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Network Layer 4-
Chapter 4
Network Layer
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All material copyright 1996- J.F Kurose and K.W. Ross, All Rights Reserved
Computer Networking: A Top Down Approach 4 th^ edition. Jim Kurose, Keith Ross Addison-Wesley, July
Network Layer 4-
Chapter 4: Network Layer
Chapter goals:
❒ understand principles behind network layer
services:
❍ network layer service models
❍ forwarding versus routing
❍ how a router works
❍ routing (path selection)
❍ dealing with scale
❍ advanced topics: IPv6, mobility
❒ instantiation, implementation in the Internet
Network Layer 4-
Chapter 4: Network Layer
❒ 4. 1 Introduction
❒ 4.2 Virtual circuit and
datagram networks
❒ 4.3 What’s inside a
router
❒ 4.4 IP: Internet
Protocol
❍ Datagram format ❍ IPv4 addressing ❍ ICMP ❍ IPv
❒ 4.5 Routing algorithms
❍ Link state ❍ Distance Vector ❍ Hierarchical routing
❒ 4.6 Routing in the
Internet
❍ RIP
❍ OSPF
❍ BGP
❒ 4.7 Broadcast and
multicast routing
Network Layer 4-
Network layer
❒ transport segment from
sending to receiving host
❒ on sending side
encapsulates segments
into datagrams
❒ on rcving side, delivers
segments to transport
layer
❒ network layer protocols
inevery host, router
❒ router examines header
fields in all IP datagrams
passing through it
application transportnetwork data link physical
application transportnetwork data link physical
networkdata link physical (^) network data link physical
networkdata link physical network data link physical
network data linkphysical
data linknetwork physical
network data link physical
data linknetwork physical
network data link physical
network data link network physical data linkphysical
Two Key Network-Layer Functions
❒ forwarding: move
packets from router’s
input to appropriate
router output
❒ routing: determine
route taken by
packets from source
to dest.
❍ routing algorithms
analogy:
❒ routing: process of
planning trip from
source to dest
❒ forwarding: process
of getting through
single interchange
32
0111
value in arriving packet’s header
routing algorithm
local forwarding table header value output link 0100 0101 0111 1001
3 2 2 1
Interplay between routing and forwarding
Network Layer 4-
Connection setup
❒ 3 rd^ important function insome network architectures:
❍ ATM, frame relay, X.
❒ before datagrams flow, two end hostsand intervening
routers establish virtual connection
❍ routers get involved
❒ network vs transport layer connection service:
❍ network: between two hosts (may also involve
inervening routers in case of VCs)
❍ transport: between two processes
Network Layer 4-
Network service model
Q: Whatservice model for “channel” transporting
datagrams from sender to receiver?
Example services for
individual datagrams:
❒ guaranteed delivery
❒ guaranteed delivery
with less than 40 msec
delay
Example services for a
flow of datagrams:
❒ in-order datagram
delivery
❒ guaranteed minimum
bandwidth to flow
❒ restrictions on
changes in inter-
packet spacing
Network Layer 4-
Network layer service models:
Network Architecture
Internet
ATM
ATM
ATM
ATM
Service Model
best effort
CBR
VBR
ABR
UBR
Bandwidth
none
constant rate guaranteed rate guaranteed minimum none
Loss
no
yes
yes
no
no
Order
no
yes
yes
yes
yes
Timing
no
yes
yes
no
no
Congestion feedback
no (inferred via loss) no congestion no congestion yes
no
Guarantees?
Network Layer 4-
Chapter 4: Network Layer
❒ 4. 1 Introduction
❒ 4.2 Virtual circuit and
datagram networks
❒ 4.3 What’s inside a
router
❒ 4.4 IP: Internet
Protocol
❍ Datagram format ❍ IPv4 addressing ❍ ICMP ❍ IPv
❒ 4.5 Routing algorithms
❍ Link state ❍ Distance Vector ❍ Hierarchical routing
❒ 4.6 Routing in the
Internet
❍ RIP
❍ OSPF
❍ BGP
❒ 4.7 Broadcast and
multicast routing
Network layer connection and
connection-less service
❒ datagram network provides network-layer
connectionless service
❒ VC network provides network-layer
connection service
❒ analogous to the transport-layer services,
but:
❍ service: host-to-host
❍ no choice: network provides one or the other
❍ implementation: in network core
Virtual circuits
❒ call setup, teardown for each callbefore data can flow
❒ each packet carries VC identifier (not destination host address)
❒ every router on source-dest path maintains “state” for
each passing connection ❒ link, router resources (bandwidth, buffers) may be
allocated to VC (dedicated resources = predictable service)
“source-to-dest path behaves much like telephone
circuit”
❍ performance-wise ❍ network actions along source-to-dest path
Network Layer 4-
Datagram or VC network: why?
Internet (datagram)
❒ data exchange among computers ❍ “elastic” service, no strict timing req.
❒ “smart” end systems (computers) ❍ can adapt, perform control, error recovery ❍ simple inside network, complexity at “edge”
❒ many link types
❍ different characteristics ❍ uniform service difficult
ATM (VC)
❒ evolved from telephony ❒ human conversation: ❍ strict timing, reliability requirements ❍ need for guaranteed service ❒ “dumb” end systems ❍ telephones ❍ complexity inside network
Network Layer 4-
Chapter 4: Network Layer
❒ 4. 1 Introduction
❒ 4.2 Virtual circuit and
datagram networks
❒ 4.3 What’s inside a
router
❒ 4.4 IP: Internet
Protocol
❍ Datagram format ❍ IPv4 addressing ❍ ICMP ❍ IPv
❒ 4.5 Routing algorithms
❍ Link state ❍ Distance Vector ❍ Hierarchical routing
❒ 4.6 Routing in the
Internet
❍ RIP
❍ OSPF
❍ BGP
❒ 4.7 Broadcast and
multicast routing
Network Layer 4-
Router Architecture Overview
Two key router functions:
❒ run routing algorithms/protocol (RIP, OSPF, BGP)
❒ forwarding datagrams from incoming to outgoing link
Network Layer 4-
Input Port Functions
Decentralized switching : ❒ given datagram dest., lookup output port using forwarding table in input port memory ❒ goal: complete input port processing at ‘line speed’ ❒ queuing: if datagrams arrive faster than forwarding rate into switch fabric
Physical layer: bit-level reception
Data link layer: e.g., Ethernet see chapter 5
Three types of switching fabrics Switching Via Memory
First generation routers:
❒ traditional computers with switching under direct
control of CPU
❒ packet copied to system’s memory
❒ speed limited by memory bandwidth (2 bus
crossings per datagram)
Input Port
Output Port
Memory
System Bus
Network Layer 4-
Switching Via a Bus
❒ datagram from input port memory
to output port memory via a shared
bus
❒ bus contention: switching speed
limited by bus bandwidth
❒ 32 Gbps bus, Cisco 5600: sufficient
speed for access and enterprise
routers
Network Layer 4-
Switching Via An Interconnection
Network
❒ overcome bus bandwidth limitations
❒ Banyan networks, other interconnection nets
initially developed to connect processors in
multiprocessor
❒ advanced design: fragmenting datagram into fixed
length cells, switch cells through the fabric.
❒ Cisco 12000: switches 60 Gbps through the
interconnection network
Network Layer 4-
Output Ports
❒ Buffering required when datagrams arrive from
fabric faster than the transmission rate
❒ Scheduling discipline chooses among queued
datagrams for transmission
Network Layer 4-
Output port queueing
❒ buffering when arrival rate via switch exceeds
output line speed
❒ queueing (delay) and loss due to output port buffer overflow!
How much buffering?
❒ RFC 3439 rule of thumb: average buffering
equal to “typical” RTT (say 250 msec) times
link capacity C
❍ e.g., C = 10 Gps link: 2.5 Gbit buffer
❒ Recent recommendation: withN flows,
buffering equal to RTT .C
N
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!
Network Layer 4-
Chapter 4: Network Layer
❒ 4. 1 Introduction
❒ 4.2 Virtual circuit and
datagram networks
❒ 4.3 What’s inside a
router
❒ 4.4 IP: Internet
Protocol
❍ Datagram format ❍ IPv4 addressing ❍ ICMP ❍ IPv
❒ 4.5 Routing algorithms
❍ Link state ❍ Distance Vector ❍ Hierarchical routing
❒ 4.6 Routing in the
Internet
❍ RIP
❍ OSPF
❍ BGP
❒ 4.7 Broadcast and
multicast routing
Network Layer 4-
IP Addressing: introduction
❒ IP address: 32-bit
identifier for host,
routerinterface ❒ interface: connection
between host/router
and physical link
❍ router’s typically have multiple interfaces ❍ host typically has one interface ❍ IP addresses associated with each interface
223.1.1.
223.1.1.
223.1.1.
223.1.1.4 223.1.2.
223.1.2.
223.1.2.
223.1.3.1 223.1.3.
223.1.3.
223.1.1.1 = 11011111 00000001 00000001 00000001
(^223 1 )
Network Layer 4-
Subnets
❒ IP address:
❍ subnet part (high order bits) ❍ host part (low order bits)
❒ What’s a subnet?
❍ device interfaces with same subnet part of IP address ❍ can physically reach each other without intervening router
223.1.1.
223.1.1.
223.1.1.
223.1.1.4 223.1.2.
223.1.2.
223.1.2.
223.1.3.1 223.1.3.
223.1.3.
network consisting of 3 subnets
subnet
Network Layer 4-
Subnets
223.1.1.0/24 (^) 223.1.2.0/
223.1.3.0/
Recipe
❒ To determine the
subnets, detach each
interface from its
host or router,
creating islands of
isolated networks.
Each isolated network
is called a subnet.
Subnet mask: /
Subnets
How many? 223.1.1.
223.1.1.
223.1.1.
223.1.2.1 223.1.2.
223.1.2.
223.1.3.1 223.1.3.
223.1.3.
223.1.1.
223.1.7.
223.1.7. 223.1.8.1 223.1.8.
223.1.9.
223.1.9.
IP addressing: CIDR
CIDR: Classless InterDomain Routing
❍ subnet portion of address of arbitrary length
❍ address format: a.b.c.d/x, where x is # bits in
subnet portion of address
subnet part
host part
Network Layer 4-
IP addresses: how to get one?
Q: How doeshost get IP address?
❒ hard-coded by system admin in a file
❍ Wintel: control-panel->network->configuration-
>tcp/ip->properties
❍ UNIX: /etc/rc.config
❒ DHCP: Dynamic Host Configuration Protocol:
dynamically get address from as server
❍ “plug-and-play”
Network Layer 4-
DHCP: Dynamic Host Configuration Protocol
Goal: allow host todynamically obtain its IP address
from network server when it joins network
Can renew its lease on address in use Allows reuse of addresses (only hold address while connected an “on” Support for mobile users who want to join network (more shortly)
DHCP overview:
❍ host broadcasts “DHCP discover” msg
❍ DHCP server responds with “DHCP offer” msg
❍ host requests IP address: “DHCP request” msg
❍ DHCP server sends address: “DHCP ack” msg
Network Layer 4-
DHCP client-server scenario
223.1.1.
223.1.1.
223.1.1.
223.1.1.4 223.1.2.
223.1.2.
223.1.2.
223.1.3.1 223.1.3.
223.1.3.
A
B
E
DHCP
server
arriving DHCP client needs address in this network
Network Layer 4-
DHCP client-server scenario
DHCP server: 223.1.2.5 arriving client
time
DHCP discover src : 0.0.0.0, 68 dest.: 255.255.255.255, yiaddr: 0.0.0. transaction ID: 654 DHCP offer src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2. transaction ID: 654 Lifetime: 3600 secs DHCP request src: 0.0.0.0, 68 dest:: 255.255.255.255, 67 yiaddrr: 223.1.2. transaction ID: 655 Lifetime: 3600 secs DHCP ACK src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2. transaction ID: 655 Lifetime: 3600 secs
IP addresses: how to get one?
Q: How doesnetwork get subnet part of IP
addr?
A: gets allocated portion of its provider ISP’s
address space
ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/
Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/ Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/ Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/ ... ….. …. …. Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/
Hierarchical addressing: route aggregation
“Send me anything with addresses beginning 200.23.16.0/20”
200.23.16.0/
200.23.18.0/
200.23.30.0/
Fly-By-Night-ISP
Organization 0
Organization 7
Internet
Organization 1
ISPs-R-Us “Send me anythingwith addresses beginning 199.31.0.0/16”
200.23.20.0/
Organization 2
.. .
Hierarchical addressing allows efficient advertisement of routing information:
Network Layer 4-
NAT: Network Address Translation
❒ 16-bit port-number field:
❍ 60,000 simultaneous connections with a single
LAN-side address!
❒ NAT is controversial:
❍ routers should only process up to layer 3
❍ violates end-to-end argument
- NAT possibility must be taken into account by app designers, eg, P2P applications
❍ address shortage should instead be solved by
IPv
Network Layer 4-
NAT traversal problem
❒ client want to connect to
server with address 10.0.0.
❍ server address 10.0.0.1 local to LAN (client can’t use it as destination addr) ❍ only one externally visible NATted address: 138.76.29.
❒ solution 1: statically
configure NAT to forward
incoming connection
requests at given port to
server
❍ e.g., (123.76.29.7, port 2500) always forwarded to 10.0.0. port 25000
10.0.0.
10.0.0.
NAT router
138.76.29.
Client
Network Layer 4-
NAT traversal problem
❒ solution 2: Universal Plug and
Play (UPnP) Internet Gateway
Device (IGD) Protocol. Allows
NATted host to:
learn public IP address
enumerate existing port
mappings
add/remove port mappings
(with lease times)
i.e., automate static NAT port
map configuration
10.0.0.
10.0.0.
NAT
router
138.76.29.
IGD
Network Layer 4-
NAT traversal problem
❒ solution 3: relaying (used in Skype)
❍ NATed server establishes connection to relay
❍ External client connects to relay
❍ relay bridges packets between to connections
10.0.0.
NAT
router
138.76.29.
Client
- connection to relay initiated by NATted host
- connection to relay initiated by client
- relaying established
Chapter 4: Network Layer
❒ 4. 1 Introduction
❒ 4.2 Virtual circuit and
datagram networks
❒ 4.3 What’s inside a
router
❒ 4.4 IP: Internet
Protocol
❍ Datagram format ❍ IPv4 addressing ❍ ICMP ❍ IPv
❒ 4.5 Routing algorithms
❍ Link state ❍ Distance Vector ❍ Hierarchical routing
❒ 4.6 Routing in the
Internet
❍ RIP
❍ OSPF
❍ BGP
❒ 4.7 Broadcast and
multicast routing
ICMP: Internet Control Message Protocol
❒ used by hosts & routers to communicate network-level information ❍ error reporting: unreachable host, network, port, protocol ❍ echo request/reply (used by ping) ❒ network-layer “above” IP: ❍ ICMP msgs carried in IP datagrams ❒ ICMP message: type, code plus first 8 bytes of IP datagram causing error
Type Code description 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header
Network Layer 4-
Traceroute and ICMP
❒ Source sends series of UDP segments to dest ❍ First has TTL = ❍ Second has TTL=2, etc. ❍ Unlikely port number ❒ When nth datagram arrives to nth router: ❍ Router discards datagram ❍ And sends to source an ICMP message (type 11, code 0) ❍ Message includes name of router& IP address
❒ When ICMP message arrives, source calculates RTT ❒ Traceroute does this 3 times Stopping criterion ❒ UDP segment eventually arrives at destination host ❒ Destination returns ICMP “host unreachable” packet (type 3, code 3) ❒ When source gets this ICMP, stops.
Network Layer 4-
Chapter 4: Network Layer
❒ 4. 1 Introduction
❒ 4.2 Virtual circuit and
datagram networks
❒ 4.3 What’s inside a
router
❒ 4.4 IP: Internet
Protocol
❍ Datagram format ❍ IPv4 addressing ❍ ICMP ❍ IPv
❒ 4.5 Routing algorithms
❍ Link state ❍ Distance Vector ❍ Hierarchical routing
❒ 4.6 Routing in the
Internet
❍ RIP
❍ OSPF
❍ BGP
❒ 4.7 Broadcast and
multicast routing
Network Layer 4-
IPv
❒ Initial motivation: 32-bit address space soon
to be completely allocated.
❒ Additional motivation:
❍ header format helps speed processing/forwarding
❍ header changes to facilitate QoS
IPv6 datagram format:
❍ fixed-length 40 byte header
❍ no fragmentation allowed
Network Layer 4-
IPv6 Header (Cont)
Priority: identify priority among datagrams in flow Flow Label: identify datagrams in same “flow.”
(concept of“flow” not well defined).
Next header: identify upper layer protocol for data
Other Changes from IPv
❒ 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 IPv
❒ Not all routers can be upgraded simultaneous
❍ no “flag days”
❍ How will the network operate with mixed IPv4 and
IPv6 routers?
❒ Tunneling: IPv6 carried as payload in IPv
datagram among IPv4 routers
Network Layer 4-
Routing Algorithm classification
Global or decentralized
information?
Global: ❒ all routers have complete topology, link cost info ❒ “link state” algorithms Decentralized: ❒ router knows physically- connected neighbors, link costs to neighbors ❒ iterative process of computation, exchange of info with neighbors ❒ “distance vector” algorithms
Static or dynamic?
Static:
❒ routes change slowly
over time
Dynamic:
❒ routes change more
quickly
❍ periodic update
❍ in response to link
cost changes
Network Layer 4-
Chapter 4: Network Layer
❒ 4. 1 Introduction
❒ 4.2 Virtual circuit and
datagram networks
❒ 4.3 What’s inside a
router
❒ 4.4 IP: Internet
Protocol
❍ Datagram format ❍ IPv4 addressing ❍ ICMP ❍ IPv
❒ 4.5 Routing algorithms
❍ Link state ❍ Distance Vector ❍ Hierarchical routing
❒ 4.6 Routing in the
Internet
❍ RIP
❍ OSPF
❍ BGP
❒ 4.7 Broadcast and
multicast routing
Network Layer 4-
A Link-State Routing Algorithm
Dijkstra’s algorithm
❒ net topology, link costs known to all nodes ❍ accomplished via “link state broadcast” ❍ all nodes have same info ❒ computes least cost paths from one node (‘source”) to all other nodes ❍ gives forwarding table for that node ❒ iterative: after k iterations, know least cost path to k dest.’s
Notation:
❒ c(x,y): link cost from node
x to y; = ∞ if not direct neighbors
❒ D(v): current value of cost
of path from source to dest. v
❒ p(v): predecessor node
along path from source to v
❒ N': set of nodes whose
least cost path definitively known
Network Layer 4-
Dijsktra’s Algorithm
1 Initialization: 2 N' = {u} 3 for all nodes v 4 if v adjacent to u 5 then D(v) = c(u,v) 6 else D(v) = ∞ 7 8 Loop 9 find w not in N' such that D(w) is a minimum 10 add w to N' 11 update D(v) for all v adjacent to w and not in N' : 12 D(v) = min( D(v), D(w) + c(w,v) ) 13 /* new cost to v is either old cost to v or known 14 shortest path cost to w plus cost from w to v */ 15 until all nodes in N'
Dijkstra’s algorithm: example
Step 0 1 2 3 4 5
N'
u ux uxy uxyv uxyvw uxyvwz
D(v),p(v) 2,u 2,u 2,u
D(w),p(w) 5,u 4,x 3,y 3,y
D(x),p(x) 1,u
D(y),p(y) ∞ 2,x
D(z),p(z) ∞ ∞ 4,y 4,y 4,y
u
x y
v w
z
Dijkstra’s algorithm: example (2)
u
x y
v w
z
Resulting shortest-path tree from u:
v x y w z
(u,v) (u,x) (u,x) (u,x) (u,x)
destination (^) link
Resulting forwarding table in u:
Network Layer 4-
Dijkstra’s algorithm, discussion
Algorithm complexity: n nodes
❒ each iteration: need to check all nodes, w, not in N
❒ n(n+1)/2 comparisons: O(n^2 )
❒ more efficient implementations possible: O(nlogn)
Oscillations possible:
❒ e.g., link cost = amount of carried traffic
A
D
C
B
(^1) 1+e
0 e
e
A
D
C
B
2+e (^0)
1+e 1
A
D
C
B
(^0) 2+e
(^1) 1+e
A
D
C
B
2+e (^0)
0 e
1+e 1
initially
… recompute routing
… recompute … recompute
Network Layer 4-
Chapter 4: Network Layer
❒ 4. 1 Introduction
❒ 4.2 Virtual circuit and
datagram networks
❒ 4.3 What’s inside a
router
❒ 4.4 IP: Internet
Protocol
❍ Datagram format ❍ IPv4 addressing ❍ ICMP ❍ IPv
❒ 4.5 Routing algorithms
❍ Link state ❍ Distance Vector ❍ Hierarchical routing
❒ 4.6 Routing in the
Internet
❍ RIP
❍ OSPF
❍ BGP
❒ 4.7 Broadcast and
multicast routing
Network Layer 4-
Distance Vector Algorithm
Bellman-Ford Equation (dynamic programming)
Define
d x (y) := cost of least-cost path from x to y
Then
d x (y) = min {c(x,v) + d v (y) }
where min is taken over all neighbors v of x
v
Network Layer 4-
Bellman-Ford example
u
x y
v w
z
Clearly, dv (z) = 5, dx(z) = 3, dw(z) = 3
du (z) = min { c(u,v) + dv (z),
c(u,x) + dx(z),
c(u,w) + dw(z) }
= min {2 + 5,
Node that achieves minimum is next
hop in shortest path ➜ forwarding table
B-F equation says:
Distance Vector Algorithm
❒ D x (y) = estimate of least cost from x to y
❒ Node x knows cost to each neighbor v:
c(x,v)
❒ Node x maintains distance vector D x =
[D x (y): y є N ]
❒ Node x also maintains its neighbors’
distance vectors
❍ For each neighbor v, x maintains
D v = [D v (y): y є N ]
Distance vector algorithm (4)
Basic idea:
❒ Each node periodically sends its own distance
vector estimate to neighbors
❒ When a node x receives new DV estimate from
neighbor, it updates its own DV using B-F equation:
D (^) x(y) ← minv {c(x,v) + D (^) v (y)} for each node y ∊ N
❒ Under minor, natural conditions, the estimate
D (^) x(y) converge to the actual least cost dx(y)
Network Layer 4-
Chapter 4: Network Layer
❒ 4. 1 Introduction
❒ 4.2 Virtual circuit and
datagram networks
❒ 4.3 What’s inside a
router
❒ 4.4 IP: Internet
Protocol
❍ Datagram format ❍ IPv4 addressing ❍ ICMP ❍ IPv
❒ 4.5 Routing algorithms
❍ Link state ❍ Distance Vector ❍ Hierarchical routing
❒ 4.6 Routing in the
Internet
❍ RIP
❍ OSPF
❍ BGP
❒ 4.7 Broadcast and
multicast routing
Network Layer 4-
Hierarchical Routing
scale: with 200 million
destinations:
❒ can’t store all dest’s in routing tables! ❒ routing table exchange would swamp links!
administrative autonomy
❒ internet = network of networks ❒ each network admin may want to control routing in its own network
Our routing study thus far - idealization
❒ all routers identical
❒ network “flat”
… not true in practice
Network Layer 4-
Hierarchical Routing
❒ aggregate routers into
regions, “autonomous
systems” (AS)
❒ routers in same AS run
same routing protocol
❍ “intra-AS” routing protocol ❍ routers in different AS can run different intra- AS routing protocol
Gateway router
❒ Direct link to router in
another AS
Network Layer 4-
3b
1d
3a
1c
AS3 2a
AS
AS
1a
2c 2b
1b
Intra-AS Routing algorithm
Inter-AS Routing algorithm
Forwarding table
3c
Interconnected ASes
❒ forwarding table
configured by both
intra- and inter-AS
routing algorithm
❍ intra-AS sets entries for internal dests ❍ inter-AS & Intra-As sets entries for external dests
3b
1d
3a
1c
AS3 2a
AS
AS
1a
2c 2b
1b
3c
Inter-AS tasks
❒ suppose router in AS
receives datagram
dest outside of AS
❍ router should
forward packet to
gateway router, but
which one?
AS1 must:
1. learn which dests
reachable through
AS2, which through
AS
2. propagate this
reachability info to all
routers in AS
Job of inter-AS routing!
Example: Setting forwarding table in router 1d
❒ suppose AS1 learns (via inter-AS protocol) that subnet
x reachable via AS3 (gateway 1c) but not via AS2.
❒ inter-AS protocol propagates reachability info to all
internal routers.
❒ router 1d determines from intra-AS routing info that
its interfaceI is on the least cost path to 1c.
❍ installs forwarding table entry(x,I)
3b
1d
3a
1c
AS3 2a
AS
AS
1a
2c 2b
1b
3c
x
Network Layer 4-
Example: Choosing among multiple ASes
❒ now suppose AS1 learns from inter-AS protocol that
subnetx is reachable from AS3and from AS2.
❒ to configure forwarding table, router 1d must
determine towards which gateway it should forward
packets for dest x.
❍ this is also job of inter-AS routing protocol!
3b
1d
3a
1c
AS3 2a
AS
AS
1a
2c 2b
1b
3c
x
Network Layer 4-
Learn from inter-AS protocol that subnet x is reachable via multiple gateways
Use routing info from intra-AS protocol to determine costs of least-cost paths to each of the gateways
Hot potato routing: Choose the gateway that has the smallest least cost
Determine from forwarding table the interface I that leads to least-cost gateway. Enter (x,I) in forwarding table
Example: Choosing among multiple ASes
❒ now suppose AS1 learns from inter-AS protocol that
subnetx is reachable from AS3and from AS2.
❒ to configure forwarding table, router 1d must
determine towards which gateway it should forward
packets for dest x.
❍ this is also job of inter-AS routing protocol!
❒ hot potato routing: send packet towards closest of
two routers.
Network Layer 4-
Chapter 4: Network Layer
❒ 4. 1 Introduction
❒ 4.2 Virtual circuit and
datagram networks
❒ 4.3 What’s inside a
router
❒ 4.4 IP: Internet
Protocol
❍ Datagram format ❍ IPv4 addressing ❍ ICMP ❍ IPv
❒ 4.5 Routing algorithms
❍ Link state ❍ Distance Vector ❍ Hierarchical routing
❒ 4.6 Routing in the
Internet
❍ RIP
❍ OSPF
❍ BGP
❒ 4.7 Broadcast and
multicast routing
Network Layer 4-
Intra-AS Routing
❒ also known as Interior Gateway Protocols (IGP)
❒ most common Intra-AS routing protocols:
❍ RIP: Routing Information Protocol
❍ OSPF: Open Shortest Path First
❍ IGRP: Interior Gateway Routing Protocol (Cisco
proprietary)
Chapter 4: Network Layer
❒ 4. 1 Introduction
❒ 4.2 Virtual circuit and
datagram networks
❒ 4.3 What’s inside a
router
❒ 4.4 IP: Internet
Protocol
❍ Datagram format ❍ IPv4 addressing ❍ ICMP ❍ IPv
❒ 4.5 Routing algorithms
❍ Link state ❍ Distance Vector ❍ Hierarchical routing
❒ 4.6 Routing in the
Internet
❍ RIP
❍ OSPF
❍ BGP
❒ 4.7 Broadcast and
multicast routing
RIP ( Routing Information Protocol)
❒ distance vector algorithm
❒ included in BSD-UNIX Distribution in 1982
❒ distance metric: # of hops (max = 15 hops)
C D
A B
u v w
x
y
z
destination hops u 1 v 2 w 2 x 3 y 3 z 2
From router A to subsets:
Network Layer 4-
OSPF (Open Shortest Path First)
❒ “open”: publicly available
❒ uses 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
Network Layer 4-
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)
❒ For each link, multiple cost metrics for different
TOS (e.g., satellite link cost set “low” for best effort;
high for real time)
❒ integrated uni- and multicast support:
❍ Multicast OSPF (MOSPF) uses same topology data
base as OSPF
❒ hierarchical OSPF in large domains.
Network Layer 4-
Hierarchical OSPF
Network Layer 4-
Hierarchical OSPF
❒ two-level hierarchy: local area, backbone.
❍ Link-state advertisements only in area
❍ each nodes has detailed area topology; only know
direction (shortest path) to nets in other areas.
❒ 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.
Chapter 4: Network Layer
❒ 4. 1 Introduction
❒ 4.2 Virtual circuit and
datagram networks
❒ 4.3 What’s inside a
router
❒ 4.4 IP: Internet
Protocol
❍ Datagram format ❍ IPv4 addressing ❍ ICMP ❍ IPv
❒ 4.5 Routing algorithms
❍ Link state ❍ Distance Vector ❍ Hierarchical routing
❒ 4.6 Routing in the
Internet
❍ RIP
❍ OSPF
❍ BGP
❒ 4.7 Broadcast and
multicast routing
Internet inter-AS routing: BGP
❒ BGP (Border Gateway Protocol):the de
facto standard
❒ BGP provides each AS a means to:
1. Obtain subnet reachability information from
neighboring ASs.
2. Propagate reachability information to all AS-
internal routers.
3. Determine “good” routes to subnets based on
reachability information and policy.
❒ allows subnet to advertise its existence to
rest of Internet:“I am here”
Network Layer 4-
BGP basics
❒ pairs of routers (BGP peers) exchange routing info
over semi-permanent TCP connections: BGP sessions
❍ BGP sessions need not correspond to physical
links.
❒ when AS2 advertises prefix to AS1:
❍ AS2promises it will forward any addresses
datagrams towards that prefix.
❍ AS2 can aggregate prefixes in its advertisement
3b
1d
3a
1c
AS3 2a
AS
AS
1a
2c
2b
1b
3c
eBGP session iBGP session
Network Layer 4-
Distributing reachability info
❒ using eBGP session between 3a and 1c, AS3 sends
prefix reachability info to AS1.
❍ 1c can then use iBGP do distribute new prefix
info to all routers in AS
❍ 1b can then re-advertise new reachability info
to AS2 over 1b-to-2a eBGP session
❒ when router learns of new prefix, creates entry
for prefix in its forwarding table.
3b
1d
3a
1c
AS3 2a
AS
AS
1a
2c
2b
1b
3c
eBGP session iBGP session
Network Layer 4-
Path attributes & BGP routes
❒ advertised prefix includes BGP attributes.
❍ prefix + attributes = “route”
❒ two important attributes:
❍ AS-PATH: contains ASs through which prefix
advertisement has passed: e.g, AS 67, AS 17
❍ NEXT-HOP: indicates specific internal-AS router
to next-hop AS. (may be multiple links from
current AS to next-hop-AS)
❒ when gateway router receives route
advertisement, uses import policy to
accept/decline.
Network Layer 4-
BGP route selection
❒ router may learn about more than 1 route
to some prefix. Router must select route.
❒ elimination rules:
1. local preference value attribute: policy
decision
2. shortest AS-PATH
3. closest NEXT-HOP router: hot potato routing
4. additional criteria
BGP messages
❒ BGP messages exchanged using TCP.
❒ BGP messages:
❍ OPEN: opens TCP connection to peer and
authenticates sender
❍ UPDATE: advertises new path (or withdraws old)
❍ KEEPALIVE keeps connection alive in absence of
UPDATES; also ACKs OPEN request
❍ NOTIFICATION: reports errors in previous msg;
also used to close connection
BGP routing policy
❒ A,B,C are provider networks
❒ X,W,Y are customer (of provider networks)
❒ X is dual-homed: attached to two networks
❍ X does not want to route from B via X to C
❍ .. so X will not advertise to B a route to C
A
B
C
W
X
Y
legend :
customer network:
provider network
