RPC, Presentation Layer

• x-kernel RPC:
  • Blast
  • Chan
  • Select
• Presentation Layer:
  • Encoding
  • Lossless Compression
  • Lossy Compression
  • Security

x-kernel RPC

• 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"

Blast protocol

• 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 (gives up after a time)
• at most 32 fragments per packet, since the SRR has a 32-bit bitmask

Chan protocol

• 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

Presentation Layer: Encoding Principles

• which data types are we going to encode?
  • ints, char, arrays, pointers, ...
  • length, content type, text/html, ...
• how are we going to represent them?
  • big-endian
  • little-endian
  • ASCII (American Standard Code for Information Interchange)
• how is the receiver going to decode?
  • tags
  • tag-less
• compiled or interpreted?

Presentation Layer: Encoding

• XDR:
  • 32-bit encoding for basic types
  • 32-bit multiples for other types
  • arrays encoded by length followed by contents
  • same routines used for encoding or decoding
  • Sun RPC
• ASN.1
  • ISO standard
  • variable, value-dependent encoding size: (tag, length, value)
  • tagged, generally interpreted
  • SNMP

Compression

• most real data has redundancy, i.e. is encoded using more bits than the theoretical minimum
• trivial example: the sequence 1111111111111... (n ones) can be encoded using 1 + log n bits instead of n bits
• compression:
  • encodes the data using fewer bits (if possible)
  • sends (or stores) the data
  • reconstructs the original (or a sufficient approximation) from the compressed data
• lossless compression: exact reconstruction
• lossy compression: approximate reconstruction

Lossless Compression

• run-length encoding, e.g. encoding 1111111111111... (n ones) as the value (1) and the number of occurrences (n)
• Differential Pulse Code Modulation (DPCM) sends the differences between successive values of a slowly changing signal, using fewer bits
• Dictionary-based compression:
  • Lempel-Ziv (Unix compress)
    • builds a table of substrings which appear (repeatedly) in the text
    • encodes each substring by its dictionary position
  • GIF replaces 24-bit color pixels with 8-bit pixels by selecting the "most important" 256 colors and only using those

Presentation Layer: Lossy Compression

• JPEG:
  • discrete cosine transform (almost lossless)
  • quantization: reduced precision of high spatial frequencies
  • run-length encoding of quantized data
• MPEG:
  • JPEG on each frame
  • 3 frame types:
    • I (Intrapicture), like JPEG
    • P (Predicted), specified in terms of the differences from prior I frame
    • B (Bidirectional Predicted), specified in terms of the differences from prior I/P frame to next I/P frame

Presentation Layer: Security

• trusted system(s): own computer, partner's computer
• untrusted system: network
• plaintext encrypted to give cyphertext, decrypted to give back plaintext
• secret key is required for decryption, may be required for encryption:
  • if encryption and decryption keys are same, encryption key must be kept secret: this is secret key cryptography
  • if encryption and decryption keys are same, encryption key may be published: this is public key cryptography
  • in both cases, the decryption key must be kept secret to guarantee privacy

Security Algorithms, Protocols

• one-time pad: unbreakable, but key security is difficult (secret key)
• DES: hard to break 56-bit key, commercial use (secret key)
• Kerberos: security protocol, distributes and renews session keys based on host keys (secret key)
• RSA: arbitrary size keys, public keys
• message integrity: can I prove in court that this message was sent to me by person X?