exam2final

static

routes do not change or change slowly over time

dynamic

routes change more quickly

global information

all routers have compete topology, link cost info

decentralized information

router knows physically connected neighbors, link costs to neighbors

Dijkstra’s Algorithm

net topology, link costs known to all nodes

computes least cost paths from one node to all other nodes

iterative

split horizon

when a node sends a routing update to its neighbors, it does not send those routes it learned from each neighbor back to that neighbor

Count to Infinity problem

failures can cause DV to count to infinity, while seeking a path to an unreachable node

area border routers

summarize distances to nets in own area, advertise to other Area Border routers

backbone routers

run OSPF routing limited to backbone

boundary routers

connect to other AS’s

AS-PATH

contains ASs through which prefix advertisement has passed

NEXT-HOP

indicates specific internal-AS router to next-hop AS

gateway router receiving route advertisement uses ________ to accept/decline

import policy

policy intra/inter AS

inter-AS: admin wants control over how its traffic routed, who routes through its net

intra-AS: single admin, so no policy decisions needed

scale intra/inter

hierarchical routing saves table size, reduced update traffic

performance intra/inter

intra-AS: can focus on performance

inter-AS: policy may dominate over performance

Hot Potato Routing

suppose there are 2 or more best inter-routes, then choose route with closest NEXT-HOP

Multicast Routing Goal

find tree/tress connecting routers having local mcast group members

shared-tree

same tree used by all group members

source-based

different tree from each sender to rcvrs

Reverse Path Forwarding

rely on router’s knowledge of unicast shortest path from it to sender

DVMRP

distance vector multicast routing protocol

flood and prune

reverse path forwarding, source based tree

Where is the Link Layer Implemented

• in each and every host

• link layer implemented in “adaptor“

• attaches into host’s system buses

flow control

pacing between adjacent sending and receiving nodes

half duplex full duplex

with half duplex, nodes at both ends of link can transmit but not at same time

Slotted Aloha Assumption

• all frames same size

• time divided into equal size slots

• host start to transmit only at slot beginning

• if 2 or more hosts transmit in slot, al hosts detect collision

Slotted ALOHA Operation

• when host obtains fresh frame, transmit in next slot

• if no collision: host can send new frame in next slot

• if collision: host retransmits frame in each subsequent slot with probablilty p until success

polling

• master host invites slave host to trainsmit in turn

• typically used with dumb slave devices

polling concerns

• polling overhead

• latency

• single point of failure (master)

MAC Address

• 32 bit IP address

  • network-layer address for interface

  • used for layer 3 forwarding

• function: used ‘locally‘ to get frame from one interface to another physically connected interface

____ learns which hosts can be reached through which interfaces

when frame received, ____ learns location of sender: incoming LAN segment

switch

VLANS trunk port

carries frames between VLANS defined over multiple physical switches

MPLS Goal

high speed IP forwarding using fixed length label instead of IP Address

load balancer: application-layer routing

• receives external client requests

• directs workload within data center

• returns results to external client

Cookies Four Components

1. cookie header line of HTTP response message

2. cookie header line in next HTTP request message

3. cookie file kept on users host, managed by users browser

4. backend database at website

____ acts as both client and server

typically ___ installed by ISP

cache

Why web caching?

• reduce client request resposne time

• reduce traffic on insitution access link

• internet dense with caches: enables poor content providers to effectively deliver content

TDMA: time division multple access

• access to channel in rounds

• each station gets fixed length sloth in ecah round (length = pkt trans time)

• unused slots go idle

• ex. 6-station LAN, 1, 3, 4 have pkt, slots 2, 5, 6, idle

TDMA pros

• variable user rate possible

• no duplexer is needed for radio transmission

TDMA cons

• precise synchronization required

• for radio transmission equalization is necessary to combat ISI

bus/hub

popular through mid 90s, all nodes in same collision doman

star

prevails today

• active switch in center

• each spoke runs separate Ethernet protocol (nodes dont collide with each other)

DNS services

• hostname to IP address transl

Why not centralize DNS?

• single point of failure

• traffic volume

• distant centralized database

• maintenance

RIP

• included in BSD UNIX distribution 1982

• distance vector algorithm

  • dist metric: # hops (max = 15 hops), each link has cost 1

  • DVs exchanged with neighbors every 30 sec in response message

  • each advertisement: list of up to 25 destination subnets

MPLS vs IP paths

IP routing: path to destination determined by destination address alone

MPLS: path to destination can be based on source and dest address

• fast reroute: precompute backup routes in case of link failure