Network Layer IV and Network Layer V

Question 1

BGP basics

Answer 1

BGP session: two BGP routers (“peers”) exchange BGP messages over
semi-permanent TCP connection:
* advertising paths to different destination network prefixes (BGP is a “path
vector” protocol)

BGP Protocol Messages (RFC 4271)
BGP routers (peers) exchange these messages over a TCP connection (port 179):

OPEN
Starts the BGP session.

Authenticates the peer.

Exchanged when the TCP connection is first established.

2. UPDATE
   Most important message.

Used to:

Advertise new paths to destination IP prefixes.

Withdraw old paths that are no longer valid.

3. KEEPALIVE
   Sent periodically to keep the connection alive.

Also serves as an acknowledgment for OPEN messages.

4. NOTIFICATION
   Sent when there’s an error (e.g., invalid message, bad routing info).

Also used to close the connection

Question 2

Internet Inter-AS Structure: Tiers

Answer 2

We can usefully think of the Internet as a tiered model (i.e. having big, medium
and small ASes), but there is no formal hierarchy.
* Majority are Internet Service Providers (ISPs) and Corporation (Enterprise) networks with
different sizes.
* Tier 3 ASes: Minor ISPs or customer networks:
* Customers of Tier 1 and Tier 2 providers
* Peer with other Tier 3 providers
* Tier 2 ASes:
* National or regional ISPs
* Customers of Tier 1 providers
* Peer with other Tier 2 providers
* e.g. BT, France Telecom
* Tier 1 ASes: International (global backbone) ISPs:
* Peer with each other
* Major operators, e.g. Sprint, AT&T
* Very highly connected with other Tier 1 ASes

ISP only wants to route traffic to/from its customer networks (does not want to
carry transit traffic between other ISPs – a typical “real world” policy)