UDP et al

Goals for today

  * See an example higher-layer protocol atop IP

  * Understand some principles of protocol design

IP review

Missing from IP:

  * Reliable transport (dropped, corrupted*, out-of-order)

  * Distinguish between multiple endpoints behind a
    single interface

    (ALT: having multiple streams between the same pair
    of endpoints)

  * Security (e.g. encryption)

  * Flow Control

  * MTU = requirement to segment

UDP Provides

  * Slightly reliable transport (Provides payload checksum)

  * Distinguish between multiple endpoints behind a
    single interface

    * Simple

    * Works well in environments where anything would

    * Minimizes latency (at expense of correctness)

    * Can host higher-level protocols

      * In particular, protocols managed by userland SW
      

Mechanisms for Protocol Construction

  * Retransmits

  * (Forward Error Correction)

  * Acks

  * Sliding Window

    * Latency hiding
    * Flow control

  * Compression

  * Implied Transmission

Compression

* Change of representation of information from a more verbose one
  to a more terse

* No universal compression scheme

* Lossy vs *lossless*

* Temporal vs spatial

* E.g. IP header
  * Temporal (addrs in same flow)
  * Spatial (src vs dest addr)

Implied Transmission

  * Don't send stuff at all if the receiver can figure it
    out without you

    * E.g. ACK-ing only certain packets

    * E.g. X sequence numbers