Chapter 5: Network Layer, Control Plane

Question 1

Network layer, have data plane and control plane, what are the different function these 2 plane gives?

Answer 1

Forwarding (Data Plane): move packets from router’s input to appropriate router output

Routing (Control Plane) : determine route taken by packets from source to destination

Question 2

2 approaches to structuring network control plane

Answer 2

1. Per router control (traditional)

2. Logically centralized control (software defined networking)

Question 3

How Per router (fig 5.1) would work?

Answer 3

individual routing components in each and every router interact in the control plane to compute forwarding tables

Question 4

explains routing protocols and path in network layer

Answer 4

determine “good” paths from sending host to receiving host, through network of routers

path: sequence of routers packets traverse from given initial source host to final destination host

Question 5

what does it mean for a path to be “good”?

Answer 5

least “cost”, “fastest”, “least” congested

Question 6

graph abstraction (G = (N,E) )

Answer 6

G = graph
N = set of routers {x, y,...} 
E = set of links {(x, y), (x, z),...}

Question 7

Routing algorithm classification (global, static, decentralized, dynamic)

Answer 7

global: all routers have complete topology , “link state” (LS) algorithm
decentralized : iterative process of computation, exchange of info with neighbors (DV) algorithm
static: routers change slowly over time
dynamic: routes change more quickly

Question 8

Dijkstra’s link-state routing algorithm

Answer 8

1. Initialization:
   - N’ ={u}
   - for all nodes v -> if v adjacent to u ->
   then D(v) = Cu,v -> else D(v) = inf
2. Loop
   - find w not in N’ such that D(w) is a min - >
   add w to N’ -> update D(v) for all adjacent to w and
   not in N’ , D(v) = min ( D(v), D(w) + cw,v ). Do until
   all nodes in N’

Question 9

Distance Vector (DV) routing algorithm

Answer 9

Based on Bellman-Ford (BF) equation (dynamic programming):
Let Dx(y): cost of least-cost path from x to y.
Dx(y) = minv{ cx,v + Dv(y) }

Question 10

Autonomous Systems (AS) is an approach to handle internet is a scale that it is today. briefly explains intra-AS and inter-AS

Answer 10

intra-AS : routing among within same AS (network)

inter-AS : routing among AS’es

Question 11

OSPF (open shortest path first) routing is one of routing algorithm used in Intra-AS. what are some characteristics of OSPF

Answer 11

• publicly available
• classic link state: each router floods OSPF link state advertisements to all outer routesrs in entire AS
• all OSPF message authenticated

Question 12

Internet inter-AS routing , BGP (Border Gateway Protocol) is the de-facto inter domain routing protocol. explains and what are the means of BGP

Answer 12

allows subnet to advertise its existence , and the destination it can reach, to rest of internet

means:
- eBGP : obtain subnet reachability information from neighboring ASes
- iBGP: propagate reachability information to all AS-internet routers

Question 13

Whats a BGP session

Answer 13

two BGP routers (“peers”) exhange BGP messages over semi permanent TCP connection

Question 14

What are BGP messages

Answer 14

messages exhanged btwn peers over TCP connection
messages:
- OPEN: Opens connection to remote BPG peer and authenticate sending BGP peers
- UPDATE: advertises new path or withdraws old
- KEEPALIVE: keeps connection alive in absence of updates. also ACKs open request
- NOTIFICATION: reports prev error messages, also to close connection