Computer Networks

Chapter 4: Advanced Internetworking

CN_4.2

Problems How do we build a routing system

that can handle hundreds of thousands of networks and billions of end nodes?

How to handle address space exhaustion of IPv4?

How to enhance the functionalities of Internet?

CN_4.3

Chapter Outline Global Internet Multicast Mobile IP

CN_4.4

Chapter Goal Understanding the scalability of

routing in the Internet Discussing IPv6 Understanding the concept of

multicasting Discussing Mobile IP

CN_4.5

The Global Internet

The tree structure of the Internet in 1990

CN_4.6

The Global Internet

A simple multi-provider Internet

CN_4.7

Interdomain Routing (BGP) Internet is organized as

autonomous systems (AS) each of which is under the control of a single administrative entity

Autonomous System (AS) corresponds to an administrative

domain examples: University, company,

backbone network A corporation’s internal network

might be a single AS, as may the network of a single Internet service provider

CN_4.8

Interdomain Routing

A network with two autonomous system

CN_4.9

Route Propagation Idea: Provide an additional way to

hierarchically aggregate routing information is a large internet. Improves scalability

Divide the routing problem in two parts: Routing within a single autonomous

system Routing between autonomous systems

Another name for autonomous systems in the Internet is routing domains Two-level route propagation hierarchy

Inter-domain routing protocol (Internet-wide standard)

Intra-domain routing protocol (each AS selects its own)

CN_4.10

EGP and BGP Inter-domain Routing Protocols

Exterior Gateway Protocol (EGP) Forced a tree-like topology onto the Internet Did not allow for the topology to become

general– Tree like structure: there is a single backbone

and autonomous systems are connected only as parents and children and not as peers

Border Gateway Protocol (BGP) Assumes that the Internet is an arbitrarily

interconnected set of ASs. Today’s Internet consists of an interconnection

of multiple backbone networks (they are usually called service provider networks, and they are operated by private companies rather than the government)

Sites are connected to each other in arbitrary ways

CN_4.11

BGP Some large corporations connect

directly to one or more of the backbone, while others connect to smaller, non-backbone service providers.

Many service providers exist mainly to provide service to “consumers” (individuals with PCs in their homes), and these providers must connect to the backbone providers

Often many providers arrange to interconnect with each other at a single “peering point”

CN_4.12

The Global Internet

A simple multi-provider InternetStub AS

Multihomed AS

Transit AS

CN_4.13

BGP-4: Border Gateway Protocol

Assumes the Internet is an arbitrarily interconnected set of AS's.

Define local traffic as traffic that originates at or terminates on nodes within an AS, and transit traffic as traffic that passes through an AS.

We can classify AS's into three types: Stub AS: an AS that has only a single connection

to one other AS; such an AS will only carry local traffic (small corporation in the figure of the previous page).

Multihomed AS: an AS that has connections to more than one other AS, but refuses to carry transit traffic (large corporation at the top in the figure of the previous page).

Transit AS: an AS that has connections to more than one other AS, and is designed to carry both transit and local traffic (backbone providers in the figure of the previous page).

CN_4.14

The goal of Inter-domain routing is to find any loop free path to the intended destination Reachability is more concerned

than optimality Finding path close to optimal is

considered to be a great achievement

Why?

BGP

CN_4.15

Scalability: An Internet backbone router must be able to forward any packet destined anywhere Having a routing table that will provide a

match for any valid IP address Autonomous nature of the domains

Not possible to calculate meaningful path costs for a path that crosses multiple ASs

A cost of 1000 across one provider might imply a great path but it might mean an unacceptable bad one from another provider

Issues of trust Provider A might be unwilling to believe

certain advertisements from provider B

BGP

CN_4.16

Each AS has: One BGP speaker that advertises:

local networks other reachable networks (transit AS

only) gives path information

In addition to the BGP speakers, the AS has one or more border “gateways” which need not be the same as the speakers

The border gateways are the routers through which packets enter and leave the AS

BGP

CN_4.17

BGP does not belong to either of the two main classes of routing protocols (distance vectors and link-state protocols)

BGP advertises complete paths as an enumerated lists of ASs to reach a particular network

BGP

CN_4.18

BGP Example

Example of a network running BGP

CN_4.19

BGP Example Speaker for AS 2 advertises reachability

to P and Q Network 128.96, 192.4.153, 192.4.32,

and 192.4.3, can be reached directly from AS 2.

Speaker for backbone network then advertises Networks 128.96, 192.4.153, 192.4.32,

and 192.4.3 can be reached along the path <AS 1, AS 2>.

Speaker can also cancel previously advertised paths

CN_4.20

BGP Issues It should be apparent that the

AS numbers carried in BGP need to be unique

For example, AS 2 can only recognize itself in the AS path in the example if no other AS identifies itself in the same way

AS numbers are 16-bit numbers assigned by a central authority

CN_4.21

Integrating Interdomain and Intradomain Routing

All routers run iBGP and an intradomain routing protocol. Border routers (A, D, E) also run eBGP to other ASs

CN_4.22

Integrating Interdomain and Intradomain Routing

BGP routing table, IGP routing table, and combined table at router B

CN_4.23

Routing Areas

A domain divided into areas

Backbone areaArea border router (ABR)

CN_4.24

Next Generation IP (IPv6)

CN_4.25

Major Features

128-bit addresses Multicast Real-time service Authentication and security Auto-configuration End-to-end fragmentation Enhanced routing functionality,

including support for mobile hosts

CN_4.26

IPv6 Addresses Classless addressing/routing (similar to CIDR) Notation: x:x:x:x:x:x:x:x (x = 16-bit hex

number) contiguous 0s are compressed:

47CD::A456:0124 IPv6 compatible IPv4 address: ::128.42.1.87

Address assignment provider-based geographic

CN_4.27

IPv6 Header 40-byte “base” header Extension headers (fixed order, mostly

fixed length) fragmentation source routing authentication and

security other options

CN_4.28

Internet Broadcast/ Multicast Routing

CN_4.29

R1

R2

R3 R4

sourceduplication

R1

R2

R3 R4

in-networkduplication

duplicatecreation/transmission

duplicate

duplicate

Broadcast Routing Deliver packets from source to all other

nodes Source duplication is inefficient:

source duplication: how does source determine recipient addresses?

CN_4.30

In-network duplication Flooding: when node receives

broadcast packet, sends copy to all neighbors Problems: cycles & broadcast storm

Controlled flooding: node only broadcasts packet if it hasn’t broadcast same packet before Node keeps track of packet ids

already broadcasted Or reverse path forwarding (RPF):

only forward packet if it arrived on shortest path between node and source

Spanning tree No redundant packets received by

any node

CN_4.31

A

B

G

DE

c

F

A

B

G

DE

c

F

(a) Broadcast initiated at A (b) Broadcast initiated at D

Spanning Tree First construct a spanning tree Nodes forward copies only along

spanning tree

CN_4.32

A

B

G

DE

c

F1

2

3

4

5

(a) Stepwise construction of spanning tree

A

B

G

DE

c

F

(b) Constructed spanning tree

Spanning Tree: Creation Center node Each node sends unicast join message

to center node Message forwarded until it arrives at a node

already belonging to spanning tree

CN_4.33

Multicast Routing: Problem Statement

Goal: find a tree (or trees) connecting routers having local multicast group members tree: not all paths between routers used source-based: different tree from each sender to

receivers shared-tree: same tree used by all group

members

Shared treeSource-based trees

CN_4.34

Approaches for building multicast trees

Approaches: source-based tree: one tree per

source shortest path trees reverse path forwarding

group-shared tree: group uses one tree minimal spanning (Steiner) center-based trees…We first look at basic approaches, then

specific protocols adopting these approaches

CN_4.35

Shortest Path Tree Multicast forwarding tree: tree of

shortest path routes from source to all receivers Dijkstra’s algorithm

R1

R2

R3

R4

R5

R6 R7

21

6

3 45

i

router with attachedgroup member

router with no attachedgroup member

link used for forwarding,i indicates order linkadded by algorithm

LEGENDS: source

CN_4.36

Reverse Path Forwarding

if (multicast datagram received on incoming link on shortest path back to sender)

then flood datagram onto all outgoing links

else ignore datagram

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

Each router has simple forwarding behavior:

CN_4.37

Reverse Path Forwarding: example

• Result is a source-specific reverse SPT– may be a bad choice with asymmetric

links

R1

R2

R3

R4

R5

R6 R7

router with attachedgroup member

router with no attachedgroup member

datagram will be forwarded

LEGENDS: source

datagram will not be forwarded

CN_4.38

Reverse Path Forwarding: pruning

Forwarding tree contains subtrees with no multicast group members no need to forward datagrams down

subtree “prune” messages sent upstream by

router with no downstream group members

R1

R2

R3

R4

R5

R6 R7

router with attachedgroup memberrouter with no attachedgroup member

prune message

LEGENDS: source

links with multicastforwarding

P

PP

CN_4.39

Shared-Tree: Steiner Tree Steiner Tree: minimum cost tree

connecting all routers with attached group members

problem is NP-complete excellent heuristics exists not used in practice:

computational complexity information about entire network

needed monolithic: rerun whenever a router

needs to join/leave

CN_4.40

Center-based trees Single delivery tree shared by all One router identified as “center” of

tree to join:

edge router sends unicast join-msg addressed to center router

join-msg “processed” by intermediate routers and forwarded towards center

join-msg either hits existing tree branch for this center, or arrives at center

path taken by join-msg becomes new branch of tree for this router

CN_4.41

Center-based trees: an example

Suppose R6 chosen as center:

R1

R2

R3

R4

R5

R6 R7

router with attachedgroup member

router with no attachedgroup member

path order in which join messages generated

LEGEND

2

1

3

1

CN_4.42

Internet Multicasting Routing: DVMRP

DVMRP: Distance Vector Multicast Routing Protocol, RFC1075

Flood and prune: reverse path forwarding, source-based tree RPF tree based on DVMRP’s own routing

tables constructed by communicating DVMRP routers

no assumptions about underlying unicast routing protocols (RIP, OSPF, etc)

initial datagram to multicast group flooded everywhere via RPF

routers not wanting group: send upstream prune messages

CN_4.43

DVMRP: continued… Soft state: DVMRP router periodically (1

min.) “forgets” branches are pruned: Multicast data again flows down

unpruned branch downstream router: reprune or else

continue to receive data routers can quickly regraft to tree

following IGMP join at leaf odds and ends

commonly implemented in commercial routers

Mbone routing done using DVMRP

CN_4.44

TunnelingQ: How to connect “islands” of

multicast routers in a “sea” of unicast routers?

Multicast datagram encapsulated inside “normal” (non-multicast-addressed) datagram

Normal IP datagram sent thru “tunnel” via regular IP unicast to receiving multicast router

receiving multicast router unencapsulates to get multicast datagram

physical topology logical topology

CN_4.45

PIM: Protocol Independent Multicast

Not dependent on any specific underlying unicast routing algorithm (works with all)

Two different multicast distribution scenarios :

Dense (密 ): group

members densely packed, in “close” proximity.

bandwidth more plentiful

Sparse(疏 ): Number of

networks with group members small wrt number of interconnected networks

group members “widely dispersed”

bandwidth not plentiful

CN_4.46

Consequences of Sparse-Dense Dichotomy:

Dense group membership

by routers assumed until routers explicitly prune

data-driven construction on multicast tree (e.g., RPF)

bandwidth and non-group-router processing profligate

Sparse: no membership until

routers explicitly join

receiver- driven construction of multicast tree (e.g., center-based)

bandwidth and non-group-router processing conservative

CN_4.47

PIM- Dense Mode

Flood-and-prune RPF, similar to DVMRP but

underlying unicast routing protocol provides RPF information for incoming datagram

less complicated (less efficient) downstream flood than DVMRP reduces reliance on underlying routing algorithm

has protocol mechanism for router to detect it is a leaf-node router

CN_4.48

PIM - Sparse Mode Center-based

approach Router sends join

message to rendezvous point (RP) intermediate routers

update state and forward join

after joining via RP, router can switch to source-specific tree increased

performance: less concentration, shorter paths

R1

R2

R3

R4

R5

R6R7

join

join

join

all data multicastfrom rendezvouspoint

rendezvouspoint

CN_4.49

PIM - Sparse Mode

Sender(s): unicast data to RP,

which distributes down RP-rooted tree

RP can extend multicast tree upstream to source

RP can send stop message if no attached receivers “no one is

listening!”

R1

R2

R4

R5

R6R7

join

join

join

all data multicastfrom rendezvouspoint

rendezvouspoint

CN_4.50

Routing for Mobile Hosts Mobile IP

home agent: Router located on the home network of the mobile hosts

home address: The permanent IP address of the mobile host. Has a network number equal to that of the home network and thus of the home agent

foreign agent: Router located on a network to which the mobile node attaches itself when it is away from its home network

CN_4.51

Routing for Mobile Hosts

CN_4.52

Routing for Mobile Hosts Problem of delivering a packet to the

mobile node How does the home agent intercept a

packet that is destined for the mobile node ? Proxy ARP

How does the home agent then deliver the packet to the foreign agent ? IP tunnel Care-of-address

How does the foreign agent deliver the packet to the mobile node ?

CN_4.53

Routing for Mobile Hosts

Route optimization in Mobile IP The route from the sending node to

mobile node can be significantly sub-optimal

One extreme exampleThe mobile node and the sending node are on the same network, but the home network for the mobile node is on the far side of the Internet– Triangle Routing Problem

CN_4.54

Routing for Mobile Hosts Solution

Let the sending node know the care-of-address of the mobile node. The sending node can create its own tunnel to the foreign agent

Home agent sends binding update message

The sending node creates an entry in the binding cache

The binding cache may become out-of-date– The mobile node moved to a different network

– Foreign agent sends a binding warning message

CN_4.55

Summary

We have talked about the following issuesScalability routing in the

InternetIPv6Broadcast/MulticastingMobile IP

End of Chapter 4