Transport Layer 3-1

Summary: TCP Congestion Control When CongWin is below Threshold, sender in slow-start phase,

window grows exponentially. When CongWin is above Threshold, sender is in congestion-

avoidance phase, window grows linearly. When a triple duplicate ACK occurs, Threshold set to CongWin/2

and CongWin set to Threshold. When timeout occurs, Threshold set to CongWin/2 and CongWin is

set to 1 MSS.

Transport Layer 3-2

Fairness goal: if K TCP sessions share same bottleneck link of bandwidth R, each should have average rate of R/K

TCP connection 1

bottleneckroutercapacity R

TCP connection 2

TCP Fairness

Transport Layer 3-3

Why is TCP fair?Two competing sessions: Additive increase gives slope of 1, as throughout increases multiplicative decrease decreases throughput proportionally

R

R

equal bandwidth share

Connection 1 throughputConn

e ctio

n 2

thro

u ghp

ut

congestion avoidance: additive increase

loss: decrease window by factor of 2congestion avoidance: additive increaseloss: decrease window by factor of 2

Transport Layer 3-4

Fairness (more)Fairness and UDP Multimedia apps

often do not use TCP do not want rate

throttled by congestion control

Instead use UDP: pump audio/video at

constant rate, tolerate packet loss

Research area: TCP friendly

Fairness and parallel TCP connections

nothing prevents app from opening parallel connections between 2 hosts.

Web browsers do this Example: link of rate R

supporting 9 connections; new app asks for 1 TCP,

gets rate R/10 new app asks for 11 TCPs,

gets R/2 !

Network Layer 4-5

Chapter 4Network Layer

A note on the use of these ppt slides:We’re making these slides freely available to all (faculty, students, readers). They’re in PowerPoint form so you can add, modify, and delete slides (including this one) and slide content to suit your needs. They obviously represent a lot of work on our part. In return for use, we only ask the following: If you use these slides (e.g., in a class) in substantially unaltered form,

that you mention their source (after all, we’d like people to use our book!) If you post any slides in substantially unaltered form on a www site, that

you note that they are adapted from (or perhaps identical to) our slides, and note our copyright of this material.

Thanks and enjoy! JFK/KWR

All material copyright 1996-2009J.F Kurose and K.W. Ross, All Rights Reserved

Computer Networking: A Top Down Approach 5th edition. Jim Kurose, Keith RossAddison-Wesley, April 2009.

Network Layer

Review: TCP Reliable data transfer: acks Pipelined protocol: in-flight packets Cumulated acks (single timer) Flow control (receiver window size) Congestion control (congestion window

size): AIMD TCP's two phase operations: Slow Start

+ Congestion Control

4-6

Network Layer 4-7

Chapter 4: Network LayerChapter goals: understand principles behind network

layer services: IP addresses (+ getting an IP address via

DHCP) Routing algorithms Network of networks (BGP, dealing with

scales) ICMP NAT (network address translation)

Network Layer 4-8

Datagram networks no call setup at network layer routers: no state about end-to-end connections

no network-level concept of “connection” packets forwarded using destination host

address packets between same source-dest pair may take

different paths

application

transportnetworkdata linkphysical

application

transportnetworkdata linkphysical

1. Send data 2. Receive data

Network Layer 4-9

The Internet Network layer

forwardingtable

Host, router network layer functions:

Routing protocols• path selection• RIP, OSPF, BGP

IP protocol• addressing conventions• datagram format• packet handling conventions

ICMP protocol• error reporting• router

“signaling”

Transport layer: TCP, UDP

Link layer

physical layer

Networklayer

IP datagram format

ver length

32 bits

data (variable length,typically a TCP

or UDP segment)

16-bit identifierheader

checksumtime to

live

32 bit source IP address

IP protocol versionnumber

header length (bytes)

max numberremaining hops

(decremented at each router)

forfragmentation/reassembly

total datagramlength (bytes)

upper layer protocolto deliver payload to

head.len

type ofservice

“type” of data flgs fragment offset

upper layer

32 bit destination IP address

Options (if any) E.g. timestamp,record routetaken, specifylist of routers to visit.

how much overhead with TCP?

20 bytes of TCP 20 bytes of IP = 40 bytes +

app layer overhead

TCP/UDPIP

Ethernet

Application

Network Layer 4-11

IP Fragmentation and Reassembly

network links have MTU (max.transfer size) - largest possible link-level frame. different link types,

different MTUs large IP datagram divided

(“fragmented”) within net one datagram becomes

several datagrams “reassembled” only at

final destination IP header bits used to

identify, order related fragments

fragmentation: in: one large datagramout: 3 smaller datagrams

reassembly

Network Layer 4-12

IP Addressing 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 interface

IP addresses associated with each interface

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

223.1.1.1 = 11011111 00000001 00000001 00000001

223 1 11

interface

Network Layer 4-13

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

223.1.1.1

223.1.1.2

223.1.1.3

223.1.1.4 223.1.2.9

223.1.2.2

223.1.2.1

223.1.3.2223.1.3.1

223.1.3.27

network consisting of 3 subnets

subnet

Network Layer 4-14

IP addressing: CIDRCIDR: 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

11001000 00010111 00010000 00000000

subnetpart

hostpart

200.23.16.0/23

Network Layer 4-15

IP addresses: how to get one?

Q: How does a host get IP address?

hard-coded by system admin in a file Windows: control-panel->network-

>configuration->tcp/ip->properties Linux (ubuntu): /etc/network/interface

DHCP: Dynamic Host Configuration Protocol: dynamically get address from as server “plug-and-play”

Network Layer 4-16

DHCP: Dynamic Host Configuration Protocol

Goal: allow host to dynamically obtain its IP address from network server when it joins networkCan renew its lease on address in useAllows reuse of addresses (only hold address while

connected an “on”)Support for mobile users who want to join network (more

shortly)DHCP overview:

1. host broadcasts “DHCP discover” msg2. DHCP server responds with “DHCP offer”

msg3. host requests IP address: “DHCP request”

msg4. DHCP server sends address: “DHCP ack”

msg

Network Layer 4-17

Graph abstraction of a network

u

yx

wv

z2

21 3

1

12

53

5• c(x,x’) = cost of link (x,x’)

- e.g., c(w,z) = 5

• cost could always be 1, or inversely related to bandwidth,or inversely related to congestion

Cost of path (x1, x2, x3,…, xp) = c(x1,x2) + c(x2,x3) + … + c(xp-1,xp)

Question: What’s the least-cost path between u and z ?

Routing algorithm: algorithm that finds least-cost path

Graph: G = (N,E)N = set of routers = { u, v, w, x, y, z }

E = set of links ={ (u,v), (u,x), (v,x), (v,w), (x,w), (x,y), (w,y), (w,z), (y,z) }

Network Layer 4-18

Routing algorithmsGlobal or decentralized

information?

Global: all routers have complete

topology, link cost info “link state” algorithms (OSPF)

Decentralized: router knows physically-

connected neighbors, link costs to neighbors

iterative process of computation, exchange of info with neighbors

“distance vector” algorithms (RIP)

Static or dynamic?

Static: routes change slowly

over time

Dynamic: routes change more

quickly periodic update in response to link

cost changes

Network Layer 4-19

1

23

0111

value in arrivingpacket’s header

routing algorithm

local forwarding tableheader value output link

0100010101111001

3221

Interplay between routing, forwarding

Network Layer 4-20

Hierarchical routing for scalability

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 4-21

Hierarchical addressing: route aggregation

“Send me anythingwith addresses

beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us “Send me anythingwith addresses

beginning 199.31.0.0/16”

200.23.20.0/23Organization 2

...

...

Hierarchical addressing allows efficient advertisement of routing information:

Network Layer 4-22

Hierarchical addressing: more specific routesISPs-R-Us has a more specific route to Organization 1

“Send me anythingwith addresses beginning 200.23.16.0/20”

200.23.16.0/23

200.23.18.0/23

200.23.30.0/23

Fly-By-Night-ISP

Organization 0

Organization 7Internet

Organization 1

ISPs-R-Us “Send me anythingwith addresses beginning 199.31.0.0/16or 200.23.18.0/23”

200.23.20.0/23Organization 2

...

...

Network Layer 4-23

Hierarchical routing for scalability 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 4-24

3b

1d

3a1c

2aAS3

AS1AS21a

2c2b

1b

Intra-ASRouting algorithm

Inter-ASRouting algorithm

Forwardingtable

3c

Interconnected ASs

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

Network Layer 4-25

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 4-26

BGP basics pairs of routers (BGP peers) exchange routing

info over TCP connections (called BGP sessions) BGP sessions need not correspond to

physical links. when AS2 advertises prefix “200.23.16.0/23” to

AS1: AS2 promises it will forward datagrams

towards that prefix. AS2 can aggregate prefixes in its

advertisement

3b

1d

3a1c

2aAS3

AS1

AS21a

2c2b

1b

3ceBGP sessioniBGP session

Network Layer 4-27

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 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.

3b

1d

3a1c

2aAS3

AS1

AS21a

2c2b

1b

3ceBGP sessioniBGP session

Any dest w/ IP addr AS1should be

routed to 1c

Any dest w/ IP addr AS1should be

routed to 2a

Network Layer 4-28

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 local import policy to accept/decline.

Network Layer 4-29

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

decision2. shortest AS-PATH 3. closest NEXT-HOP router: hot potato

routing4. additional criteria

Network Layer 4-30

Why different Intra- and Inter-AS routing ? Policy: Inter-AS: admin wants control over how its traffic

routed, who routes through its net. Intra-AS: single admin, so no policy decisions

neededScale: hierarchical routing saves table size, reduced

update trafficPerformance: Intra-AS: can focus on performance Inter-AS: policy may dominate over performance

Network Layer 4-31

NAT: Network Address Translation Motivation: local network uses just one IP address

as far as outside world is concerned: range of addresses not needed from ISP: just one

IP address for all devices can change addresses of devices in local network

without notifying outside world can change ISP without changing addresses of

devices in local network devices inside local net not explicitly

addressable, visible by outside world (a security plus).

Network Layer 4-32

NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

10.0.0.4

138.76.29.7

local network(e.g., home network)

10.0.0/24

rest ofInternet

Datagrams with source or destination in this networkhave 10.0.0/24 address for

source, destination (as usual)

All datagrams leaving localnetwork have same single source

NAT IP address: 138.76.29.7,different source port numbers

Network Layer 4-33

NAT: Network Address Translation

10.0.0.1

10.0.0.2

10.0.0.3

S: 10.0.0.1, 3345D: 128.119.40.186, 80

110.0.0.4

138.76.29.7

1: host 10.0.0.1 sends datagram to 128.119.40.186, 80

NAT translation tableWAN side addr LAN side addr138.76.29.7, 5001 10.0.0.1, 3345…… ……

S: 128.119.40.186, 80 D: 10.0.0.1, 3345

4

S: 138.76.29.7, 5001D: 128.119.40.186, 80

2

2: NAT routerchanges datagramsource addr from10.0.0.1, 3345 to138.76.29.7, 5001,updates table

S: 128.119.40.186, 80 D: 138.76.29.7, 5001

33: Reply arrives dest. address: 138.76.29.7, 5001

4: NAT routerchanges datagramdest addr from138.76.29.7, 5001 to 10.0.0.1, 3345

Network Layer 4-34

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 IPv6

Network Layer 4-35

NAT traversal problem client wants to connect to

server with address 10.0.0.1 server address 10.0.0.1

local to LAN (client can’t use it as destination addr)

only one externally visible NATted address: 138.76.29.7

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 10.0.0.1 port 25000

10.0.0.1

10.0.0.4

NAT router

138.76.29.7

Client ?

Network Layer 4-36

NAT traversal problem solution 2: Universal Plug

and Play (UPnP) Internet Gateway Device (IGD) Protocol. Allows NATted host to: learn public IP address

(138.76.29.7) add/remove port

mappings (with lease times)

i.e., automate static NAT port map configuration

10.0.0.1

10.0.0.4

NAT router

138.76.29.7

IGD

Network Layer 4-37

NAT traversal problem solution 3: relaying (used in Skype)

NATed client establishes connection to relay External client connects to relay relay bridges packets between to

connections

138.76.29.7Client

10.0.0.1

NAT router

1. connection torelay initiatedby NATted host

2. connection torelay initiatedby client 3. relaying

established

Network Layer 4-38