Note Well: This protocol specified in this document has been superseded by the protocol specified in &xep0124;.

This specification documents a method to allow Jabber clients to access Jabber servers from behind existing firewalls. Although several similar methods have been proposed, this approach should work through all known firewall configurations which allow outbound HTTP access.

In general, a firewall is a box that protects a network from outsiders, by controlling the IP connections that are allowed to pass through the box. Often, a firewall will also allow access outside only by proxy, either explicit proxy support or implicit through Network Address Translation (NAT).

In the interest of security, many firewall administrators do not allow outbound connections to unknown and unused ports. Until Jabber becomes more widely deployed, port 5222/tcp (for Jabber client connections) will often be blocked.

The best solution for sites that are concerned about security is to run their own Jabber server, either inside the firewall, or in a DMZ DMZ definition at searchwebmanagement.com network. However, there are network configuration where an external Jabber server must still be used and port 5222/tcp outbound cannot be allowed. In these situations, different methods for connecting to a Jabber server are required. Several methods exist today for doing this traversal. Most rely on the fact that a most firewalls are configured to allow access through port 80/tcp. Although some less-complicated firewalls will allow any protocol to traverse this port, many will proxy, filter, and verify requests on this port as HTTP. Because of this, a normal Jabber connection on port 80/tcp will not suffice.

In addition, many firewalls/proxy servers will also not allow or not honor HTTP Keep-alives (as defined in section 19.7.1.1 of &rfc2068;) and will consider long-lived socket connections as security issues. Because of this the traditional Jabber connection model, where one socket is one stream is one session, will not work reliably.

In light of all of the ways that default firewall rules can interfere with Jabber connectivity, a lowest-common denominator approach was selected. HTTP is used to send XML as POST requests and receieve pending XML within the responses. Additional information is prepended in the request body to ensure an equivalent level of security to TCP/IP sockets.

The client makes HTTP requests periodically to the server. Whenever the client has something to send, that XML is included in the body of the request. When the server has something to send to the client, it must be contained in the body of the response.

In some browser/platform combinations, sending cookies from the client is not possible due to design choices and limitations in the browser. Therefore, a work-around was needed to support clients based on these application platforms.

All requests to the server are HTTP POST requests, with Content-Type: application/x-www-form-urlencoded. Responses from the server have Content-Type: text/xml. Both the request and response bodies are UTF-8 encoded text, even if an HTTP header to the contrary exists. All responses contain a Set-Cookie header with an identifier, which is sent along with future requests as described below. This identifier cookie must have a name of 'ID'. The first request to a server always uses 0 as the identifier. The server must always return a 200 response code, sending any session errors as specially-formatted identifiers.

The client sends requests with bodies in the following format: identifier ; key [ ; new_key] , [xml_body] If the identifier is zero, key indicates an initial key. In this case, new_key should not be specified, and must be ignored.

Identifier	Purpose
identifier	To uniquely identify the session server-side. This field is only used to identify the session, and provides no security.
key	To verify this request is from the originator of the session. The client generates a new key in the manner described below for each request, which the server then verifies before processing the request.
new_key	The key algorithm can exhaust valid keys in a sequence, which requires a new key sequence to be used in order to continue the session. The new key is sent along with the last used key in the old sequence.
xml_body	The body of text to send. Since a POST must be sent in order for the server to respond with recent messages, a client may send a request without an xml_body in order to just retrieve new incoming packets. This is not required to be a full XML document or XML fragment, it does not need to start or end on element boundaries.

The identifier is everything before the first semicolon, and must consist of the characters [A-Za-z0-9:-]. The identifier returned from the first request is the identifier for the session. Any new identifier that ends in ':0' indicates an error, with the entire identifier indicating the specific error condition. Any new identifier that does not end in ':0' is a server programming error, the client should discontinue the session. For new sessions, the client identifier is considered to be 0.

The key is a client security feature to allow TCP/IP socket equivalent security. It does not protect against intermediary attacks, but does prevent a person who is capable of listening to the HTTP traffic from sending messages and receiving incoming traffic from another machine.

The key algorithm should be familiar with those with knowledge of Jabber zero-knowledge authentication.

Note: Base64 encoding is defined in &rfc3548;. SHA1 is defined in &rfc3174;.

No framing is implied by a single request or reply. A single request can have no content sent, in which case the body contains only the identifier followed by a comma. A reply may have no content to send, in which case the body is empty. Zero or more XMPP packets may be sent in a single request or reply, including partial XMPP packets.

The absense of a long-lived connection requires the server to consider client traffic as a heartbeat to keep the session alive. If a server-configurable period of time passes without a successful POST request sent by the client, the server must end the client session. Any client requests using the identifier associated with that now dead session must return an error of '0:0'.

The maximum period of time to keep a client session active without an incoming POST request is not defined, but five minutes is the recommended minimum. The maximum period of time recommended for clients between requests is two minutes; if the client has not sent any XML out for two minutes, a request without an XML body should be sent. If a client is disconnecting from the server, a closing <stream:stream> must be sent to end the session. Failure to do this may have the client continue to be represented to other users as available.

If the server disconnects the user do to a session timeout, the server MUST bounce pending IQ requests and either bounce or store offline incoming messages.

The following is the sequence used for client communication

1. The client generates some initial K(0, seed) and runs the algorithm above 'n' times to determine the initial key sent to the server, K(n, seed)
2. The client sends the request to the server to start the stream, including an identifier with a value of zero and K(n, seed)
3. The server responds with the session identifier in the headers (within the Set-Cookie field).
4. For each further request done by the client, the identifier from the server and K(n - 1, seed) are sent along.
5. The server verifies the incoming value by generating K(1, incoming_value), and verifying that value against the value sent along with the last client request. If the values do not match, the request should be ignored or logged, with an error code being returned of -3:0. The request must not be processed, and must not extend the session keepalive.
6. The client may send a new key K(m, seed') at any point, but should do this for n > 0 and must do this for n = 0. If K(0, seed) is sent without a new key, the client will not be able to continue the session.

• This method works over HTTPS, which is good from the standpoint of functionality, but bad in the sense that a massive amount of hardware would be needed to support reasonable polling intervals for non-trivial numbers of clients.