Congestion Control, Data Encodings

• additive increase/multiplicative decrease
• available bit rate in ATM
• presentation layer
• integer encodings
• array and structure encodings
• encodings systems:
  • XDR
  • ASN.1

fast recovery

• don't want to empty the pipe when a single packet is lost
• fast retransmit the packet
• set window to W0/2+3
• for every ack, increase window by MTU
• after W0/2 worth of acks, can start retransmitting
• by the time the lost packet is acked, there are W0/2 more packets in the pipe
• allows incoming acks to "clock" new outgoing packets, avoids slow start

additive increase/multiplicative decrease

• additive increase: each round-trip time, add a fixed amount (e.g. 1 MTU) to the window
• multiplicative decrease: when congestion is detected, reduce window by constant factor
• reduces large windows more than small windows
• gives everyone equal opportunity to increase (modulo different round-trip times)
• multiplicative increase would be unstable and unfair

Presentation Layer

• above session layer (which is above transport layer)
  • data encodings
  • security
  • compression
• in practice, often distributed among:
  • the application (e.g. XDR)
  • the transport layer (e.g. SLIP compression)
  • the network layer (e.g. security)

Data Encodings

• marshal: convert an "in-memory" structure to a linearized form in a byte array
• unmarshal: convert a byte array to an in-memory structure

integer encodings

• bits-in-byte byte ordering for bit-serial channels (usually MSB first)
• little-endian: least-significant byte first (Intel, DEC)
• big-endian: most-significant byte first (Motorola, Apple, Sun, IBM, Internet)
• either: (MIPS, X-windows)
• agreement as to integer size (number of bytes), alignment, position

array and structure encodings

• structures:
  • as laid out in memory
  • padding
• arrays:
  • fixed length
  • length explicitly encoded:
    • sentinel element (e.g. '\0')
    • integer length (e.g. packet length)
• unions:
  • encoding of discriminant
  • encoding of value
  • reserve space for largest possible value?

XDR

• eXternal Data Representation
• type declaration specifies possible values
• single "type" is union of possible messages
• compiler (xdr) converts this into encoding/decoding routines
• same routine encodes and decodes depending on boolean flag
• arguments include buffer pointer, encode/decode
• size of encoding depends only on type declaration
• big-endian 32-bit integers, padding for alignment, explicit length for arrays

ASN.1

• self-describing tagged data
• no alignment
• space efficient
• tag, length, value
• length is one byte if byte < 128; otherwise, byte - 128 is length in bytes of actual length field
• SNMP (Simple Network Management Protocol)