1. 1 © NOKIA nqa-rohc.PPT/ March, 2001 / NRC (Dallas) SCRIBE - SCalable, Robust, EffIcient Dictionary-Based ComprEssion March 23, 2001 Zhigang Liu, Khiem Le, Kacheong Leung, Chris Clanton Nokia Research Center Dallas

2. 2 © NOKIA nqa-rohc.PPT/ March, 2001 / NRC (Dallas) Highlights Scheme to use Dictionary-Based Compression Algorithms when transport between compressor and decompressor is unreliable (errors, losses) Scalable: Adapt to memory size limitation Robust: Maintain correctness Errors Losses Efficient High compression efficiency Applicable to compression of text-based protocols, in particular SIP Applicable to cellular environments Draft at http://38.197.106.103/draft-liu-rohc-scribe- 00.txthttp://38.197.106.103/draft-liu-rohc-scribe- 00.txt

3. 3 © NOKIA nqa-rohc.PPT/ March, 2001 / NRC (Dallas) Main Concepts of SCRIBE Cross-Session Dictionary Checkpoint Profile-Specific Dictionary Content Deletion Sliding Handshake

4. 4 © NOKIA nqa-rohc.PPT/ March, 2001 / NRC (Dallas) Cross-Session Dictionary Dictionary built up in previous sessions is used to jumpstart the compression/decompression of subsequent sessions Use of cross-session dictionary leads to higher compression of initial messages Higher compression of initial messages helps overcome the call set up latency problem contributes more heavily to the overall compression ratio, when the initial messages are large (especially true for SIP)

5. 5 © NOKIA nqa-rohc.PPT/ March, 2001 / NRC (Dallas) Checkpoint Checkpoint Dictionary A well defined dictionary snapshot stored by compressor and decompressor Integrity from strong CRC calculated over the dictionary Can be referred to by a version number Checkpoint Establishment Version number assigned Strong CRC check Handshake Rollback Rollback to an established checkpoint Triggered by e.g. detection of dictionary incoherency

6. 6 © NOKIA nqa-rohc.PPT/ March, 2001 / NRC (Dallas) Dictionary Download Done only when needed, e.g. mobile terminal roams to new area Normally, last established checkpoint is downloaded Download done at registration time or at beginning of session Mechanisms similar to those existing in cellular systems If scheme used in cellular systems, SCRIBE download can be piggybacked on existing download mechanisms

7. 7 © NOKIA nqa-rohc.PPT/ March, 2001 / NRC (Dallas) Profile-Specific Dictionary Dictionary consists of 3 parts Static: Populated from knowledge of the protocol, protocol-specific, but common to all users; e.g. "From", "To" Profile-Specific: Populated from knowledge of the user/device profile; specific to user or class of user; e.g. SIP URL, device capabilities Dynamic: Populated from observation of messages exchanged Static and profile-specific parts can be pre- populated

8. 8 © NOKIA nqa-rohc.PPT/ March, 2001 / NRC (Dallas) Content Deletion When memory limit is reached, no new item can be added unless some content is deleted Mobile devices tend to have more memory limitations When the dictionary is maintained across sessions, memory limit will be eventually reached, even for high-end devices Common algorithm to select the content to be deleted ---> Implicit agreement, no extra signaling messages are needed One simple algorithm is to select the oldest part of the dictionary

9. 9 © NOKIA nqa-rohc.PPT/ March, 2001 / NRC (Dallas) Sliding Handshake Used to update dictionary during session establish checkpoint Cumulative and selective acks