ICS156 final review

Xiaowei Yang

What this course is about

Reinforcing basic networking concepts

Practical networking knowledge Today

Review concepts Final

Concepts, and labs

Basic networking concepts

Layering architecture Encapsulation and decapsulation Multiplexing and demultiplexing Addressing and routing

Protocols

Link layer Ethernet, ARP, and learning bridge

Network layer IP and ICMP Routing protocols: RIP, OSPF, and BGP

Transport layer UDP and TCP

Layering Architecture

The task of communication is to send data from one entity to another entity.

The complexity of the communication task is reduced by using multiple protocol layers.

Each protocol is implemented independently Each protocol is responsible for a specific

subtask

Internet Protocol Suite

The Internet protocol suite has four layers: Application, Transport, Network, and Data Link Layer

End systems (hosts) implement all four layers. Routers only need the bottom two layers.

Application

Transport

Network Operating system / router

User-level programs

Data Link

Functions of the Layers Data Link Layer:

Service: Reliable transfer of frames over a linkMedia Access Control on a LAN

Functions: Framing, media access control, error checking

Network Layer: Service: Move packets from source host to

destination host Functions: Routing, addressing

Transport Layer: Service: Delivery of data between hosts Functions: Connection establishment/termination,

error control, flow control

Application Layer: Service: Application specific (delivery of email,

retrieval of HTML documents, reliable transfer of file) Functions: Application specific

Encapsulation/decapsulation

As data is moving down the protocol stack, each protocol is adding layer-specific control information

HTTP

TCP

IP

Ethernet

User data

User dataHTTP Header

TCP Header

TCP HeaderIP Header

TCP HeaderIP HeaderEthernetHeader

EthernetTrailer

IP datagram

TCP segment

Ethernet frame

User dataHTTP Header

User dataHTTP Header

User dataHTTP Header

Multiplexing/Demultiplexing

Application dataTCP HeaderEthernet Header Ethernet Trailer

Ethernet frame

IP Header

DS ECNversion(4 bits)

headerlength

Total Length (in bytes)(16 bits)

Identification (16 bits)flags

(3 bits)Fragment Offset (13 bits)

Source IP address (32 bits)

Destination IP address (32 bits)

TTL Time-to-Live(8 bits)

Protocol(8 bits)

Header Checksum (16 bits)

32 bits

A field in a lower-layer protocol header indicates to which upper layer module to deliver a packet.

IPv4 addressing

network prefixnetwork prefix host numberhost number

An IP address is 4-byte long. It encodes both a network number

(network prefix) and an interface number (host number). network prefix identifies a network the host number identifies a specific host

(actually, interface on the network).

0 31

Classless Inter-domain routing (CIDR)

Network prefix is of variable length Addresses are allocated

hierarchically. Routers can aggregate multiple

address prefixes into one routing entry to minimize routing table size

CIDR network prefix is variable length

A network mask specifies the number of bits used to identify a network in an IP address.

1000111110000000 10001001 10010000

1111111111111111 1111111 00000000

128 143 137 144

255 255 255 0

Addr

Mask

CIDR notation

CIDR notation of an IP address: 128.143.137.144/24

CIDR notation can nicely express blocks of addresses A contiguous block of addresses

[128.195.0.0, 128.195.255.255] can be represented by an address prefix 128.195.0.0/16

Hierarchical address allocation

ISP obtains an address block 128.0.0.0/8 [128.0.0.0, 128.255.255.255]

ISP allocates 128.195.0.0/16 ([128.195.0.0, 128.195.255.255]) to the university.

University allocates 128.195.4.0/24 ([128.195.4.0, 128.195.4.255]) to the CS department’s network

A host on the CS department’s network gets one IP address 128.195.4.150

128.0.0.0 128.255.255.255128.195.0.0 128.196.255.255

128.195.4.0 128.195.4.255 128.195.4.150

IP forwarding

1. Hosts on the same subnet can directly reach each other without using a router.

2. To send a packet to a destination host on a different subnet, a host will direct the packet to its default router.

3. A router looks up the destination address in its routing table to determine the next hop. Longest prefix match

4. Routing protocols build the contents of a router’s routing table.

How does a host figure out whether a destination address is on the same subnet?

Short answer: Uses its routing table

Long answer: A host applies its network mask to both its

address and a destination address. If the network prefixes are the same, the destination address is on the same subnet.

A host with multiple interfaces will compare the network prefix of each interface to determine which subnet the destination address is on.

How does a host directly send a packet to a destination address?

A host sends an Ethernet frame using the hardware address of the destination host.

Address resolution protocol (ARP) is used to map an IP address to an Ethernet address.

How does a host send a packet to a default router? How does a router send a packet to a next hop router?

neon.tcpip-lab.edu"Neon"

128.143.71.21

argon.tcpip-lab.edu"Argon"128.143.137.144

router137.tcpip-lab.edu"Router137"

128.143.137.1

router71.tcpip-lab.edu"Router71"128.143.71.1

Ethernet NetworkEthernet Network

Router

Sending a packet from Argon to Neon

Review of Protocols

Link layer Ethernet, ARP Learning bridge

Learning forwarding state Spanning tree

Network layer IP and ICMP Routing protocols: RIP, OSPF, and BGP

Transport layer UDP and TCP

What’s a protocol? a human protocol and a computer network

protocol:

Hi

Hi

Got thetime?

2:00

TCP connection req

TCP connection response

Get http://www.ics.uci.edu

<file>time

What is a protocol? Protocols define

1. Format of messages2. Order of messages sent and received

among network entities3. Actions taken on message

transmission and receipt When reviewing, learning, or

implementing a protocol, pay attention to the above details.

ARP

Function: translates an IP address into a hardware address

Message format Message types:

ARP request, ARP response When an ARP request/response is

sent What happens after an ARP

request/response is received ARP cache is updated

Bridge spanning tree protocol

Function: to organize bridges into spanning tree to avoid loops

Message format When a BPDU is sent What actions are taken when a

BPDU is received

BPDU contents

Each bridge is sending out BPDUs that contain the following fields: root bridge ID, cost to root bridge, sending bridge ID, and sending port ID.

root bridge (what the sender thinks it is) root path cost for sending bridge

Identifies sending bridgeIdentifies the sending port

root IDroot ID costcost bridge IDbridge ID port IDport ID

Ordering of Messages

We say M1 advertises a better path than M2 (“M1<<M2”) if

(R1 < R2), Or (R1 == R2) and (C1 < C2), Or (R1 == R2) and (C1 == C2) and (B1 < B2), Or (R1 == R2) and (C1 == C2) and (B1 == B2) and (P1 < P2)

R1R1 C1C1 B1 B1

M1 M2

P1 P1 R2R2 C2C2 B2 B2 ID P2 ID P2

Initially, all bridges assume they are the root bridge.

Each bridge with bridge ID B sends BPDUs of this form on its LANs from each port P:

Initialization message

BB 00 BB PP

When a BPDU is received

Each bridge looks at the BPDUs received on all its ports and its own transmitted BPDUs.

Root bridge is the smallest received root ID that has been received so far (Whenever a smaller ID arrives, the root is updated).

When a BPDU is received (cont.)

Compare and update BPDU If a bridge sends a BPDU from port P1 with

four-tuple (R1, C1, B1, P1) receives a better BPDU (R2, C2, B2, P2), it will update its BPDU to (R2, C2 + cost, B1, P1).

Cost is a configuration parameter, and can be set for each port.

Elect root port On each bridge, the port where the “best

BPDU” (via relation “<<“) was received is elected as the root port of the bridge.

When to send a BPDU

B will send its updated BPDU (R2, C2 + cost, B1, P1) on port P1 only if its BPDU is better than any BPDU that B received from port P1.

In this case, B also assumes that it is the designated bridge for the LAN to which P1 connects

And port P1 is the designated port for that LAN

Selecting the Ports for the Spanning Tree

B can decide which ports are in the spanning tree: B’s root port and all designated ports

are part of the spanning tree. Those parts are in forwarding state.

Other ports are not part of the spanning tree. Those ports are in blocking state.

Data packets are only broadcasted on spanning tree ports if forwarding state is unavailable Learning bridge algorithm

Example

Assume that all bridges send out their BPDU’s once per second, and assume that all bridges send their BPDUs at the same time

Assume that all bridges are turned on simultaneously at time T=0 sec. Messages are sent out every second.

Brige2

LAN 1

LAN 2

LAN 3

LAN 4

Brige1

Brige5

Brige3 Brige4A

B

A B

AB A

BA

B

Example: BPDU’s sent by the bridges

Bridge 1

Bridge 2

Bridge 3

Bridge 4

Bridge 5

T=1sec Send:A: (B1,0,B1,A)B: (B1,0,B1,B)Recv:A:(B5,0,B5,A)(B2,0,B2,B)B:(B3,0,B3,B)(B4,0,B4,A)

Send:A: (B2,0,B2,A)B: (B2,0,B2,B)Recv:A:B: (B1,0,B1,A)(B5,0,B5,A)

Send:A:(B3,0,B3,A)B:(B3,0,B3,B)Recv:A: (B5,0,B5,B)(B4,0,B4,B)B: (B1,0,B1,B) (B4,0,B4,A)

Send:A:(B4,0,B4,A)B:(B4,0,B4,B)Recv:A: (B3,0,B3,B) (B1,0,B1,B)B: (B3,0,B3,A) (B5,0,B5,B)

Send:A:(B5,0,B5,A)B:(B5,0,B5,B)Recv:A: (B2,0,B2,B)(B1,0,B1,A)B: (B3,0,B3,A)(B4,0,B4,B)

Example: BPDU’s sent by the bridges

Bridge 1

Bridge 2

Bridge 3

Bridge 4

Bridge 5

T=2sec D-port: A,BSend:A: (B1,0,B1,A)B: (B1,0,B1,B)Recv:

R-port: BD-port: ASend:A: (B1,1,B2,A)Recv:A: B:(B1,0,B1,A)

R-port: BD-port: ASend:A: (B1,1,B3,A)Recv:A:(B1,1,B4,B)(B1,1,B5,B)B:(B1,0,B1,B)

R-port: AD-port: BSend:B: (B1,1,B4,B)Recv:A:(B1,0,B1,B)B:(B1,1,B3,A)(B1,1,B5,B)

R-port: AD-port: BSend:B: (B1,1,B5,B)Recv:A:(B1,0,B1,A)B:(B1,1,B3,A)(B1,1,B4,B)

Example: BPDU’s sent by the bridges

Bridge 1

Bridge 2

Bridge 3

Bridge 4

Bridge 5

T=3sec D-port: A,BSend:A: (B1,0,B1,A)B: (B1,0,B1,B)Recv:

R-port: BD-port: ASend:A: (B1,1,B2,A)Recv:A: B:(B1,0,B1,A)

R-port: BD-port: ASend:A: (B1,1,B3,A)Recv:A:B:(B1,0,B1,B)

R-port: ABlocked: B

Recv:A:(B1,0,B1,B)B:(B1,1,B3,A)

R-port: ABlocked: B

Recv:A:(B1,0,B1,A)B:(B1,1,B3,A)

Network layer protocols

IP Forwarding: longest prefix match Route entry aggregation IP fragmentation

ICMP When and why an ICMP message is sent

Routing protocols RIP: distance vector protocol

Count-to-infinity problem OSPF: link state protocol BGP: path vector protocol how each protocol works:

Order of messages sent and received Actions taken when messages are sent or received. In

particular, how next hop is selected. The underlying routing algorithm

Transport layer protocol

UDP Connectionless, unreliable, datagram

service TCP

Connection-oriented, reliable, byte-stream service

Things to know about TCP

How a TCP connection is established and terminated.

How TCP sends data reliably Cumulative acknowledgement, error/loss detection,

and retransmission Flow control: how a TCP sender avoids

overflowing a receiver buffer Receiver’s advertisement window to limit sender’s

sliding window Congestion control: how a TCP sender avoids

overflowing a router buffer Dynamically compute congestion window to limit

sender’s sliding window

Three-Way Handshake to establish a TCP connection

aida.poly.edu mng.poly.edu

S 1031880193:1031880193(0)win 16384 <mss 1460, ...>

S 172488586:172488586(0)

ack 1031880194 win 8760 <mss 1460>

ack 172488587 win 17520

TCP Connection Termination

aida.poly.edu mng.poly.edu

F 172488734:172488734(0)

ack 1031880221 win 8733

. ack 172488735 win 17484

. ack 1031880222 win 8733

F 1031880221:1031880221(0)ack 172488735 win 17520

How TCP sends data reliably Sender associates each byte sent

with a sequence number A receiver acknowledges the

highest number of bytes received. A TCP sender retransmits a segment

when it assumes that the segment has been lost:1. No ACK has been received and a

timeout occurs2. Multiple ACKs have been received for

the same segment

Flow control: sliding window algorithm

A window size variable specifies the maximum amount of data a sender can send without receiving an acknowledgement from a receiver.

A receiver uses the advertisement window size field to tell a sender the available receiving buffer

A sender’s window size is set to less than the advertisement window size.

1 2 3 4 5 6 7 8 9 10 11

window size = 6

sent but notacknowledged can be sent

sent andacknowledged

can't sent

Congestion control

The sender has two internal parameters: Congestion Window (cwnd) Slow-start threshhold Value

(ssthresh) The sender’s sending window size is

set to the minimum of (cwnd, receiver advertised win)

Slow start and congestion avoidance

Congestion control operates in two modes: slow start (cwnd < ssthresh)

Probe the available bandwidth congestion avoidance (cwnd >=

ssthresh) Try not to overload the network.

Slow Start

Initial value: cwnd = 1 (or 2) Note: Unit is a segment size. In real implementation, cwnd = 2MSS

Each time an ACK is received by a sender, the congestion window is increased by 1 segment: cwnd = cwnd + 1

Congestion Avoidance

If cwnd >= ssthresh then each time an ACK is received, increment cwnd as follows:

cwnd = cwnd + 1/cwnd In byte units: cwnd = cwnd +

MSS*MSS/cwnd

cwnd is increased by one only if all cwnd segments have been acknowledged.

Example of Slow Start/Congestion Avoidance

Assume that ssthresh = 8

cwnd = 1

cwnd = 2

cwnd = 4

cwnd = 8

cwnd = 9

cwnd = 10

0

2

4

6

8

10

12

14

t=0

t=2

t=4

t=6

Roundtrip times

Cw

nd

(in

seg

men

ts)

ssthresh

Summary

Basic networking concepts How protocols work

Note: We cannot review all protocols in this session, but you are responsible for all protocols we covered in lectures and labs (1-6).

Things that will show up in the final

Basic concepts Knowledge about lab

You are expected to understand data traces Protocols Questions:

My office hours TA lab hours Discussion board Email

What you’ve learned in this course will

Help you find a job in the field of networking

Set up and manage your own home network

Equip you with the basic knowledge and skills to understand new network protocols