BGP + Medium Sharing

Border Gateway Protocol (BGP)

• Inter-domain routing• Why not distance-vector?– Shortest path may not necessarily follow policies

• Why not link state?– Everyone knows everything – privacy goes for a toss!

• How else can we avoid loops and dead-ends?– Path vector!

Business Relationships

peer peerprovider customer

Relations between ASes Customers pay provider Peers don’t pay each other

Business Implications

Routing Follows the Money!

• Peers provide transit between their customers• Peers do not provide transit to each other

traffic allowed traffic not allowed

Selection vs Export• Selection policies– determines which paths I want my traffic to take

• Export policies– determines whose traffic I am willing to carry

Sharing the Medium

• How do multiple hosts share a medium?

Channel Partitioning

TDMA, FDMA

Taking Turns

Polling, Token Passing

Random Access

ALOHA, CSMA

Random Access

• No “sensing” of medium– ALOHA

• Sense the medium (CSMA/CD)– Listen to the channel – Transmit if free, else wait – Upon collision• Stop sending, inform others• Wait for a random time

Why would collisions occur if you listen before transmission?

Implications of CD

• How long does it take to detect collision?

Dictates minimum frame size, and maximum distance!

CSMA/CD

• E = Time spent transmitting / (Time spent transmitting + time wasted due to collision)

• = (P/B) / ((P/B) + KD)• P: packet length• B: bandwidth• D: Distance

High for large transmission time, or low propagation time