Outline: RPC

RPC Model

local procedure call involves:
1. calling a specific procedure with arguments (if any)
2. arbitrarily long suspension of the caller
3. the caller is awakened when the called procedure returns results (if any)
remote procedure call involves:
1. calling a specific procedure on a specific processor and passing any arguments
2. arbitrarily long suspension of the caller
3. the caller is awakened when the called procedure returns any (or no) results, or the communication failed

transaction ID (XID) identifies an exchange
Asymmetric protocol: client sends requests, receives replies, server receives requests, sends replies.
arguments are:
- marshaled (converted to a standard form) on the client
- sent with the request
- unmarshaled (checking for errors) on the server
return values and status are:
- marshaled on the server
- sent with the reply
- unmarshaled on the client

Developed and standardized by Sun Microsystems.
Functions: synchronization of requests and replies, reliability, and demultiplexing.
"RPC" is generic for "remote procedure call", Sun RPC identifies a specific protocol.

Client sends request, waits. Server receives request, processes, {\em always} sends a reply. Client receives reply, starts up caller thread.
A reply always carries a status as well as data.
Request is retransmitted until a reply with matching transaction ID is received (reliability).

Problems: Transaction ID not stored in client, only stored for short time in server.
If client re-uses the same ID quickly, the server might think it's a retransmission, and re-send the old reply.
If a long-delayed request arrives at the server, the server might not realize it's a duplicate.
RPC provides two fields, credentials and verifier, that can be used for authentication.

Sun RPC uses 3 32-bit fields for demultiplexing: program number, version number, and procedure number.
Sun RPC runs over UDP. Which UDP port does server X use? (e.g., NFS).
Port Mapper is a server that (using RPC) maps program numbers to UDP port numbers. Port Mapper binds to the well-known port 111.

Procedure Call model (synchronous). Sun RPC also includes XDR for argument and result marshaling.
Asymmetric protocol: client sends requests, receives replies, server receives requests, sends replies.
Many different RPCs.

Blast: fragmentation, acknowledgement, retransmission, but not a reliable protocol (may give up).
Chan: basic RPC, with channel ID, transaction ID, at-most-once semantics
Select: called-procedure identification
non-standard RPC protocol built from general-purpose "building blocks"

receiver sends a selective retransmission request (SRR -- a combination positive and negative ack) when:
- the last segment arrives
- a timer expires that was started when the first fragment arrives
- a retry timer expires that was started when we first sent SRR for missing packets
sender keeps packet for a fixed time, or until a completely positive SRR is received
more reliable than IP fragmentation (retransmits missing fragments), but not as reliable as TCP (will give up after a certain time)
at most 32 fragments per packet, since the SRR has a 32-bit bitmask

interfaces below to a send/receive (asynchronous) protocol, and above to an RPC (synchronous) protocol
retransmits until ack is received
reply is ack for request, next request is ack for reply
probe packets assure that the server is still working
"boot ID" prevents errors, allows message ID counter to start at 0 rather than random number