1. CMPE 252A : Computer Networks
Chen Qian
Computer Engineering
UCSC Baskin Engineering
Network layer

2. 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
Network Layer

3. Interplay between routing and forwarding1 2 3
0111
value in arriving
packet’s header
routing algorithm
local forwarding table
header value
output link
0100
0101
1001
Control plane:
routing algorithm determines
end-end-path through network
Data plane:
forwarding table determines
local forwarding at this router
Network Layer

4. Datagram forwarding table
4 billion IP addresses, so rather than list individual destination address
list range of addresses
(aggregate table entries)
routing algorithm
local forwarding table
dest address
output link
address-range 1
address-range 2
address-range 3
address-range 4 3 2 1
IP destination address in
arriving packet’s header 1 2 3
Network Layer

5. Datagram forwarding table
Destination Address Range
through
otherwise
Link Interface 1 2 3
Q: but what happens if ranges don’t divide up so nicely?
Network Layer

6. Longest prefix matching
when looking for forwarding table entry for given destination address, use longest address prefix that matches destination address.
Destination Address Range
*** *********
*********
*** *********
otherwise
Link interface 1 2 3
examples:
DA:
which interface?
DA:
which interface?
Network Layer

7. IP addressing: introduction
IP address: 32-bit identifier for host, router interface
interface: connection between host/router and physical link
router’s typically have multiple interfaces
host typically has one or two interfaces (e.g., wired Ethernet, wireless )
IP addresses associated with each interface =
223 1 1 1
Network Layer

8. Subnets IP address: what’s a subnet ? 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
subnet
network consisting of 3 subnets
Network Layer

9. Subnets
/24
/24
/24
subnet
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: /24
Network Layer

10. 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
/23
Network Layer

11. IP addresses: how to get one?
Q: how does network get subnet part of IP addr?
A: gets allocated portion of its provider ISP’s address space
ISP's block /20
Organization /23
Organization /23
Organization /23
… … ….
Organization /23
Network Layer

12. Hierarchical addressing: route aggregation
hierarchical addressing allows efficient advertisement of routing
information:
Organization 0
/23
Organization 1
“Send me anything
with addresses
beginning
/20”
/23
Organization 2
/23 .
Fly-By-Night-ISP .
Internet
Organization 7
/23
“Send me anything
with addresses
beginning
/16”
ISPs-R-Us
Network Layer

13. Hierarchical addressing: more specific routes
ISPs-R-Us has a more specific route to Organization 1
Organization 0
/23
“Send me anything
with addresses
beginning
/20”
Organization 2
/23 .
Fly-By-Night-ISP .
Internet
Organization 7
/23
“Send me anything
with addresses
beginning /16
or /23”
ISPs-R-Us
Organization 1
/23
Network Layer

14. 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

15. Dijsktra’s Algorithm 1 Initialization: 2 N' = {u} 3 for all nodes v
if v adjacent to u
then D(v) = c(u,v)
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' :
D(v) = min( D(v), D(w) + c(w,v) )
13 /* new cost to v is either old cost to v or known
shortest path cost to w plus cost from w to v */
15 until all nodes in N'
Network Layer

16. Dijkstra’s algorithm: example
D(v)
p(v)
D(w)
p(w)
D(x)
p(x)
D(y)
p(y)
D(z)
p(z)
Step N' u ∞
7,u
3,u
5,u 1 uw ∞
11,w
6,w
5,u 2
uwx
14,x
11,w
6,w 3
uwxv
14,x
10,v 4
uwxvy
12,y 5
uwxvyz w 3 4 v x u 5 7 y 8 z 2 9
notes:
construct shortest path tree by tracing predecessor nodes
ties can exist (can be broken arbitrarily)
Network Layer

17. Dijkstra’s algorithm: another example
Step 1 2 3 4 5 N' u ux
uxy
uxyv
uxyvw
uxyvwz
D(v),p(v)
2,u
D(w),p(w)
5,u
4,x
3,y
D(x),p(x)
1,u
D(y),p(y) ∞
2,x
D(z),p(z) ∞
4,y u y x w v z 2 1 3 5
Network Layer

18. Dijkstra’s algorithm: example (2)
resulting shortest-path tree from u: u y x w v z
resulting forwarding table in u: v x y w z
(u,v)
(u,x)
destination
link
Network Layer

19. 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(n2)
more efficient implementations possible: O(nlogn)
Network Layer

20. 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)
Network Layer

21. 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
advertisements flooded to entire AS
carried in OSPF messages directly over IP (rather than TCP or UDP
IS-IS routing protocol: nearly identical to OSPF
Network Layer

22. Hierarchical OSPF backbone boundary router backbone router area border
routers
area 3
internal
routers
area 1
area 2
Network Layer

23. Internet inter-AS routing: BGP
BGP (Border Gateway Protocol): the de facto inter-domain routing protocol
“glue that holds the Internet together”
BGP provides each AS a means to:
eBGP: obtain subnet reachability information from neighboring ASs.
iBGP: propagate reachability information to all AS-internal routers.
determine “good” routes to other networks based on reachability information and policy.
allows subnet to advertise its existence to rest of Internet: “I am here”
Network Layer

24. BGP basics
BGP session: two BGP routers (“peers”) exchange BGP messages:
advertising paths to different destination network prefixes (“path vector” protocol)
exchanged over semi-permanent TCP connections
when AS3 advertises a prefix to AS1:
AS3 promises it will forward datagrams towards that prefix
AS3 can aggregate prefixes in its advertisement 3c 3a
BGP
message 3b 2c
AS3
other
networks
AS1 1c 1a 1d 1b 2a 2b
other
networks
AS2
Network Layer

25. BGP basics: distributing path information
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 AS1
1b can then re-advertise new reachability info to AS2 over 1b-to-2a eBGP session
when router learns of new prefix, it creates entry for prefix in its forwarding table.
eBGP session 3a 3b
iBGP session 2c
AS3
other
networks 1c 2a 2b
other
networks 1a 1b
AS2 1d
AS1
Network Layer

26. BGP route selection
router may learn about more than 1 route to destination AS, selects route based on:
local preference value attribute: policy decision
shortest AS-PATH
closest NEXT-HOP router: hot potato routing
additional criteria
Network Layer

27. Next class
Please read Chapter 6 of the textbook BEFORE Class 27