Münster University of Applied Sciences

Stegerwaldstrasse 39 48565 Steinfurt Germany tuexen@fh-muenster.de

Mozilla Corporation

1835 Horse Shoe Trl Malvern PA 19355 US randell-ietf@jesup.org

hannes.tschofenig@gmx.net

This document describes a method for the Stream Control Transmission Protocol (SCTP) allowing the upper layer to perform fragmentation, reassembly, and interleaving of large ordered user messages by using the payload protocol identifier (PPID). According to the base specification supporting fragmentation of large user messages is optional. And even if an SCTP implementation supports fragmentation, interleaving of user messages is not supported by the base specification.

Introduction This document specifies a method to use PPIDs for fragmenting ordered large user messages. Using this method also allows the ability to interleave large user messages as provided by in combination with using the SCTP_FRAGMENT_INTERLEAVE level_2 as described , Section 8.1.20. Reasons to use this method include:

• The fragmentation of large user messages is only an optional feature of SCTP implementations compliant . Therefore, if an implementation does not support fragmentation, it is impossible to send large user messages requiring fragmentation.
• An SCTP implementation supporting , but not , does not allow the interleaving of large user messages.

This method does not apply to user messages sent using partial reliability as described in . The idea described in this document was already described in . In the final specification , this method is declared deprecated.

Conventions The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in BCP 14 when, and only when, they appear in all capitals, as shown here.

Sender Side Considerations An upper layer splits a user message in one or more user message fragments. The upper layer SHOULD choose the size of the user message fragments such than SCTP level fragmentation is avoided. The upper layer uses two PPIDs. It MUST use one PPID for all user messages fragments except the last one, and it MUST the other PPID for the last user message fragments. All user message fragments belonging to the same user message MUST be sent on the same stream, reliable, and ordered in the sequence they belong to the user message. User message fragments sent on different stream MAY be sent in any order. This allows the interleaving of user messages sent on different streams. User messages not requiring to be split into multiple user message fragments are sent as a single user message fragment with the PPID used for last user fragments.

Receiver Side Considerations The upper layer MUST process user message fragments received on different streams independently. All user message fragments are received by the upper layer in the correct ordering and the PPID MUST be used to reconstruct the user message boundaries. A user message fragment with the PPID marking the last user message fragment is the last fragment of a use message. The next received user message fragment on the stream is the first fragment of the next user message. An upper layer MUST deal with interleaving of user messages. Please note that notifications, if enabled, can be provided by the SCTP implementation at any time.

Socket API Considerations This document does not require and changes or additions to the Socket API described in .

IANA Considerations This document does not make any requests for IANA.

Security Considerations This document does not change the considerations given in .

References Normative References Informative References Mozilla Ericsson Muenster Univ. of Appl. Sciences The WebRTC framework specifies protocol support for direct, interactive, rich communication using audio, video, and data between two peers' web browsers. This document specifies the non-media data transport aspects of the WebRTC framework. It provides an architectural overview of how the Stream Control Transmission Protocol (SCTP) is used in the WebRTC context as a generic transport service that allows web browsers to exchange generic data from peer to peer.