Routing Protocols: RIP, OSPF, BGP, CSMA/CD, CSMA/CA

Intra-AS Routing

RIP – Based on Distance-Vector (D-V), fewer messages, converges slower, smaller size.

OSPF – Based on Link-State (L-S), converges faster, more messages, larger networks.

Distance-Vector is distributed, and each router only knows the costs to its neighbors and their distance vectors.

Link state is global and requires that each router knows the network topology and link costs to all nodes.

OSPF: Used internally inside a network, an intra-network/domain protocol used in an AS.

BGP: An internetwork protocol used between two different ASs, used at the edge of your network to connect your network to the internet.

RIP: An intra-network protocol used in an AS, used for small networks (maximum number of hops is 16).

EBGP: Runs between two routers in different ASs. Routes received from an EBGP peer can be advertised to EBGP and iBGP peers. Peers are assumed to be directly connected.

iBGP: Runs between two routers in the same AS. Routes received from an iBGP peer can't be advertised to another iBGP peer but can be advertised to an EBGP peer. There is no restriction that they have to be directly connected.

OSPF Enhancements Over RIP

1. Security: All OSPF messages are authenticated (to prevent malicious intrusion).
2. Multiple same-cost paths are allowed (only one path in RIP).
3. For each link, multiple cost metrics for different TOS (e.g., satellite link cost set “low” for best effort ToS; high for real-time ToS).
4. Hierarchical OSPF in large domains.
5. Supports more than 15 routers.

Main Purpose of BGP

eBGP – Sets routing paths between ASs. iBGP sets paths within an AS to the correct gateway. The inter-AS protocol can be configured based on the AS's policy decisions.

CSMA/CD vs. CSMA/CA

CSMA/CD relies on collision detection. It is used by Ethernet for wired LANs. The assumption is that it is easy to detect collisions on the wire and easy to retransmit. The transmitting host checks the line. If it is not busy, then it transmits and continues to listen. If it detects a collision, it will stop its transmission and send a jam signal. It then goes into a random back-off before retrying to transmit. The range of random back-off increases exponentially with each collision by 2 to the power of k for k collisions.

CSMA/CA relies on collision avoidance. It is used by 802.11 for wireless LANs. The assumption is that it is impossible to detect all collisions on the LAN (hidden terminal problem) and difficult to transmit and listen at the same time for collisions over the air. To avoid collisions, the sender checks if the LAN is clear. If so, it waits a DIF space and then transmits. Otherwise, it chooses a random back-off variable and counts down only when the channel is idle and then waits for an ACK.

Router Components

Three Main Physical Components of a Router

1. Input ports
2. Switch fabric
3. Output ports

The switch fabric speed and each output port speed should be greater than or equal to the sum of all the input ports for best performance.

Ethernet and 802.11 Comparison

Similarities

• Both have no flow control.
• Both have no error correction.
• Both have error detection via CRC.
• Both are CSMA.

Differences

• 802.11 has RDT with ACKs, retransmissions, and sequence numbers.
• 802.11 is connection-based.
• 802.11 is CA, Ethernet is CD.
• 802.11 has 3 MAC Addresses to include both the AP and the first-hop router.
• 802.11 has a 4th MAC Address for use in ad-hoc mode.
• 802.11 has special features optimized for wireless environments like RTS/CTS and power management.

802.11 Features Not in Ethernet

• Reliable data transfer: Seq# and ACKs
• Connection/association with AP: AP MacAddr, AP from and to fields
• Collision Avoidance: CTS/RTS, ACKs

Shared Features of 802.11 and Ethernet

• Error Detection and correction: CRC
• Link layer addressing: Source and destination MAC addresses