General Internet Signaling Transport (GIST) over SCTP and Datagram TLS

This document describes the usage of the General Internet Signaling Transport (GIST) protocol over the Stream Control Transmission Protocol (SCTP) . GIST, in its initial specification for connection mode operation, runs on top of a byte-stream oriented transport protocol providing a reliable, in-sequence delivery, i.e., using the Transmission Control Protocol (TCP) for signaling message transport. However, some NSLP context information has a definite lifetime, therefore, the GIST transport protocol could benefit from flexible retransmission, so stale NSLP messages that are held up by congestion can be dropped. Together with the head-of-line blocking issue and other issues with TCP, these considerations argue that implementations of GIST should support the Stream Control Transport Protocol (SCTP) as an optional transport protocol for GIST, especially if deployment over the public Internet is contemplated. Like TCP, SCTP supports reliability, congestion control and fragmentation. Unlike TCP, SCTP provides a number of functions that are desirable for signaling transport, such as multiple streams and multiple IP addresses for path failure recovery. In addition, its Partial Reliability extension (PR-SCTP) supports partial retransmission based on a programmable retransmission timer. Furthermore, Datagram Transport Layer Security (DTLS) provides a viable solution for securing datagram transport protocols, e.g., by using DTLS over SCTP . This document defines the use of SCTP as a transport protocol and the use of DTLS as a security mechanism for GIST Messaging Associations and discusses the implications on GIST State Maintenance and API between GIST and NSLPs. Furthermore, this document shows how GIST SHOULD be used to provide the additional features offered by SCTP to deliver the GIST C-mode messages (which can in turn carry NSIS Signaling Layer Protocol (NSLP) messages as payload). More specifically: How to use the multiple streams feature of SCTP. How to use the PR-SCTP extention of SCTP. How to take advantage of the multi-homing support of SCTP. The method described in this document does not require any changes of GIST or SCTP. However, SCTP implementations MUST support the optional feature of fragmentation of SCTP user messages.

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in . Other terminologies and abbreviations used in this document are taken from related specifications (e.g., and ) as follows: SCTP - Stream Control Transmission Protocol PR-SCTP - SCTP Partial Reliability Extension MRM - Message Routing Method MRI - Message Routing Information MRS - Message Routing State SCD - Stack Configuration Data MA - A GIST Messaging Association is a single connection between two explicitly identified GIST adjacent peers on the data path. A messaging association may use a specific transport protocol and known ports. If security protection is required, it may use a specific network layer security association, or use a transport layer security association internally. A messaging association is bidirectional; signaling messages can be sent over it in either direction, and can refer to flows of either direction. SCTP Association - A protocol relationship between SCTP endpoints, composed of the two SCTP endpoints and protocol state information. An association can be uniquely identified by the transport addresses used by the endpoints in the association. Two SCTP endpoints MUST NOT have more than one SCTP association between them at any given time. Stream - A sequence of user messages that are to be delivered to the upper-layer protocol in order with respect to other messages within the same stream.

The basic GIST protocol specification defines two possible protocols to be used in Messaging Associations, namely Forwards-TCP and TLS. This document adds Forwards-SCTP as another possible protocol. In Forwards-SCTP, analog to Forwards-TCP, connections between peers are opened in the forwards direction, from the querying node, towards the responder. A new MA-Protocol-ID type, "Forwards-SCTP", is defined in this document for using SCTP as GIST transport protocol. A formal definition of Forwards-SCTP is given in the following section.

This MA-Protocol-ID denotes a basic use of SCTP between peers. Support for this protocol is OPTIONAL. If this protocol is offered, MA-protocol-options data MUST also be carried in the SCD object. The MA-protocol-options field formats are: in a Query: no information apart from the field header. in a Response: 2 byte port number at which the connection will be accepted, followed by 2 pad bytes. The connection is opened in the forwards direction, from the querying node towards the responder. The querying node MAY use any source address and source port. The destination information MUST be derived from information in the Response: the address from the interface- address from the Network-Layer-Information object and the port from the SCD object as described above. Associations using Forwards-SCTP can carry messages with the transfer attribute Reliable=True. If an error occurs on the SCTP connection such as a reset, as can be detected for example by a socket exception condition, GIST MUST report this to NSLPs as discussed in Section 4.1.2 of .

This document defines the use of SCTP as a transport protocol for GIST Messaging Associations. As SCTP provides additional functionality over TCP, this section dicusses the implications of using GIST over SCTP on GIST State Maintenance. While SCTP defines uni-directional streams, for the purpose of this document, the concept of a bi-direction stream is used. Implementations MUST establish downstream and upstream (uni-directional) SCTP streams always together and use the same stream identifier in both directions. Thus, the two uni-directional streams (in opposite directions) form a bi-directional stream. Due to the multi-streaming support of SCTP, it is possible to use different SCTP streams for different resources (e.g., different NSLP sessions), rather than maintaining all messages along the same transport connection/association in a correlated fashion as TCP (which imposes strict (re)ordering and reliability per transport level). However, there are limitations to the use of multi-streaming. All GIST messages for a particular session MUST be sent over the same SCTP stream to assure the NSLP assumption of in-order delivery. Multiple sessions MAY share the same SCTP stream based on local policy. The GIST concept of Messaging Association re-use is not affected by this document or the use of SCTP. All rules defined in the GIST specification remain valid in the context of GIST over SCTP.

A variant of SCTP, PR-SCTP provides a "timed reliability" service. It allows the user to specify, on a per message basis, the rules governing how persistent the transport service should be in attempting to send the message to the receiver. Because of the chunk bundling function of SCTP, reliable and partial reliable messages can be multiplexed over a single PR-SCTP association. Therefore, a GIST over SCTP implementation SHOULD attempt to establish a PR-SCTP association instead of a standard SCTP association, if available, to support more flexible transport features for potential needs of different NSLPs.

GIST specification defines an abstract API between GIST and NSLPs. While this document does not change the API itself, the semantics of some parameters have slightly different interpretation in the context of SCTP. This section only lists those primitives and parameters, that need special consideration when used in the context of SCTP. The relevant primitives from are as follows: The Timeout parameter in API "SendMessage": According to , this parameter represents the "length of time GIST should attempt to send this message before indicating an error." When used with PR-SCTP, this parameter is used as the timeout for the "timed reliability" service of PR-SCTP. "NetworkNotification": According to , this primitive "is passed from GIST to a signalling application. It indicates that a network event of possible interest to the signalling application occurred." Here, if SCTP detects a failure of the primary path, GIST SHOULD also indicate this event to the NSLP by calling this primitive with Network-Notification-Type "Routing Status Change". This notification should be done even if SCTP was able to remain an open connection to the peer due to its multi-homing capabilities.

This section provides the bit-level format for the MA-protocol-options field that is used for SCTP protocol in the Stack-Configuration-Data object of GIST.

The SCTP port number is only supplied if sent by the responder.

In general, the multi-homing support of SCTP can be used to improve fault-tolerance in case of a path- or link-failure. Thus, GIST over SCTP would be able to deliver NSLP messages between peers even if the primary path is not working anymore. However, for the Message Routing Methods (MRMs) defined in the basic GIST specification such a feature is only of limited use. The default MRM is path-coupled, which means, that if the primary path is failing for the SCTP association, it most likely is also for the IP traffic that is signaled for. Thus, GIST would need to perform a refresh anyway to cope with the route change. Nevertheless, the use of the multi-homing support of SCTP provides GIST and the NSLP with another source to detect route changes. Furthermore, for the time between detection of the route change and recovering from it, the alternative path offered by SCTP can be used by the NSLP to make the transition more smoothly. Finally, future MRMs might have different properties and therefore benefit from multi-homing more broadly.

Streaming support in SCTP is advantageous for GIST. It allows better parallel processing, in particular by avoiding head of line blocking issue in TCP. Since a same GIST MA may be reused by multiple sessions, using TCP as transport GIST signaling messages belonging to different sessions may be blocked if another message is dropped. In the case of SCTP, this can be avoided as different sessions having different requirements can belong to different streams, thus a message loss or reordering in a stream will only affect the delivery of messages within that particular stream, and not any other streams.

The MA-Protocol-ID for DTLS denotes a basic use of datagram transport layer channel security, initially in conjunction with SCTP. It provides authentication, integrity and optionally replay protection for control packets. The use of DTLS for securing GIST over SCTP allows GIST to take the advantage of features provided by SCTP and its extensions. Note replay protection is not available for DTLS over SCTP . The usage of DTLS for GIST over SCTP is similar to TLS for GIST as specified in , where a stack-proposal containing both MA-Protocol-IDs for SCTP and DTLS during the GIST handshake phase. GIST message associations using DTLS may carry messages with transfer attributes requesting confidentiality or integrity protection. The specific DTLS version will be negotiated within the DTLS layer itself, but implementations MUST NOT negotiate to protocol versions prior to DTLS v1.0 and MUST use the highest protocol version supported by both peers. GIST nodes supporting DTLS MUST be able to negotiate the DTLS NULL and block cipher ciphers and SHOULD be able to negotiate the new cipher suites. They MAY negotiate any mutually acceptable ciphersuite that provides authentication, integrity, and confidentiality. The same rules for negotiating TLS cipher suites as specified in Section 5.7.3 of apply. No MA-protocol-options field is required for DTLS. The configuration information for the transport protocol over which DTLS is running (e.g. SCTP port number) is provided by the MA-protocol-options for that protocol.

The security considerations of , and apply. Following , replay detection of DTLS over SCTP is not supported. The usage of DTLS for securing GIST over datagram transport protocols MUST be implemented and SHOULD be used. An implementation of GIST over SCTP with no PR-SCTP support MAY use TLS for its channel security, when DTLS is not available between two GIST peers.

This specification extends by introducing two additional MA-Protocol-IDs:

The authors would like to thank John Loughney, Robert Hancock, Andrew McDonald, Martin Stiemerling, Fang-Chun Kuo, Jan Demter, Lauri Liuhto, and Michael Tuexen for their helpful suggestions.