Network Layer – part 31 Customer-Provider Routing Relationships The Global Internet consists of Autonomous Systems (AS) interconnected with each other:

Network Layer – part 3 1

Customer-Provider Routing RelationshipsCustomer-Provider Routing Relationships

The Global Internet consists of Autonomous Systems (AS) interconnected with each other: Customer: Stub AS: small corporation Customer: Multihomed AS: large corporation (no transit) Provider: Transit AS: backbone provider networks

A

B

C

w

x

y

e.g. A, B, C

e.g. x

e.g. w, y

Advertises to its neighbors that it

has no paths to any other destinations

except itself

All traffic entering must be destined for w, all traffic leaving must have originated

from w

Stub AS must be prevented from forwarding traffic between Transit ASs using Selective Route Advertisement Policy

Group of routers


Routing in the Internet

Two-level routing: Intra-AS: administrator is responsible for choice Inter-AS: unique standard

Border Gateway Protocol (BGP4)

de facto standard inter-AS routing protocol in today’s Internet provides each AS a means to:

• obtain subnet reachability information (i.e. via one of its neighboring AS) • propagate the reachability information to all routers internal to the AS• determine “good” routes to subnets based on the reachability information and on AS policy.

Allows each subnet to

advertise its existence to

the rest of the Internet


Internet AS HierarchyAS border (exterior gateway) routers

AS interior (gateway) routers


Intra-AS Routing

Also known as Interior Gateway Protocols (IGP) Most common IGPs:

RIPRIP: Routing Information Protocol (lower-tier ISPs and (lower-tier ISPs and Enterprise networks)Enterprise networks)

OSPFOSPF: Open Shortest Path First (upper-tier ISPs)(upper-tier ISPs)

IGRPIGRP: Interior Gateway Routing Protocol (Cisco proprietary)


RIP ( Routing Information Protocol)RIP ( Routing Information Protocol)

Distance vector algorithm Included in (Berkeley Software Distribution) BSD-UNIX

Distribution in 1982 Distance metric: # of hops (max = 15 hopsmax = 15 hops) = (AS < 15 hops in diameter)

Can you guess why?

Distance vectors: exchange routing updates via Response Message (also called advertisement) every 30 secevery 30 sec

Each advertisement: route to up to 25 destination subnets25 destination subnets within the AS, including the sender’s distance from each of them

Hop – no. of subnets traversed along the shortest path from Source Router to Destination Subnet, including the Destination Subnet.


RIP (Routing Information Protocol) RIP (Routing Information Protocol)

Destination Subnet Next Router Num. of hops to dest. w A 2

y B 2 z B 7

x -- 1... ... ....

w x y

z

A

C

D B

Routing table in Router Routing table in Router DD

…

ExampleExample

subnet



Destination Subnet Next Router Num. of hops to dest.w A 2

y B 2z B 7

x -- 1... ... ....

w x y

z

A

C

D B


…

ExampleExample

Destination Subnet Next Router Num. of hops to dest.z C 4w -- 1x -- 1

... ... ....

(30 secs. later.. (30 secs. later.. DD receives an receives an advertisementadvertisement from Router from Router AA ) )

Router A has a shorter path to Z!



Destination Subnet Next Router Num. of hops to dest.w A 2

y B 2z AA 4

x -- 1... ... ....

w x y

z

A

C

D B


…

ExampleExample

Destination Subnet Next Router Num. of hops to dest.z C 4w -- 1x -- 1

... ... ....

Advertisement from Router Advertisement from Router AA

Router D updates its entry for destination Z


RIP: Link Failure and Recovery RIP: Link Failure and Recovery

If no advertisement is heard after 180 secafter 180 sec --> the neighbour/link is declared dead Modifies routing table - routes via neighbor invalidated new advertisements sent to neighbors neighbours in turn send out new advertisements (if

tables changed) link failure info quickly propagates to entire net poisoned reverse used to prevent ping-pong loops

(infinite distance = 16 hops)

ExampleExample


Routing Info Protocol (RIP) Routing Info Protocol (RIP) Table processingTable processing

RIP routing tables managed by application-level application-level process called route-d (daemon)route-d (daemon)

advertisements sent in UDP packetsUDP packets, periodically repeated Able to manipulate

routing tables within the UNIX kernel

via UDP, via UDP, port 520port 520


OSPF (Open Shortest Path First)OSPF (Open Shortest Path First)

“Open” means publicly available Uses Link-State algorithm Link-State algorithm

LS packet dissemination Topology map at each node Route computation using Dijkstra's algorithm

OSPF advertisement carries one entry per neighbor router

Advertisements disseminated to entire AS (via flooding) Carried in OSPF messages directly over IP (rather than TCP or

UDP with upper-layer protocol of 89

Broadcasts information to all all not just neighboring

routers

OSPF Protocol Functionalities: reliable data transfer, link-state broadcast, check for links operability, extraction of neighboring router’s database of network-wide link state


OSPF advanced features (not in RIP)OSPF advanced features (not in RIP)

Security: all OSPF messages authenticated (to prevent malicious intrusion)

Multiple same-cost paths allowed (only one path in RIP) Integrated uni- and multicast routing support:

Multicast OSPF (MOSPF) uses same topology data base as OSPF

Hierarchical OSPF in large domains.

Allow only trusted routers

Most significant advancement! Has the ability to structure an autonomous system hierarchically


Hierarchical Open Shortest Path First (OSPF)


Hierarchical OSPFHierarchical OSPF

Two-level hierarchy: local area, backbone. Link-state advertisements are sent only within an area each node has detailed area topology; only know

direction (shortest path) to nets in other areas. Each area runs its own OSPF link-state routing algorithm

Area border routers: responsible for routing packets outside the area.

Backbone routers: run OSPF routing limited to backbone.

Boundary routers: connect to other ASs.


IGRP (Interior Gateway Routing Protocol) CISCO proprietary; successor of RIP (mid 80s) Uses the Distance Vector algorithm, like RIP several cost metrics (delay, bandwidth,

reliability, load, etc.) uses TCP to exchange routing updates Loop-free routing via Distributed Updating Alg.

(DUAL) based on diffused computation


Router Architecture Overview

Two key router functions:Two key router functions:

run routing algorithms/protocol (RIP, OSPF, BGP) switching datagrams from incoming to outgoing link

Physical layer functions

Data link layer functions

Lookup & forwarding functions

computes routing tables, performs

Network management functions


Input Port Functions

Decentralized switching: given datagram dest., lookup output port

using routing table in input port memory goalgoal: complete input port processing at

'line speed' queuingqueuing: happens if datagrams arrive

faster than forwarding rate into switch fabric

Physical layer:bit-level reception

Data link layer:e.g., Ethernetsee chapter 5


Input Port Queuing

Fabric slower than input ports combined -> queueing may occur at input queues

Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward

queueing delay and loss due to input buffer overflow!

Slot for Green packet is free, but there is HOL

blocking, so Green packet will have to wait


Three types of switching fabrics

No routing processor; 1 packet 1 packet at a timeLike shared memory multiprocessors

2n2n buses that connect n input ports to nn output ports


Switching Via Memory

First generation routers:First generation routers: packet copied by system's (single) CPUCPU speed limited by memory bandwidth memory bandwidth (2 bus crossings per datagram)

InputPort

OutputPort

Workstation’sMemory

System Bus

Modern routers:Modern routers: input port processorprocessor performs lookup, copy into memorymemory

Cisco Catalyst 8500


Switching Via Bus

datagram from input port memory to output port memory via a shared shared

busbus bus contention: switching speed

limited by bus bandwidth 1 Gbps bus, Cisco 1900: sufficient

speed for access and enterprise routers (not regional or backbone)


Switching Via An Interconnection Network

overcome bus bandwidth limitations Banyan networks, other interconnection nets

initially developed to connect processors in multiprocessor

Other Advanced design: fragmenting fragmenting datagram datagram into fixed length cells, switch cells through the fabric.

Cisco 12000: switches 60 Gbps through the interconnection network


Output Ports

Buffering required when datagrams arrive from the fabric faster than the transmission rate

Scheduling discipline chooses among queued datagrams for transmission


Output port queueing

buffering when arrival rate via switch exceeeds ouput line speed

queueing (delay) and loss due to output port buffer overflow!

It is more advantageous

to mark a packet before the buffer is

full in order to provide a

congestion signal to the

sender


END OF SESSION


IPv6 Initial motivation: 32-bit address space

completely allocated by 2008. Additional motivation:

header format helps speed processing/forwarding

header changes to facilitate QoS new anycast address: route to best of several

replicated servers IPv6 datagram format:

fixed-length 40 byte header no fragmentation allowed


IPv6 Header (Cont)Priority: identify priority among datagrams in flowFlow Label: identify datagrams in same flow. (concept of flow not well defined).Next header: identify upper layer protocol for data


Other Changes from IPv4

Checksum: removed entirely to reduce processing time at each hop

Options: allowed, but outside of header, indicated by Next Header field

ICMPv6: new version of ICMP additional message types, e.g. ''Packet Too

Big'' multicast group management functions


Transition From IPv4 To IPv6

Not all routers can be upgraded simultaneously no flag days How will the network operate with mixed

IPv4 and IPv6 routers? Two proposed approaches:

Dual Stack: some routers with dual stack (v6, v4) can translate between formats

Tunneling: IPv6 carried as payload in IPv4 datagram among IPv4 routers


Dual Stack Approach


Tunneling

IPv6 inside IPv4 where needed

Documents

Network Layer – part 31 Customer-Provider Routing Relationships The Global Internet consists of Autonomous Systems (AS) interconnected with each other: