation but instead attempts server dialback for weak identity verification.

Example 75. Dialback Key

<db:result
    from='type2.lit'
    to='type3.lit'>
  some-long-dialback-key
</db:result>

The type3.lit service then performs a DNS lookup on the type2.lit domain, opens a TCP connection at the discovered IP address and port, and establishes a stream with the authoritative server for the type2.lit service.

Example 76. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        to='type2.lit'>

The authoritative server for the type2.lit service then returns a response stream header.

Example 77. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        id='idt2_t3r'
        to='type3.lit'
        version='1.0'>

The type3.lit service then sends a dialback verification request to the authoritative server for the type2.lit domain.

Example 78. Verification Request

<db:verify
    from='type3.lit'
    id='idt2_t3o'
    to='type2.lit'>
  some-long-dialback-key
</db:verify>

Here we assume that the authoritative server for the type2.lit domain notifies the type3.lit service that the key is valid.

Example 79. Key is Valid

<db:verify
    from='type2.lit'
    id='idt2_t3o'
    to='type3.lit'
    type='valid'>
  some-long-dialback-key
</db:verify>

The type3.lit service then returns a positive server dialback result to the originating server (i.e., type2.lit).

Example 80. Server Dialback Result

<db:result
    from='type3.lit'
    to='type2.lit'
    type='valid'>
  some-long-dialback-key
</db:result>

Because the connection is successful, the type2.lit service routes the XML stanza from juliet@type2.lit to the type3.lit service.

5.4 Type 2 to Type 4

In this scenario, an XMPP user juliet@type2.lit attempts to send an XML stanza to hamlet@type4.lit:

Example 81. Test Stanza

<iq from='juliet@type2.lit/foo'
    id='t2_t4'
    to='hamlet@type4.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type2.lit service (which accepts verified connections and has a self-signed certificate) attempts to initiate a server-to-server connection with the type4.lit service (which also supports verified connections and has a CA-issued certificate).

First, the type2.lit service sends an initial stream header to type4.lit.

Example 82. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        to='type4.lit'
        version='1.0'>

Next the type4.lit service sends a response stream header to type2.lit.

Example 83. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type4.lit'
        id='idt2_t4o'
        to='type2.lit'>

Because the type4.lit service supports XMPP 1.0, it also sends stream features.

Example 84. Stream Features

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
    <required/>
  </starttls>
  <dialback xmlns='urn:xmpp:features:dialback'/>
</stream:features>

Because type4.lit requires encryption, type2.lit attempts to negotiate a STARTTLS upgrade to the stream.

Example 85. STARTTLS Request

<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

Example 86. STARTTLS Response

<proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

The servers then negotiate TLS. We assume the negotiation is successful.

The type2.lit service then opens a new stream over the encrypted connection.

Example 87. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        to='type4.lit'
        version='1.0'>

Next the type4.lit service sends a response stream header to type2.lit.

Example 88. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type4.lit'
        id='idt2_t4o2'
        to='type2.lit'>

Because the type4.lit service supports XMPP 1.0, it also sends stream features.

Example 89. Stream Features

<stream:features>
  <dialback xmlns='urn:xmpp:features:dialback'>
    <required/>
  </dialback>
</stream:features>

Notice that type4.lit requires dialback here (perhaps because of some local service policy). Therefore type2.lit sends a dialback key to type4.lit.

Example 90. Dialback Key

<db:result
    from='type2.lit'
    to='type4.lit'>
  some-long-dialback-key
</db:result>

The type4.lit service then performs a DNS lookup on the type2.lit domain, opens a TCP connection at the discovered IP address and port, and establishes a stream with the authoritative server for the type2.lit service.

Example 91. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type4.lit'
        to='type2.lit'>

The authoritative server for the type2.lit service then returns a response stream header.

Example 92. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        id='idt2_t4r'
        to='type4.lit'
        version='1.0'>

The type4.lit service then sends a dialback verification request to the authoritative server for the type2.lit domain.

Example 93. Verification Request

<db:verify
    from='type4.lit'
    id='idt2_t4o'
    to='type2.lit'>
  some-long-dialback-key
</db:verify>

Here we assume that the authoritative server for the type2.lit domain notifies the type4.lit service that the key is valid.

Example 94. Key is Valid

<db:verify
    from='type2.lit'
    id='idt2_t4o'
    to='type4.lit'
    type='valid'>
  some-long-dialback-key
</db:verify>

The type4.lit service then returns a positive server dialback result to the originating server (i.e., type2.lit).

Example 95. Server Dialback Result

<db:result
    from='type4.lit'
    to='type2.lit'
    type='valid'>
  some-long-dialback-key
</db:result>

Because the connection is successful, the type2.lit service routes the XML stanza from juliet@type2.lit to the type4.lit service.

5.5 Type 2 to Type 5

In this scenario, an XMPP user juliet@type2.lit attempts to send an XML stanza to bill@type5.lit:

Example 96. Test Stanza

<iq from='juliet@type2.lit/foo'
    id='t2_t5'
    to='bill@type5.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type2.lit service (which accepts verified connections and has a self-signed certificate) attempts to initiate a server-to-server connection with the type5.lit service (which also supports verified connections and has a CA-issued certificate).

First, the type2.lit service sends an initial stream header to type5.lit.

Example 97. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        to='type5.lit'
        version='1.0'>

Next the type5.lit service sends a response stream header to type2.lit.

Example 98. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type5.lit'
        id='idt2_t5o'
        to='type2.lit'>

Because the type5.lit service supports XMPP 1.0, it also sends stream features.

Example 99. Stream Features

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
    <required/>
  </starttls>
  <dialback xmlns='urn:xmpp:features:dialback'/>
</stream:features>

Because type5.lit requires encryption, type2.lit attempts to negotiate a STARTTLS upgrade to the stream.

Example 100. STARTTLS Request

<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

Example 101. STARTTLS Response

<proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

The servers then negotiate TLS. We assume the negotiation is successful.

The type2.lit service then opens a new stream over the encrypted connection.

Example 102. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        to='type5.lit'
        version='1.0'>

Next the type5.lit service sends a response stream header to type2.lit.

Example 103. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type5.lit'
        id='idt2_t5o2'
        to='type2.lit'>

Because the type5.lit service supports XMPP 1.0, it also sends stream features.

Example 104. Stream Features

<stream:features>
  <dialback xmlns='urn:xmpp:features:dialback'>
    <required/>
  </dialback>
</stream:features>

Notice that type5.lit requires dialback here (perhaps because of some local service policy). Therefore type2.lit sends a dialback key to type5.lit.

Example 105. Dialback Key

<db:result
    from='type2.lit'
    to='type5.lit'>
  some-long-dialback-key
</db:result>

The type5.lit service then performs a DNS lookup on the type2.lit domain, opens a TCP connection at the discovered IP address and port, and establishes a stream with the authoritative server for the type2.lit service.

Example 106. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type5.lit'
        to='type2.lit'>

The authoritative server for the type2.lit service then returns a response stream header.

Example 107. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        id='idt2_t5r'
        to='type5.lit'
        version='1.0'>

The type5.lit service then sends a dialback verification request to the authoritative server for the type2.lit domain.

Example 108. Verification Request

<db:verify
    from='type5.lit'
    id='idt2_t5o'
    to='type2.lit'>
  some-long-dialback-key
</db:verify>

Here we assume that the authoritative server for the type2.lit domain notifies the type5.lit service that the key is valid.

Example 109. Key is Valid

<db:verify
    from='type2.lit'
    id='idt2_t5o'
    to='type5.lit'
    type='valid'>
  some-long-dialback-key
</db:verify>

The type5.lit service then returns a positive server dialback result to the originating server (i.e., type2.lit).

Example 110. Server Dialback Result

<db:result
    from='type5.lit'
    to='type2.lit'
    type='valid'>
  some-long-dialback-key
</db:result>

Because the connection is successful, the type2.lit service routes the XML stanza from juliet@type2.lit to the type5.lit service.

5.6 Type 2 to Type 6

In this scenario, an XMPP user juliet@type2.lit attempts to send an XML stanza to chris@type6.lit.

Example 111. Test Stanza

<iq from='juliet@type2.lit/foo'
    id='t2_t6'
    to='chris@type6.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type2.lit service (which supports verified connections and has a self-signed certificate) attempts to initiate a server-to-server connection with the type6.lit service (which accepts only trusted connections, has a CA-issued certificate, and does not support Server Dialback).

First, the type2.lit service sends an initial stream header to type6.lit.

Example 112. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        to='type6.lit'>

Next the type6.lit service sends a response stream header to type2.lit. Notice that the response stream header does not include the dialback namespace, since the type6.lit service does not support Server Dialback.

Example 113. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type6.lit'
        id='idt2_t6o'
        to='type2.lit'
        version='1.0'>

The type6.lit service also sends stream features. Because the type6.lit service does not accept untrusted connections, it returns stream features with a notation that STARTTLS is required.

Example 114. Stream Features

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
    <required/>
  </starttls>
</stream:features>

Because type6.lit requires encryption, type2.lit attempts to negotiate a STARTTLS upgrade to the stream.

Example 115. STARTTLS Request

<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

Example 116. STARTTLS Response

<proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

The servers then attempt negotiate TLS. We assume the negotiation fails because type2.lit presents a self-signed certificate but type6.lit requires trusted federation relying on a common root CA.

Because the connection is unsuccessful, the type2.lit service returns a stanza error to juliet@type2.lit, which should be <remote-server-timeout/>.

Example 117. Error Stanza

<iq from='chris@type6.lit'
    id='t2_t6'
    to='juliet@type2.lit/foo'
    type='error'>
  <error type='cancel'>
    <remote-server-timeout
        xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>

6. Connections from Type 3 Services

6.1 Type 3 to Type 1

In this scenario, an XMPP user romeo@type3.lit attempts to send an XML stanza to citizen@type1.lit:

Example 118. Test Stanza

<iq from='romeo@type3.lit/foo'
    id='t3_t1'
    to='citizen@type1.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type3.lit service (which accepts verified connections and has a CA-issued certificate) attempts to initiate a server-to-server connection with the type1.lit service (which supports verified connections only and does not have a certificate).

First, the type3.lit service sends an initial stream header to type1.lit.

Example 119. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        to='type1.lit'
        version='1.0'>

Next the type1.lit service sends a response stream header to type3.lit.

Example 120. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type1.lit'
        id='idt3_t1o'
        to='type3.lit'>

Because the type1.lit service does not support XMPP 1.0, it does not send stream features. Therefore the type3.lit attempts to complete server dialback verification.

Example 121. Dialback Key

<db:result
    from='type3.lit'
    to='type1.lit'>
  some-long-dialback-key
</db:result>

The type1.lit service then performs a DNS lookup on the type3.lit domain, opens a TCP connection at the discovered IP address and port, and establishes a stream with the authoritative server for the type3.lit service.

Example 122. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type1.lit'
        to='type3.lit'>

The authoritative server for the type3.lit service then returns a response stream header.

Example 123. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        id='idt3_t1r'
        to='type1.lit'
        version='1.0'>

The type1.lit service then sends a dialback verification request to the authoritative server for the type3.lit domain.

Example 124. Verification Request

<db:verify
    from='type1.lit'
    id='idt3_t1o'
    to='type3.lit'>
  some-long-dialback-key
</db:verify>

Here we assume that the authoritative server for the type3.lit domain notifies the type1.lit service that the key is valid.

Example 125. Key is Valid

<db:verify
    from='type3.lit'
    id='idt3_t1o'
    to='type1.lit'
    type='valid'>
  some-long-dialback-key
</db:verify>

The type1.lit service then returns a positive server dialback result to the originating server (i.e., type3.lit).

Example 126. Server Dialback Result

<db:result
    from='type1.lit'
    to='type3.lit'
    type='valid'>
  some-long-dialback-key
</db:result>

Because the connection is successful, the type3.lit service routes the XML stanza from romeo@type3.lit to the type1.lit service.

6.2 Type 3 to Type 2

In this scenario, an XMPP user romeo@type3.lit attempts to send an XML stanza to juliet@type2.lit:

Example 127. Test Stanza

<iq from='romeo@type3.lit/foo'
    id='t3_t2'
    to='juliet@type2.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type3.lit service (which accepts verified connections and has a CA-issued certificate) attempts to initiate a server-to-server connection with the type2.lit service (which supports verified connections and has a self-signed certificate).

First, the type3.lit service sends an initial stream header to type2.lit.

Example 128. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        to='type2.lit'
        version='1.0'>

Next the type2.lit service sends a response stream header to type3.lit.

Example 129. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        id='idt3_t2o'
        to='type3.lit'>

Because the type3.lit service supports XMPP 1.0, it also sends stream features.

Example 130. Stream Features

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
  <dialback xmlns='urn:xmpp:features:dialback'/>
</stream:features>

We assume that type2.lit does not attempt STARTTLS negotiation but instead attempts server dialback for weak identity verification.

Example 131. Dialback Key

<db:result
    from='type3.lit'
    to='type2.lit'>
  some-long-dialback-key
</db:result>

The type2.lit service then performs a DNS lookup on the type3.lit domain, opens a TCP connection at the discovered IP address and port, and establishes a stream with the authoritative server for the type3.lit service.

Example 132. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        to='type3.lit'>

The authoritative server for the type3.lit service then returns a response stream header.

Example 133. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        id='idt3_t2r'
        to='type2.lit'
        version='1.0'>

The type2.lit service then sends a dialback verification request to the authoritative server for the type3.lit domain.

Example 134. Verification Request

<db:verify
    from='type2.lit'
    id='idt3_t2o'
    to='type3.lit'>
  some-long-dialback-key
</db:verify>

Here we assume that the authoritative server for the type3.lit domain notifies the type2.lit service that the key is valid.

Example 135. Key is Valid

<db:verify
    from='type3.lit'
    id='idt3_t1o'
    to='type2.lit'
    type='valid'>
  some-long-dialback-key
</db:verify>

The type2.lit service then returns a positive server dialback result to the originating server (i.e., type3.lit).

Example 136. Server Dialback Result

<db:result
    from='type2.lit'
    to='type3.lit'
    type='valid'>
  some-long-dialback-key
</db:result>

Because the connection is successful, the type3.lit service routes the XML stanza from romeo@type3.lit to the type2.lit service.

6.3 Type 3 to Type 3

In this scenario, an XMPP user romeo@type3.lit attempts to send an XML stanza to user@example.lit:

Example 137. Test Stanza

<iq from='romeo@type3.lit/foo'
    id='t3_t3'
    to='user@example.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type3.lit service (which accepts verified connections and has a CA-issued certificate) attempts to initiate a server-to-server connection with the example.lit service (which also supports verified connections and has a CA-issued certificate).

First, the type3.lit service sends an initial stream header to example.lit.

Example 138. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        to='example.lit'
        version='1.0'>

Next the example.lit service sends a response stream header to type3.lit.

Example 139. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='example.lit'
        id='idt3_t3o'
        to='type3.lit'>

Because the type3.lit service supports XMPP 1.0, it also sends stream features.

Example 140. Stream Features

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
  <dialback xmlns='urn:xmpp:features:dialback'/>
</stream:features>

We assume that example.lit does not attempt STARTTLS negotiation but instead attempts server dialback for weak identity verification.

Example 141. Dialback Key

<db:result
    from='type3.lit'
    to='example.lit'>
  some-long-dialback-key
</db:result>

The example.lit service then performs a DNS lookup on the type3.lit domain, opens a TCP connection at the discovered IP address and port, and establishes a stream with the authoritative server for the type3.lit service.

Example 142. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='example.lit'
        to='type3.lit'>

The authoritative server for the type3.lit service then returns a response stream header.

Example 143. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        id='idt3_t3r'
        to='example.lit'
        version='1.0'>

The example.lit service then sends a dialback verification request to the authoritative server for the type3.lit domain.

Example 144. Verification Request

<db:verify
    from='example.lit'
    id='idt3_t3o'
    to='type3.lit'>
  some-long-dialback-key
</db:verify>

Here we assume that the authoritative server for the type3.lit domain notifies the example.lit service that the key is valid.

Example 145. Key is Valid

<db:verify
    from='type3.lit'
    id='idt3_t1o'
    to='example.lit'
    type='valid'>
  some-long-dialback-key
</db:verify>

The example.lit service then returns a positive server dialback result to the originating server (i.e., type3.lit).

Example 146. Server Dialback Result

<db:result
    from='example.lit'
    to='type3.lit'
    type='valid'>
  some-long-dialback-key
</db:result>

Because the connection is successful, the type3.lit service routes the XML stanza from romeo@type3.lit to the example.lit service.

6.4 Type 3 to Type 4

In this scenario, an XMPP user romeo@type3.lit attempts to send an XML stanza to hamlet@type4.lit:

Example 147. Test Stanza

<iq from='romeo@type3.lit/foo'
    id='t3_t4'
    to='hamlet@type4.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type3.lit service (which accepts verified connections and has a CA-issued certificate) attempts to initiate a server-to-server connection with the type4.lit service (which also supports verified connections and has a CA-issued certificate).

First, the type3.lit service sends an initial stream header to type4.lit.

Example 148. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        to='type4.lit'
        version='1.0'>

Next the type4.lit service sends a response stream header to type3.lit.

Example 149. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type4.lit'
        id='idt3_t4o'
        to='type3.lit'>

Because the type4.lit service supports XMPP 1.0, it also sends stream features.

Example 150. Stream Features

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
    <required/>
  </starttls>
  <dialback xmlns='urn:xmpp:features:dialback'/>
</stream:features>

Because type4.lit requires encryption, type3.lit attempts to negotiate a STARTTLS upgrade to the stream.

Example 151. STARTTLS Request

<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

Example 152. STARTTLS Response

<proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

The servers then negotiate TLS. We assume the negotiation is successful.

The type3.lit service then opens a new stream over the encrypted connection.

Example 153. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        to='type4.lit'
        version='1.0'>

Next the type4.lit service sends a response stream header to type3.lit.

Example 154. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type4.lit'
        id='idt3_t4o2'
        to='type3.lit'>

Because the type4.lit service supports XMPP 1.0, it also sends stream features.

Example 155. Stream Features

<stream:features>
  <dialback xmlns='urn:xmpp:features:dialback'>
    <required/>
  </dialback>
</stream:features>

Notice that type4.lit requires dialback here (perhaps because of some local service policy). Therefore type3.lit sends a dialback key to type4.lit.

Example 156. Dialback Key

<db:result
    from='type3.lit'
    to='type4.lit'>
  some-long-dialback-key
</db:result>

The type4.lit service then performs a DNS lookup on the type3.lit domain, opens a TCP connection at the discovered IP address and port, and establishes a stream with the authoritative server for the type3.lit service.

Example 157. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type4.lit'
        to='type3.lit'>

The authoritative server for the type3.lit service then returns a response stream header.

Example 158. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        id='idt3_t4r'
        to='type4.lit'
        version='1.0'>

The type4.lit service then sends a dialback verification request to the authoritative server for the type3.lit domain.

Example 159. Verification Request

<db:verify
    from='type4.lit'
    id='idt3_t4o'
    to='type3.lit'>
  some-long-dialback-key
</db:verify>

Here we assume that the authoritative server for the type3.lit domain notifies the type4.lit service that the key is valid.

Example 160. Key is Valid

<db:verify
    from='type3.lit'
    id='idt3_t4o'
    to='type4.lit'
    type='valid'>
  some-long-dialback-key
</db:verify>

The type4.lit service then returns a positive server dialback result to the originating server (i.e., type3.lit).

Example 161. Server Dialback Result

<db:result
    from='type4.lit'
    to='type3.lit'
    type='valid'>
  some-long-dialback-key
</db:result>

Because the connection is successful, the type3.lit service routes the XML stanza from romeo@type3.lit to the type4.lit service.

6.5 Type 3 to Type 5

In this scenario, an XMPP user romeo@type3.lit attempts to send an XML stanza to bill@type5.lit:

Example 162. Test Stanza

<iq from='romeo@type3.lit/foo'
    id='t3_t5'
    to='bill@type5.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type3.lit service (which accepts verified connections and has a CA-issued certificate) attempts to initiate a server-to-server connection with the type5.lit service (which also supports encrypted connections and has a CA-issued certificate).

First, the type3.lit service sends an initial stream header to type5.lit.

Example 163. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        to='type5.lit'
        version='1.0'>

Next the type5.lit service sends a response stream header to type3.lit.

Example 164. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type5.lit'
        id='idt3_t5o'
        to='type3.lit'>

Because the type5.lit service supports XMPP 1.0, it also sends stream features.

Example 165. Stream Features

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
    <required/>
  </starttls>
  <dialback xmlns='urn:xmpp:features:dialback'/>
</stream:features>

Because type5.lit requires encryption, type3.lit attempts to negotiate a STARTTLS upgrade to the stream.

Example 166. STARTTLS Request

<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

Example 167. STARTTLS Response

<proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

The servers then negotiate TLS. We assume the negotiation is successful.

The type3.lit service then opens a new stream over the encrypted connection.

Example 168. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        to='type5.lit'
        version='1.0'>

Next the type5.lit service sends a response stream header to type3.lit.

Example 169. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type5.lit'
        id='idt3_t5o2'
        to='type3.lit'>

Because the type5.lit service supports XMPP 1.0, it also sends stream features.

Example 170. Stream Features

<stream:features>
  <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
    <mechanism>EXTERNAL</mechanism>
    <required/>
  </mechanisms>
</stream:features>

Notice that type5.lit requires use of SASL EXTERNAL here (because the certificate presented by type3.lit was issued by a common root CA). Therefore type3.lit attempts to complete SASL negotiation.

Example 171. SASL Mechanism Selection

<auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
      mechanism='EXTERNAL'/>dHlwZTMubGl0</auth>

The type5.lit service determines that the authorization identity provided by type3.lit matches the information in the presented certificate and therefore returns success.

Example 172. SASL Success

<success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

The type3.lit service then opens a new stream over the encrypted connection.

Example 173. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        to='type5.lit'
        version='1.0'>

Next the type5.lit service sends a response stream header to type3.lit.

Example 174. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type5.lit'
        id='idt3_t5o3'
        to='type3.lit'>

Because the type5.lit service supports XMPP 1.0, it also sends stream features (which in this case are empty).

Example 175. Stream Features

<stream:features/>

Because the connection is successful, the type3.lit service routes the XML stanza from romeo@type3.lit to the type5.lit service.

6.6 Type 3 to Type 6

In this scenario, an XMPP user romeo@type3.lit attempts to send an XML stanza to chris@type6.lit:

Example 176. Test Stanza

<iq from='romeo@type3.lit/foo'
    id='t3_t6'
    to='chris@type6.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type3.lit service (which accepts verified connections and has a CA-issued certificate) attempts to initiate a server-to-server connection with the type6.lit service (which requires trusted communications and has a CA-issued certificate).

First, the type3.lit service sends an initial stream header to type6.lit.

Example 177. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        to='type6.lit'
        version='1.0'>

Next the type6.lit service sends a response stream header to type3.lit.

Example 178. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type6.lit'
        id='idt3_t6o'
        to='type3.lit'>

Because the type6.lit service supports XMPP 1.0, it also sends stream features.

Example 179. Stream Features

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
    <required/>
  </starttls>
  <dialback xmlns='urn:xmpp:features:dialback'/>
</stream:features>

Because type6.lit requires encryption, type3.lit attempts to negotiate a STARTTLS upgrade to the stream.

Example 180. STARTTLS Request

<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

Example 181. STARTTLS Response

<proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

The servers then negotiate TLS. We assume the negotiation is successful.

The type3.lit service then opens a new stream over the encrypted connection.

Example 182. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        to='type6.lit'
        version='1.0'>

Next the type6.lit service sends a response stream header to type3.lit.

Example 183. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type6.lit'
        id='idt3_t6o2'
        to='type3.lit'>

Because the type6.lit service supports XMPP 1.0, it also sends stream features.

Example 184. Stream Features

<stream:features>
  <mechanisms xmlns='urn:ietf:params:xml:ns:xmpp-sasl'>
    <mechanism>EXTERNAL</mechanism>
    <required/>
  </mechanisms>
</stream:features>

Notice that type6.lit requires use of SASL EXTERNAL here (because the certificate presented by type3.lit was issued by a common root CA). Therefore type3.lit attempts to complete SASL negotiation.

Example 185. SASL Mechanism Selection

<auth xmlns='urn:ietf:params:xml:ns:xmpp-sasl'
      mechanism='EXTERNAL'/>dHlwZTMubGl0</auth>

The type6.lit service determines that the authorization identity provided by type3.lit matches the information in the presented certificate and therefore returns success.

Example 186. SASL Success

<success xmlns='urn:ietf:params:xml:ns:xmpp-sasl'/>

The type3.lit service then opens a new stream over the encrypted connection.

Example 187. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        to='type6.lit'
        version='1.0'>

Next the type6.lit service sends a response stream header to type3.lit.

Example 188. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type6.lit'
        id='idt3_t6o3'
        to='type3.lit'>

Because the type6.lit service supports XMPP 1.0, it also sends stream features (which in this case are empty).

Example 189. Stream Features

<stream:features/>

Because the connection is successful, the type3.lit service routes the XML stanza from romeo@type3.lit to the type6.lit service.

7. Connections from Type 4 Services

7.1 Type 4 to Type 1

In this scenario, an XMPP user hamlet@type4.lit attempts to send an XML stanza to citizen@type1.lit:

Example 190. Test Stanza

<iq from='hamlet@type4.lit/foo'
    id='t4_t1'
    to='citizen@type1.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type4.lit service (which requires encrypted connections and has a self-signed certificate) attempts to initiate a server-to-server connection with the type1.lit service (which supports verified connections only and does not have a certificate).

First, the type4.lit service sends an initial stream header to type1.lit.

Example 191. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type4.lit'
        to='type1.lit'
        version='1.0'>

Next the type1.lit service sends a response stream header to type4.lit.

Example 192. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type1.lit'
        id='idt4_t1o'
        to='type4.lit'>

Because the type1.lit service does not support XMPP 1.0, it does not send stream features. Because the type4.lit service requires encryption via TLS, it cannot proceed further with the stream negotiation and closes the stream.

Example 193. Stream Close

</stream:stream>

The type1.lit service closes the stream as well.

Example 194. Stream Close

</stream:stream>

Because the connection is unsuccessful, the type4.lit service returns a stanza error to hamlet@type4.lit, which should be <remote-server-timeout/>.

Example 195. Error Stanza

<iq from='citizen@type1.lit'
    id='t4_t1'
    to='hamlet@type4.lit/foo'
    type='error'>
  <error type='cancel'>
    <remote-server-timeout
        xmlns='urn:ietf:params:xml:ns:xmpp-stanzas'/>
  </error>
</iq>

7.2 Type 4 to Type 2

In this scenario, an XMPP user hamlet@type4.lit attempts to send an XML stanza to juliet@type2.lit:

Example 196. Test Stanza

<iq from='hamlet@type4.lit/foo'
    id='t4_t2'
    to='juliet@type2.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type4.lit service (which requires encrypted connections and has a self-signed certificate) attempts to initiate a server-to-server connection with the type2.lit service (which supports verified connections and has a self-signed certificate).

First, the type4.lit service sends an initial stream header to type2.lit.

Example 197. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type4.lit'
        to='type2.lit'
        version='1.0'>

Next the type2.lit service sends a response stream header to type4.lit.

Example 198. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        id='idt4_t2o'
        to='type4.lit'>

Because the type2.lit service supports XMPP 1.0, it also sends stream features.

Example 199. Stream Features

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
  <dialback xmlns='urn:xmpp:features:dialback'/>
</stream:features>

Because the type4.lit service requires encryption, it attempts STARTTLS negotiation.

Example 200. STARTTLS Request

<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

Example 201. STARTTLS Response

<proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

The servers then negotiate TLS. We assume the negotiation is successful.

The type4.lit service then opens a new stream over the encrypted connection.

Example 202. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type4.lit'
        to='type2.lit'
        version='1.0'>

Next the type2.lit service sends a response stream header to type4.lit.

Example 203. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        id='idt4_t2o2'
        to='type4.lit'>

Because the type4.lit service supports XMPP 1.0, it also sends stream features.

Example 204. Stream Features

<stream:features>
  <dialback xmlns='urn:xmpp:features:dialback'>
    <required/>
  </dialback>
</stream:features>

Notice that type2.lit requires dialback here (perhaps because of some local service policy). Therefore type4.lit sends a dialback key to type2.lit.

Example 205. Dialback Key

<db:result
    from='type4.lit'
    to='type2.lit'>
  some-long-dialback-key
</db:result>

The type2.lit service then performs a DNS lookup on the type4.lit domain, opens a TCP connection at the discovered IP address and port, and establishes a stream with the authoritative server for the type4.lit service.

Example 206. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type2.lit'
        to='type4.lit'>

The authoritative server for the type4.lit service then returns a response stream header.

Example 207. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type4.lit'
        id='idt4_t2r'
        to='type2.lit'
        version='1.0'>

The type2.lit service then sends a dialback verification request to the authoritative server for the type4.lit domain.

Example 208. Verification Request

<db:verify
    from='type2.lit'
    id='idt4_t2o'
    to='type4.lit'>
  some-long-dialback-key
</db:verify>

Here we assume that the authoritative server for the type4.lit domain notifies the type2.lit service that the key is valid.

Example 209. Key is Valid

<db:verify
    from='type4.lit'
    id='idt4_t2o'
    to='type2.lit'
    type='valid'>
  some-long-dialback-key
</db:verify>

The type2.lit service then returns a positive server dialback result to the originating server (i.e., type4.lit).

Example 210. Server Dialback Result

<db:result
    from='type2.lit'
    to='type4.lit'
    type='valid'>
  some-long-dialback-key
</db:result>

Because the connection is successful, the type4.lit service routes the XML stanza from hamlet@type4.lit to the type2.lit service.

7.3 Type 4 to Type 3

In this scenario, an XMPP user hamlet@type4.lit attempts to send an XML stanza to romeo@type3.lit:

Example 211. Test Stanza

<iq from='hamlet@type4.lit/foo'
    id='t4_t3'
    to='romeo@type3.lit'
    type='get'>
  <ping xmlns='urn:xmpp:ping'/>
</iq>

Therefore the type4.lit service (which requires encrypted connections and has a self-signed certificate) attempts to initiate a server-to-server connection with the type3.lit service (which supports verified connections and has a CA-issued certificate).

First, the type4.lit service sends an initial stream header to type3.lit.

Example 212. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type4.lit'
        to='type3.lit'
        version='1.0'>

Next the type3.lit service sends a response stream header to type4.lit.

Example 213. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        id='idt4_t3o'
        to='type4.lit'>

Because the type3.lit service supports XMPP 1.0, it also sends stream features.

Example 214. Stream Features

<stream:features>
  <starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>
  <dialback xmlns='urn:xmpp:features:dialback'/>
</stream:features>

Because the type4.lit service requires encryption, it attempts STARTTLS negotiation.

Example 215. STARTTLS Request

<starttls xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

Example 216. STARTTLS Response

<proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'/>

The servers then negotiate TLS. We assume the negotiation is successful.

The type4.lit service then opens a new stream over the encrypted connection.

Example 217. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type4.lit'
        to='type3.lit'
        version='1.0'>

Next the type3.lit service sends a response stream header to type4.lit.

Example 218. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        id='idt4_t3o2'
        to='type4.lit'>

Because the type4.lit service supports XMPP 1.0, it also sends stream features.

Example 219. Stream Features

<stream:features>
  <dialback xmlns='urn:xmpp:features:dialback'>
    <required/>
  </dialback>
</stream:features>

Notice that type3.lit requires dialback here (perhaps because of some local service policy). Therefore type4.lit sends a dialback key to type3.lit.

Example 220. Dialback Key

<db:result
    from='type4.lit'
    to='type3.lit'>
  some-long-dialback-key
</db:result>

The type3.lit service then performs a DNS lookup on the type4.lit domain, opens a TCP connection at the discovered IP address and port, and establishes a stream with the authoritative server for the type4.lit service.

Example 221. Initial Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type3.lit'
        to='type4.lit'>

The authoritative server for the type4.lit service then returns a response stream header.

Example 222. Response Stream Header

<stream:stream
        xmlns='jabber:server'
        xmlns:db='jabber:server:dialback'
        xmlns:stream='http://etherx.jabber.lit/streams'
        from='type4.lit'
        id='idt4_t3r'
        to='type3.lit'
        version='1.0'>

The type3.lit service then sends a dialback verification request to the authoritative server for the type4.lit domain.

Example 223. Verification Request

<db:verify
    from='type3.lit'
    id='idt4_t3o'
    to='type4.lit'>
  some-long-dialback-key
</db:verify>

Here we assume that the authoritative server for the type4.lit domain notifies the type3.lit service that the key is valid.

Example 224. Key is Valid

<db:verify
    from='type4.lit'
    id='idt4_t3o'
    to='type3.lit'
    type='valid'>
  some-long-dialback-key
</db:verify>