Ch 4. The Network Layer Myungchul Kim mckim@icu.ac.kr

Ch 4. The Network Layer

Myungchul Kim

mckim@icu.ac.kr

2

– Datagram– Routers do not run application- and transport- layer protocols– Forwarding vs routing– Forwarding: router-local action of transfering a packet from an in

put link to the appropriate out link– Routing: network-wide process determining the end-to-end paths

that packets take from source to destination– Thr routing algorithm determines the values that are inserted into

the routers’ forwarding table.– Packet switch

Link-layer switch Router

3

4

5

Network service model

– Defines the characteristics of end-to-end transport of packets between sending and receiving end systems.

Guaranteed delivery Guaranteed delivery with bounded delay In-order packet delivery Guranteed minimal bandwidth Guaranteed maximum jitter Security service

– Best-effort service: no service at all– ATM service model

Constant bit rate (CBR): as if a dedicated fixed-bandwidth transmission link

Available bit rate (ABR): cells cannot be reordered and a min cell transmission rate is guaranteed

6

7

Virtual circuit and datagram networks

Network layer vs transport layer– Host-to-host services, process-to-process services– Network layer: host-to-host connectionless service (datagram ne

tworks), host-to-host connection service (virtual-circuit networks)– End systems for transport layer vs routers and end systems for n

etwork layers

8

Virtual circuit networks– ATM and frame relay– Virtual circuit

A path VC numbers Entries in the forwarding table Page 345

9

Forwarding table

12 22 32

1 23

VC number

interfacenumber

Incoming interface Incoming VC # Outgoing interface Outgoing VC #

1 12 3 222 63 1 18 3 7 2 171 97 3 87… … … …

Forwarding table innorthwest router:

Routers maintain connection state information!

10

– VC setup -> Data transfer -> VC tear down by ATM’s Q.2931 signaling protocol

– Connection set up at the transport layer?

11

Datagram networks– Prefix match at the page 348– Longest prefix matching rule– Forwarding tables can be modifed at any time -> packets go diffe

rent paths and arrive out of order

12

Forwarding table

Destination Address Range Link Interface

11001000 00010111 00010000 00000000 through 0 11001000 00010111 00010111 11111111

11001000 00010111 00011000 00000000 through 1 11001000 00010111 00011000 11111111

11001000 00010111 00011001 00000000 through 2 11001000 00010111 00011111 11111111

otherwise 3

4 billion possible entries

13

What’s inside a router

14

Input ports– A shadow copy of the forwarding table is typically stored at ech i

nput port and updated by the routing processor– Increase lookup speeds: content addressable memories (CAM)

allows a 32-bit IP address to be presented to the CAM, which returns the content of the forwarding table entry for that address in essentially constant time.

15

Switching fabric

16

Output ports

17

Where does queueing occur– Packet queue can form at both the input ports and the output por

ts– Packet loss – Packet scheduler at the output port must choose one packet am

ong those queued for transmission First-come-first-served Weighted fair queueing For quality-of-service guarantees

18

19

20

IP: forwarding and addressing in the Internet

21

22

IP datagram fragmentation– Maximum transmission unit: a hard limit on the length of an IP da

tagram– Jolt2 attack: none of fragments has an offset of zero or overlappi

ng IP fragments

23

24

IPv4 addressing– 32 bits long (4 bytes)– Dotted-decimal notation– Globally unique– subnet

25

– Classless interdomain routing (CIDR)– a.b.c.d/x network portion of th IP address = prefix– Classfule addressing: C(/24) = 254 hosts, B(/16) = 65,634 hosts,

broadcast = 255.255.255.255

26

Obtaining a block of addresses– Internet Corporation for Assigned Name and Numbers (ICANN)

Allocate IP addresses to regional Internet registries Manage the DNS root servers

Obtaining a host address: the Dynamic Host Configuration Protocol (DHCP)

– DHCP server discovery– DHCP server offer(s)– DHCP request– DHCP ACK– A TCP connection maintanence problem for a mobile node

27

28

29

Network address translation (NAT)– Private addresses have meaning within that network– The NAT router behaves to the outside world as a single device

with a single IP address.

30

– Arguments on Network address translation (NAT) Prot numbers for addressing processes not for addressing ho

sts Routers are supposed to process packets only up to layer 3 Violates the end-to-end arguments IPv6

31

Internet Control Message Protocol (ICMP)– Error reporting– Ping program– Source quench message– Tracerout

32

33

IPv6– IPv5 (ST-2 similar to RSVP)– Datagram format

Expanded addressing capabilities: unicast, multicast, anycast address

A streamlined 40-byte header Flow labeling and priority

– IPv4 vs IPv6 Fragmentation/reassembly Header checksum Options

34

35

– Transition from IPv4 to IPv6 A flag day Dual-stack approach Tunneling

– The US Office of Management and Budget (OMB): to IPv6 by June 2008

– Europe’s Third Generation Partnership Program (3GPP) 2007.

– Difficult to change network-layer protocols

36

37

38

IP security– IPsec– Virtual Private Networks (VPN)

Cryptographic agreement on algorithms and keys Encryption of IP datagram payload Data integrity Origin authentication

39

Routing algoritms

– Default router: the first-hop router– The least cost path

– Global routing algorithm: link-state (LS) algorithms– Decentralized routing algorithm: distance-vector (DV) algorithms

– Static routing algorithms vs dynamic routing

– Load-sensitive algorithms vs load-insensitive

40

Hierarchical routing– Autonomous systems (ASs)– Gateway routers– Within an AS, all routers run the same intra-AS routing protocol.– The ASs run the same inter-AS routing protocol.

41

Routing in the Internet

RIP (routing information protocol)– DV protocol– Hop count as a cost metric (max 15)– Routing updates every 30 seconds

42

OSPF(open shortest path first)– LS protocol– Link’s state updates every 30 minutes– Advantages:

Security: MD5 Multiple same-cost paths Integrated support for unicast and multicast routing Support for hierarchy within a single routing domain

43

44

BGP (Border Gateway Protocol)– Obtain subnet reachablility information from neighboring ASs– Propagate the reachablility information to all routers interanl to th

e AS– Determine “good” routes to subnets based on the reachability inf

ormation on AS policy.

45

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

46

BGP routing policy (2)

A advertises path AW to B B advertises path BAW to X Should B advertise path BAW to C?

– No way! B gets no “revenue” for routing CBAW since neither W nor C are B’s customers

– B wants to force C to route to w via A– B wants to route only to/from its customers!

A

B

C

W X

Y

legend:

customer network:

provider network

47

Broadcast and Multicast Routing

Broadcast routing algorithms– N-way unicast– Uncontrolled flooding -> broadcast storm

48

– Controlled flooding Sequence-number-controlled flooding Reverse path forwarding (RPF)

49

– Spanning-tree broadcast

50

51

Multicast– To a subset of network nodes– Class D multicast IP address for multicast group– Internet Group Management Protocol and mulcast routing protoc

ols

52

– Multicast routing algorithms Multicast routing using a group-shared tree Multicast routing using a source-based tree with pruning

53

– Multicast routing in the Internet Distance Vector Multicast Routing Protocol (DVMRP) Protocol-Independent Multicast (PIM) routing protocol