High-Speed Networking

• Computer Architecture
• Integrated Layer Processing
• Service Models:
  • Real-Time networking
  • Reservations
  • Admission Control
  • RSVP

Faster Networks

Computer Architecture

• in the past, the computer was generally faster than the network
• it takes a very fast computer to inject 1Gb/s of TCP data into the network:
  • limited memory bandwidth
  • multiple transfers of data to and from memory before data gets to device:
    • user space to system space
    • checksumming, CRC
    • DMA
  • I/O bus bandwidth may be less than memory bandwidth
  • adapter may not be able to transfer at peak rate

Integrated Layer Processing

• Suppose I have a number of data manipulations to do on incoming and outgoing data:
  1. error checking -- checksumming or CRC
  2. encryption or decryption
  3. conversion from network to host byte-order
• If I do them in three separate loops, I have to load the data from memory 3 times, store it two times
• If I merge them into a single complex loop, I only have to load and store the data once
• modularity and clean implementation run counter to efficiency
• can the process be automated? (loop fusion)

In-Class Exercise

char * pkt, *end;
short * sp;
long * lp;
cksum = 0;
for (sp = (short *)pkt;
     sp < (short *)end; sp++) {
  cksum += *sp;
}
for (lp = ((long *)pkt) + 10;
     lp < ((long *)end; lp++) {
  *lp = ntohl (*lp);
}

Service Models

• Internet gives best-effort service -- does not even attempt to give guarantees about:
  • maximum percentage drop
  • maximum delay
  • maximum delay variation (jitter)
  • minimum bandwidth
• best effort is inadequate for real-time applications such as telephony

Application needs

• maximum delay variation lets application buffer data and play it back continuously
• maximum delay allows real-time interaction
• real-time (and multicasting) don't generally support retransmission, so:
  • transmissions must be tolerant of data loss (e.g. by degrading quality)
  • network must provide guarantees about packet loss
• some applications need bandwidth guarantees, e.g. voice-grade telephony needs 64Kb/s

Reservations

• best-effort is very efficient in network utilization, but does not fit all applications
• a reservation is an agreement (a contract) between the host and the network
• network must perform admission control to
  • admit the new reservation if the resources are available
  • not admit otherwise
• reservations are less efficient because network resources might remain unused
• mixing best-effort (low priority) and reserved (high priority) traffic gives good utilization with needed guarantees

RSVP

• reservation protocol for IPv4
• connectionless, unlike ATM (connection)
• routers have soft state which expires after 1 minute (similar to ARP, RIP)
• reservation must be repeated every 30 seconds, therefore flexible
• receiver sends reservation
• suitable for multicast