chapter 4: network layercs456/s08/week6.pdf · 1 chapter 4: network layer 4. 1 introduction 4.2...
TRANSCRIPT
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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
❍ IPv6
❒ 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
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Router Architecture Overview
Two key router functions:
❒ run routing algorithms/protocol (RIP, OSPF, BGP)
❒ forwarding datagrams from incoming to outgoing link
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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., Ethernetsee chapter 5
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Three types of switching fabrics
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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)
InputPort
OutputPort
Memory
System Bus
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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
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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
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Output Ports
❒ Buffering required when datagrams arrive from fabric faster than the transmission rate
❒ Scheduling discipline chooses among queued datagrams for transmission
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Output port queueing
❒ buffering when arrival rate via switch exceeds output line speed
❒ queueing (delay) and loss due to output port buffer overflow!
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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: with N flows, buffering equal to RTT C.
N
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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!
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Link Layer
❒ 5.1 Introduction and services
❒ 5.2 Error detection and correction
❒ 5.3Multiple access protocols
❒ 5.4 Link-Layer Addressing
❒ 5.5 Ethernet
❒ 5.6 Hubs and switches
❒ 5.7 PPP
❒ 5.8 Link Virtualization: ATM and MPLS
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Virtualization of networks
Virtualization of resources: powerful abstraction in systems engineering:
❒ computing examples: virtual memory, virtual devices
❍ Virtual machines: e.g., java
❍ IBM VM os from 1960’s/70’s
❒ layering of abstractions: don’t sweat the details of the lower layer, only deal with lower layers abstractly
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The Internet: virtualizing networks
ARPAnet satellite net
gateway
Internetwork layer (IP): ❒ addressing: internetwork appears
as single, uniform entity, despite underlying local network heterogeneity
❒ network of networks
Gateway: ❒ “embed internetwork packets in
local packet format or extract them”
❒ route (at internetwork level) to next gateway
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Cerf & Kahn’s Internetwork Architecture
What is virtualized?
❒ two layers of addressing: internetwork and local network
❒ new layer (IP) makes everything homogeneous at internetwork layer
❒ underlying local network technology
❍ cable
❍ satellite
❍ 56K telephone modem
❍ today: ATM, MPLS
… “invisible” at internetwork layer. Looks like a link layer technology to IP!
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ATM and MPLS
❒ ATM, MPLS separate networks in their own right
❍ different service models, addressing, routing from Internet
❒ viewed by Internet as logical link connecting IP routers
❍ just like dialup link is really part of separate network (telephone network)
❒ ATM, MPLS: of technical interest in their own right
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Asynchronous Transfer Mode: ATM
❒ 1990’s/00 standard for high-speed (155Mbps to 622 Mbps and higher) Broadband Integrated Service Digital Network architecture
❒ Goal: integrated, end-end transport of carry voice, video, data
❍ meeting timing/QoS requirements of voice, video (versus Internet best-effort model)
❍ “next generation” telephony: technical roots in telephone world
❍ packet-switching (fixed length packets, called “cells”) using virtual circuits
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ATM: network or link layer?Vision: end-to-end
transport: “ATM from desktop to desktop”
❍ ATM is a network technology
Reality: used to connect IP backbone routers
❍ “IP over ATM”
❍ ATM as switched link layer, connecting IP routers
ATMnetwork
IPnetwork
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Multiprotocol label switching (MPLS)
❒ initial goal: speed up IP forwarding by using fixed length label (instead of IP address) to do forwarding
❍ borrowing ideas from Virtual Circuit (VC) approach
❍ but IP datagram still keeps IP address!
PPP or Ethernet header
IP header remainder of link-layer frameMPLS header
label Exp S TTL
20 3 1 5
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MPLS capable routers
❒ a.k.a. label-switched router
❒ forwards packets to outgoing interface based only on label value (don’t inspect IP address)❍ MPLS forwarding table distinct from IP forwarding tables
❒ signaling protocol needed to set up forwarding❍ RSVP-TE
❍ forwarding possible along paths that IP alone would not allow (e.g., source-specific routing) !!
❍ use MPLS for traffic engineering
❒ must co-exist with IP-only routers
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R1R2
D
R3R4R5
0
1
00
A
R6
in out outlabel label dest interface
6 - A 0
in out outlabel label dest interface
10 6 A 1
12 9 D 0
in out outlabel label dest interface
10 A 0
12 D 0
1
in out outlabel label dest interface
8 6 A 0
0
8 A 1
MPLS forwarding tables
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Chapter 6 outline
6.1 Introduction
Wireless
❒ 6.2 Wireless links, characteristics
❍ CDMA
❒ 6.3 IEEE 802.11 wireless LANs (“wi-fi”)
❒ 6.4 Cellular Internet Access
❍ architecture
❍ standards (e.g., GSM)
Mobility
❒ 6.5 Principles: addressing and routing to mobile users
❒ 6.6 Mobile IP
❒ 6.7 Handling mobility in cellular networks
❒ 6.8 Mobility and higher-layer protocols
6.9 Summary
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What is mobility?
❒ spectrum of mobility, from the network perspective:
no mobility high mobility
mobile wireless user, using same access point
mobile user, passing through multiple access point while maintaining ongoing connections (like cell phone)
mobile user, connecting/ disconnecting from network using DHCP.
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Mobility: Vocabulary
home network: permanent “home” of mobile(e.g., 128.119.40/24)
Permanent address:address in home network, can always be used to reach mobilee.g., 128.119.40.186
home agent: entity that will perform mobility functions on behalf of mobile, when mobile is remote
wide area network
correspondent
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Mobility: more vocabulary
Care-of-address: address in visited network.(e.g., 79,129.13.2)
wide area network
visited network: network in which mobile currently resides (e.g., 79.129.13/24)
Permanent address: remains constant (e.g., 128.119.40.186)
foreign agent: entity in visited network that performs mobility functions on behalf of mobile.
correspondent: wants to communicate with mobile
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How do you contact a mobile friend:
❒ search all phone books?
❒ call her parents?
❒ expect her to let you know where he/she is?
I wonder where Alice moved to?
Consider friend frequently changing addresses, how do you find her?
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Mobility: approaches
❒ Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange.
❍ routing tables indicate where each mobile located
❍ no changes to end-systems
❒ Let end-systems handle it:
❍ indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote
❍ direct routing: correspondent gets foreign address of mobile, sends directly to mobile
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Mobility: approaches
❒ Let routing handle it: routers advertise permanent address of mobile-nodes-in-residence via usual routing table exchange.
❍ routing tables indicate where each mobile located
❍ no changes to end-systems
❒ let end-systems handle it:
❍ indirect routing: communication from correspondent to mobile goes through home agent, then forwarded to remote
❍ direct routing: correspondent gets foreign address of mobile, sends directly to mobile
not scalable
to millions ofmobiles
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Mobility: registration
End result:
❒ Foreign agent knows about mobile
❒ Home agent knows location of mobile
wide area network
home networkvisited network
1
mobile contacts foreign agent on entering visited network
2
foreign agent contacts home agent home: “this mobile is resident in my network”
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Mobility via Indirect Routing
wide area network
homenetwork
visitednetwork
3
2
41
correspondent addresses packets using home address of mobile
home agent intercepts packets, forwards to foreign agent
foreign agent receives packets, forwards to mobile
mobile replies directly to correspondent
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Indirect Routing: comments
❒ Mobile uses two addresses:
❍ permanent address: used by correspondent (hence mobile location is transparent to correspondent)
❍ care-of-address: used by home agent to forward datagrams to mobile
❒ foreign agent functions may be done by mobile itself
❒ triangle routing: correspondent-home-network-mobile
❍ inefficient when
correspondent, mobile
are in same network
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Indirect Routing: moving between networks
❒ suppose mobile user moves to another network
❍ registers with new foreign agent
❍ new foreign agent registers with home agent
❍ home agent update care-of-address for mobile
❍ packets continue to be forwarded to mobile (but with new care-of-address)
❒ mobility, changing foreign networks transparent: on going connections can be maintained!
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Mobility via Direct Routing
wide area network
homenetwork
visitednetwork
4
2
41correspondent requests, receives foreign address of mobile
correspondent forwards to foreign agent
foreign agent receives packets, forwards to mobile
mobile replies directly to correspondent
3
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Mobility via Direct Routing: comments
❒ overcome triangle routing problem
❒ non-transparent to correspondent:correspondent must get care-of-address from home agent
❍ what if mobile changes visited network?
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wide area network
1
foreign net visited at session start
anchorforeignagent
2
4
new foreignagent
35
correspondentagent
correspondent
new foreignnetwork
Accommodating mobility with direct routing
❒ anchor foreign agent: FA in first visited network
❒ data always routed first to anchor FA
❒ when mobile moves: new FA arranges to have data forwarded from old FA (chaining)
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Chapter 6 outline
6.1 Introduction
Wireless
❒ 6.2 Wireless links, characteristics
❍ CDMA
❒ 6.3 IEEE 802.11 wireless LANs (“wi-fi”)
❒ 6.4 Cellular Internet Access
❍ architecture
❍ standards (e.g., GSM)
Mobility
❒ 6.5 Principles: addressing and routing to mobile users
❒ 6.6 Mobile IP
❒ 6.7 Handling mobility in cellular networks
❒ 6.8 Mobility and higher-layer protocols
6.9 Summary
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Mobile IP
❒ RFC 3344
❒ has many features we’ve seen:
❍ home agents, foreign agents, foreign-agent registration, care-of-addresses, encapsulation (packet-within-a-packet)
❒ three components to standard:
❍ indirect routing of datagrams
❍ agent discovery
❍ registration with home agent
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Mobile IP: indirect routing
Permanent address: 128.119.40.186
Care-of address: 79.129.13.2
dest: 128.119.40.186
packet sent by correspondent
dest: 79.129.13.2 dest: 128.119.40.186
packet sent by home agent to foreign agent: a packet within a packet
dest: 128.119.40.186
foreign-agent-to-mobile packet
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Mobile IP: agent discovery
❒ agent advertisement: foreign/home agents advertise service by broadcasting ICMP messages (typefield = 9)
RBHFMGV bits reserved
type = 16
type = 9 code = 0 = 9
checksum
router address
standard ICMP fields
mobility agent advertisement
extension
length sequence #
registration lifetime
0 or more care-of-addresses
0 8 16 24
R bit: registration required
H,F bits: home and/or foreign agent
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Mobile IP: registration example
visited network: 79.129.13/24 home agent
HA: 128.119.40.7 foreign agent
COA: 79.129.13.2
COA: 79.129.13.2 ….
ICMP agent adv. Mobile agent
MA: 128.119.40.186
registration req.
COA: 79.129.13.2 HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 9999 identification:714 ….
registration req.
COA: 79.129.13.2 HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 9999 identification: 714 encapsulation format ….
registration reply
HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 4999 Identification: 714 encapsulation format ….
registration reply
HA: 128.119.40.7 MA: 128.119.40.186 Lifetime: 4999 Identification: 714 ….
time
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Chapter 6 outline
6.1 Introduction
Wireless
❒ 6.2 Wireless links, characteristics
❍ CDMA
❒ 6.3 IEEE 802.11 wireless LANs (“wi-fi”)
❒ 6.4 Cellular Internet Access
❍ architecture
❍ standards (e.g., GSM)
Mobility
❒ 6.5 Principles: addressing and routing to mobile users
❒ 6.6 Mobile IP
❒ 6.7 Handling mobility in cellular networks
❒ 6.8 Mobility and higher-layer protocols
6.9 Summary
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Mobile Switching
Center
Public telephonenetwork, andInternet
Mobile Switching
Center
Components of cellular network architecture
� connects cells to wide area net� manages call setup (more later!)� handles mobility (more later!)
MSC
� covers geographical region� base station (BS) analogous to 802.11 AP� mobile users attach to network through BS� air-interface:physical and link layer protocol between mobile and BS
cell
wired network
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Components of cellular network architecture
correspondent
MSC
MSC
MSC MSC
MSC
wired public telephonenetwork
different cellular networks,operated by different providers
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Handling mobility in cellular networks
❒ home network: network of cellular provider you subscribe to (e.g., Sprint PCS, Verizon)
❍ home location register (HLR): database in home network containing permanent cell phone #, profile information (services, preferences, billing), information about current location (could be in another network)
❒ visited network: network in which mobile currently resides
❍ visitor location register (VLR): database with entry for each user currently in network
❍ could be home network
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Public switched telephonenetwork
mobileuser
homeMobile
Switching Center
HLR home network
visitednetwork
correspondent
Mobile Switching
Center
VLR
GSM: indirect routing to mobile
1 call routed to home network
2
home MSC consults HLR,gets roaming number ofmobile in visited network
3
home MSC sets up 2nd leg of callto MSC in visited network
4
MSC in visited network completescall through base station to mobile
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Mobile Switching
Center
VLR
old BSSnew BSS
old routing
newrouting
GSM: handoff with common MSC
❒ Handoff goal: route call via new base station (without interruption)
❒ reasons for handoff:
❍ stronger signal to/from new BSS (continuing connectivity, less battery drain)
❍ load balance: free up channel in current BSS
❍ GSM doesn’t mandate why to perform handoff (policy), only how (mechanism)
❒ handoff initiated by old BSS
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Mobile Switching
Center
VLR
old BSS
1
3
24
5 6
78
GSM: handoff with common MSC
new BSS
1. old BSS informs MSC of impending handoff, provides list of 1+ new BSSs
2. MSC sets up path (allocates resources) to new BSS
3. new BSS allocates radio channel for use by mobile
4. new BSS signals MSC, old BSS: ready 5. old BSS tells mobile: perform handoff to
new BSS6. mobile, new BSS signal to activate new
channel7. mobile signals via new BSS to MSC:
handoff complete. MSC reroutes call8 MSC-old-BSS resources released
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home network
Home MSC
PSTN
correspondent
MSC
anchor MSC
MSCMSC
(a) before handoff
GSM: handoff between MSCs
❒ anchor MSC: first MSC visited during cal
❍ call remains routed through anchor MSC
❒ new MSCs add on to end of MSC chain as mobile moves to new MSC
❒ IS-41 allows optional path minimization step to shorten multi-MSC chain
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home network
Home MSC
PSTN
correspondent
MSC
anchor MSC
MSCMSC
(b) after handoff
GSM: handoff between MSCs
❒ anchor MSC: first MSC visited during cal
❍ call remains routed through anchor MSC
❒ new MSCs add on to end of MSC chain as mobile moves to new MSC
❒ IS-41 allows optional path minimization step to shorten multi-MSC chain
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Mobility: GSM versus Mobile IP
Care-of-address
Routable address for telephone call segment between home MSC and visited MSC, visible to neither the mobile nor the correspondent.
Mobile Station Roaming Number (MSRN), or “roaming number”
Foreign agentVisited MSC: responsible for setting up calls to/from mobile nodes in cells associated with MSC. VLR: temporary database entry in visited system, containing subscription information for each visiting mobile user
Visited Mobile services Switching Center.Visitor Location Record (VLR)
Visited network
Network other than home system where mobile user is currently residing
Visited System
Home agentHome MSC: point of contact to obtain routable address of mobile user. HLR: database in home system containing permanent phone number, profile information, current location of mobile user, subscription information
Gateway Mobile Switching Center, or “home MSC”. Home Location Register (HLR)
Home network
Network to which mobile user’s permanent phone number belongs
Home system
Mobile IP elementComment on GSM element GSM element
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Wireless, mobility: impact on higher layer protocols
❒ logically, impact should be minimal …
❍ best effort service model remains unchanged
❍ TCP and UDP can (and do) run over wireless, mobile
❒ … but performance-wise:
❍ packet loss/delay due to bit-errors (discarded packets, delays for link-layer retransmissions), and handoff
❍ TCP interprets loss as congestion, will decrease congestion window un-necessarily
❍ delay impairments for real-time traffic
❍ limited bandwidth of wireless links
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Chapter 3Transport Layer
Computer Networking: A Top Down Approach 4th edition. Jim Kurose, Keith RossAddison-Wesley, July 2007.
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 1996-2007J.F Kurose and K.W. Ross, All Rights Reserved
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Chapter 3: Transport Layer
Our goals:
❒ understand principles behind transport layer services:
❍ multiplexing/demultiplexing
❍ reliable data transfer
❍ flow control
❍ congestion control
❒ learn about transport layer protocols in the Internet:
❍ UDP: connectionless transport
❍ TCP: connection-oriented transport
❍ TCP congestion control
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Chapter 3 outline
❒ 3.1 Transport-layer services
❒ 3.2 Multiplexing and demultiplexing
❒ 3.3 Connectionless transport: UDP
❒ 3.4 Principles of reliable data transfer
❒ 3.5 Connection-oriented transport: TCP
❍ segment structure
❍ reliable data transfer
❍ flow control
❍ connection management
❒ 3.6 Principles of congestion control
❒ 3.7 TCP congestion control
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Transport services and protocols
❒ provide logical communicationbetween app processes running on different hosts
❒ transport protocols run in end systems
❍ send side: breaks app messages into segments, passes to network layer
❍ rcv side: reassembles segments into messages, passes to app layer
❒ more than one transport protocol available to apps
❍ Internet: TCP and UDP
applicationtransportnetworkdata linkphysical
applicationtransportnetworkdata linkphysical
logical end-end transport
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Transport vs. network layer
❒ network layer: logical communication between hosts
❒ transport layer: logical communication between processes
❍ relies on, enhances, network layer services
Household analogy:
12 kids sending letters to 12 kids
❒ processes = kids
❒ app messages = letters in envelopes
❒ hosts = houses
❒ transport protocol = Ann and Bill
❒ network-layer protocol = postal service
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Internet transport-layer protocols
❒ reliable, in-order delivery (TCP)
❍ congestion control
❍ flow control
❍ connection setup
❒ unreliable, unordered delivery: UDP
❍ no-frills extension of “best-effort” IP
❒ services not available: ❍ delay guarantees
❍ bandwidth guarantees
applicationtransportnetworkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
networkdata linkphysical
applicationtransportnetworkdata linkphysical
logical end-end transport
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Chapter 3 outline
❒ 3.1 Transport-layer services
❒ 3.2 Multiplexing and demultiplexing
❒ 3.3 Connectionless transport: UDP
❒ 3.4 Principles of reliable data transfer
❒ 3.5 Connection-oriented transport: TCP
❍ segment structure
❍ reliable data transfer
❍ flow control
❍ connection management
❒ 3.6 Principles of congestion control
❒ 3.7 TCP congestion control
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Multiplexing/demultiplexing
application
transport
network
link
physical
P1 application
transport
network
link
physical
application
transport
network
link
physical
P2P3 P4P1
host 1 host 2 host 3
= process= socket
delivering received segmentsto correct socket
Demultiplexing at rcv host:gathering data from multiplesockets, enveloping data with header (later used for demultiplexing)
Multiplexing at send host:
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How demultiplexing works❒ host receives IP datagrams
❍ each datagram has source IP address, destination IP address
❍ each datagram carries 1 transport-layer segment
❍ each segment has source, destination port number
❒ host uses IP addresses & port numbers to direct segment to appropriate socket
source port # dest port #
32 bits
applicationdata
(message)
other header fields
TCP/UDP segment format
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Connectionless demultiplexing
❒ Create sockets with port numbers:
DatagramSocket mySocket1 = new DatagramSocket(12534);
DatagramSocket mySocket2 = new DatagramSocket(12535);
❒ UDP socket identified by two-tuple:
(dest IP address, dest port number)
❒ When host receives UDP segment:
❍ checks destination port number in segment
❍ directs UDP segment to socket with that port number
❒ IP datagrams with different source IP addresses and/or source port numbers directed to same socket
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Connectionless demux (cont)
DatagramSocket serverSocket = new DatagramSocket(6428);
ClientIP:B
P2
clientIP: A
P1P1P3
serverIP: C
SP: 6428
DP: 9157
SP: 9157
DP: 6428
SP: 6428
DP: 5775
SP: 5775
DP: 6428
SP provides “return address”
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Connection-oriented demux
❒ TCP socket identified by 4-tuple:
❍ source IP address
❍ source port number
❍ dest IP address
❍ dest port number
❒ recv host uses all four values to direct segment to appropriate socket
❒ Server host may support many simultaneous TCP sockets:
❍ each socket identified by its own 4-tuple
❒ Web servers have different sockets for each connecting client
❍ non-persistent HTTP will have different socket for each request
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Connection-oriented demux(cont)
ClientIP:B
P1
clientIP: A
P1P2P4
serverIP: C
SP: 9157
DP: 80
SP: 9157
DP: 80
P5 P6 P3
D-IP:C
S-IP: A
D-IP:C
S-IP: B
SP: 5775
DP: 80
D-IP:C
S-IP: B
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Connection-oriented demux: Threaded Web Server
ClientIP:B
P1
clientIP: A
P1P2
serverIP: C
SP: 9157
DP: 80
SP: 9157
DP: 80
P4 P3
D-IP:C
S-IP: A
D-IP:C
S-IP: B
SP: 5775
DP: 80
D-IP:C
S-IP: B
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Chapter 3 outline
❒ 3.1 Transport-layer services
❒ 3.2 Multiplexing and demultiplexing
❒ 3.3 Connectionless transport: UDP
❒ 3.4 Principles of reliable data transfer
❒ 3.5 Connection-oriented transport: TCP
❍ segment structure
❍ reliable data transfer
❍ flow control
❍ connection management
❒ 3.6 Principles of congestion control
❒ 3.7 TCP congestion control
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UDP: User Datagram Protocol [RFC 768]
❒ “no frills,” “bare bones”Internet transport protocol
❒ “best effort” service, UDP segments may be:
❍ lost
❍ delivered out of order to app
❒ connectionless:
❍ no handshaking between UDP sender, receiver
❍ each UDP segment handled independently of others
Why is there a UDP?❒ no connection establishment
(which can add delay)
❒ simple: no connection state at sender, receiver
❒ small segment header
❒ no congestion control: UDP can blast away as fast as desired
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UDP: more
❒ often used for streaming multimedia apps
❍ loss tolerant
❍ rate sensitive
❒ other UDP uses❍ DNS
❍ SNMP
❒ reliable transfer over UDP: add reliability at application layer
❍ application-specific error recovery!
source port # dest port #
32 bits
Applicationdata
(message)
UDP segment format
length checksumLength, in
bytes of UDPsegment,includingheader
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UDP checksum
Sender:❒ treat segment contents as
sequence of 16-bit integers
❒ checksum: addition (1’s complement sum) of segment contents
❒ sender puts checksum value into UDP checksum field
Receiver:❒ compute checksum of received
segment
❒ check if computed checksum equals checksum field value:
❍ NO - error detected
❍ YES - no error detected. But maybe errors nonetheless?
Goal: detect “errors” (e.g., flipped bits) in transmitted segment