Another OSI UL efficiency idea
D_P_Sanford <@mwmgate1.mitre.org:D_P_Sanford@caasd1> Tue, 17 May 1994 16:35 UTC
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Subject: Another OSI UL efficiency idea
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---------------------------- Forwarded with Changes --------------------------- From: P.Furniss@ulcc.ac.uk at -smtp- Date: 5/16/94 11:15PM *To: thinosi@ulcc.ac.uk at -smtp- Subject: Another OSI UL efficiency idea ------------------------------------------------------------------------------- If I have interpreted this correctly, by using a hashed value, this essentially becomes a application stack combined with a compression algorithm. Based on work that has gone on in the aeronautical environment (ASN.1/PER compared with DLAC), are conclusions are that for field encoded data (e.g. not free text) if you make all optional fields optional to send, write an efficiently encodeable abstract syntax module and encode using PER unaligned, there is little enough noise in the signal that compression algorithms don't provide any appreciable benefit. Dave This is an outline of an efficiency approach recently discussed informally in the UK. It has some remarkable properties - no compatability problems, and could be orthogonal to other approaches being discussed. Most upper-layer connection/association establishment exchanges are between systems that communicate fairly frequently. Most, if not all, of the parameters of the S-CONNECT, P-CONNECT, A-ASSOCIATE received will be identical to some previous connection attempt. The efficiency enhancement proposed here takes advantage of this. Although the efficiency gain will not always be achieved, it will usually will give considerable bandwidth improvement. It will also gain a round trip. There appear to be NO compatibility problems. Mechanism The initiating system creates the CN+CP+AARQ+user-data pdu internally, before establishing a Transport connection. It creates a 32-bit (or 16, tbd) hash of this buffered value, and puts this hashed value in the user-data of the T-CONNECT request. The responding system compares the hash value received on the T- CONNECT indication against a local database of stored values. If this succeeds, it knows from the database what the upper-layer connect message would be, and can immediately determine its answer (AC+CPA+AARE+userdata), including presentation negotiation and any application-layer initialisation semantics. It hashes this reply and puts the reply-hash on the user data of the T-CONNECT response. The initiating system on getting the reply-hash in the T-CONNECT confirm does its own lookup and determines what the upper-layer response is going to be. It sends a minimal pdu on T-DATA to indicate the reply-hash has been recognised and can then proceed directly to data-phase of the connection/association. The minimal pdu can be non-blocking. If the lookup at the responder fails, the responding system remembers the received value and puts a field on the user data of the T-CONNECT response that means the hash was not known. On receiving this value, the initiator sends the CN+payload as normal on T-DATA. When this is received by the responder, it can add the received value to the database and construct the reply. There is a new parameter in the Session AC pdu, initially with a dummy value. The complete reply is hashed and the hash-value substituted for the dummy. The reply is sent as normal, on T-DATA. The initiator can now build its database for the reply. Features The cleanest architectural form of this is to treat it purely as a Session matter. The values to be hashed are the Session CN+userdata and the AC(with a dummy hash value)+userdata. This simple version may be the best bet, at least initially. However, the mechanism will have wider use if various values that are likely, if present, to be different between each connection can be omitted from the hashing. Most of these will be quite small (invocation identifiers for example) and could be sent in the T-CONNECT userdata. The omission from the hashing could be done by each protocol being allocated a one-octet bitmap, carried in the T-CONNECT userdata with the hash value, that defines which fields have been excluded from the hash. Each protocol, including application protocols, would need to define the bit:variable field relationship. It is assumed that upper-layer selectors, AE-titles, version and functional unit negotiation/declaration, application context and the abstract and transfer syntax would be included in the hashing. An application where a large number of systems spoke to each other with AE-titles being exchanged would require large databases of hash values, and it might not be worth bothering - trading bandwidth and a round-trip for a disk search is doubtful. However, in many applications the database will be quite moderate. There is a slight possibility of false hits in the hash database - two different CN+userdata messages producing the same hash value. It can be assumed these will be detected by the initiator, which will find the reply does not agree with what was expected. The initiator then assumes the optimisation has failed, and proceeds to use the normal protocol. Back-compatibility is achieved provided existing implementations are ignoring the user-data on T-CONNECT (not used by Session). The initiator, finding the T-CONNECT confirm has nothing in its userdata, proceeds to use the normal protocol. This approach avoids any loss of upper-layer function - the full presentation context negotiation mechanism (for example) is still available. This optmisation obviously could be combined with other proposed upper- layer efficiency approches.
- Another OSI UL efficiency idea Peter Furniss
- Another OSI UL efficiency idea D_P_Sanford
- Re: Another OSI UL efficiency idea Peter Furniss
- Re: Another OSI UL efficiency idea Bancroft Scott
- Re: Another OSI UL efficiency idea Peter Furniss