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![Services, Layering and Encapsulation
source
Transport-layer protocol transfers M (e.g., reliably) from
one process to another, using services of network layer
H
t
M
▪ network-layer protocol encapsulates
transport-layer segment [Ht | M] with
network layer-layer header Hn to create a
network-layer datagram
• Hn used by network layer protocol to
implement its service
application
transport
network
link
physical
destination
M
application
transport
network
link
physical
M
H
t
H
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Network-layer protocol transfers transport-layer segment
[Ht | M] from one host to another, using link layer services
Introduction: 1-31](https://image.slidesharecdn.com/lecture2networks-250803205808-95fd4e4b/75/lecture2-Networks-pptx-31-2048.jpg)
![Services, Layering and Encapsulation
source
H
t
M
▪ link-layer protocol encapsulates network
datagram [Hn| [Ht |M], with link-layer header
Hl to create a link-layer frame
application
transport
network
link
physical
destination
M
application
transport
network
link
physical
M
H
t
H
n
Link-layer protocol transfers datagram [Hn| [Ht |M] from
host to neighboring host, using network-layer services
M
H
t
H
n
H
l
Network-layer protocol transfers transport-layer segment
[Ht | M] from one host to another, using link layer services
Introduction: 1-32](https://image.slidesharecdn.com/lecture2networks-250803205808-95fd4e4b/75/lecture2-Networks-pptx-32-2048.jpg)
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- 1. Advanced Communication Systems andNetworks ELE 553 Lecture 2 Material : Presentation slides + book sections 1.3.3 , 1.4.1
- 2. Chapter 1: introduction Overview: •Network core: internet structure • Performance: loss, delay, throughput • Protocol layers, service models Introduction: 1-2
- 3. Internet structure: a“network of networks” • hosts connect to Internet via access Internet Service Providers (ISPs) • access ISPs in turn must be interconnected • so that any two hosts (anywhere!) can send packets to each other • resulting network of networks is very complex • evolution driven by economics, national policies rather than by performance considerations The access ISP can provide either wired or wireless connectivity, using an array of access technologies including DSL, cable, FTTH, Wi-Fi, and cellular. Example Tedata , Vodfafone , university , company,….. mobile network home network enterprise network national or global ISP local or regional ISP datacenter network content provider network
- 4. Internet structure: a“network of networks” Question: given millions of access ISPs, how to connect them together? overarching goal is to interconnect the access ISPs so that all end systems can send packets to each other access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … … Introduction: 1-4
- 5. … … … … … Internet structure: a“network of networks” Question: given millions of access ISPs, how to connect them together? access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … … connecting each access ISP to each other directly doesn’t scale: O(N2 ) connections. Introduction: 1-5
- 6. Internet structure: a“network of networks” Option: connect each access ISP to one global transit ISP? Customer and provider ISPs have economic agreement. global ISP access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … … Introduction: 1-6
- 7. ISP A ISP C ISPB Internet structure: a “network of networks” access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … … But if one global ISP is viable business, there will be competitors …. The access ISP is said to be a customer and the global transit ISP is said to be a provider This called a two-tier hierarchy with global transit providers residing at the top tier and access ISPs at the bottom tier Introduction: 1-7
- 8. ISP A ISP C ISPB Internet structure: a “network of networks” access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net access net … … … … … … But if one global ISP is viable business, there will be competitors …. who will want to be connected IXP peering link Internet exchange point IXP Introduction: 1-8
- 9. ISP A ISP C ISPB Internet structure: a “network of networks” access net access net access net access net access net access net access net access net access net access net access net … … … … … … … and regional networks may arise to connect access nets to ISPs IXP IXP access net access net regional ISP access net access net access net Introduction: 1-9
- 10. ISP A ISP C ISPB Internet structure: a “network of networks” access net access net access net access net access net access net access net access net access net access net access net … … … … … … … and content provider networks (e.g., Google, Microsoft, Akamai) may run their own network, to bring services, content close to end users IXP IXP access net access net access net access net access net Content provider network regional ISP Introduction: 1-10
- 11. Internet structure: a“network of networks” multi-tier hierarchy acce ss ISP acce ss ISP acce ss ISP acce ss ISP acce ss ISP acce ss ISP acce ss ISP acce ss ISP At “center”: small # of well-connected large networks ▪ “tier-1” commercial ISPs (e.g., Level 3, Sprint, AT&T, NTT), national & international coverage ▪ content provider networks (e.g., Google, Facebook): private network that connects its data centers to Internet, often bypassing tier-1, regional ISPs Regional ISP Regional ISP Tier 1 ISP Tier 1 ISP IXP Google IXP IXP Introduction: 1-11
- 12. Chapter 1: roadmap Introduction:1-12 • What is the Internet? • What is a protocol? • Network edge: hosts, access network, physical media • Network core: packet/circuit switching, internet structure • Performance: loss, delay, throughput • Security • Protocol layers, service models • History
- 13. How do packetdelay and loss occur? In Packet switching • packets queue in router buffers, waiting for turn for transmission ▪ queue length grows when arrival rate to link (temporarily) exceeds output link capacity ▪packet loss occurs when memory to hold queued packets fills up A B packet being transmitted (transmission delay) packets in buffers (queueing delay) free (available) buffers: arriving packets dropped (loss) if no free buffers Introduction: 1-13
- 14. Packet delay: foursources dproc: nodal processing ▪check bit errors ▪determine output link ▪typically < microsecs dqueue: queueing delay ▪time waiting at output link for transmission ▪depends on congestion level of router propagation nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop A B transmission Introduction: 1-14
- 15. Packet delay: foursources propagation nodal processing queueing dnodal = dproc + dqueue + dtrans + dprop A B transmission dtrans: transmission delay: ▪L: packet length (bits) ▪R: link transmission rate (bps)BW ▪dtrans = L/R dprop: propagation delay: ▪d: length of physical link ▪s: propagation speed (~2x108 m/sec) ▪dprop = d/s dtrans and dprop very different Introduction: 1-15
- 16. Nodel delay atrouter A dnodal = dproc + dqueue + dtrans + dprop • D proc • D queue • D trans=L/R R is BW of link between A &B • D prop= d/s d propagation speed s link length
- 17. Caravan analogy ▪car ~bit; caravan ~ packet; toll service ~ link transmission ▪toll booth takes 12 sec to service car (bit transmission time) ▪“propagate” at 100 km/hr ▪Q: How long until caravan is lined up before 2nd toll booth? ▪time to “push” entire caravan through toll booth onto highway = 12*10 = 120 sec ▪time for last car to propagate from 1st to 2nd toll both: 100km/(100km/hr) = 1 hr ▪A: 62 minutes toll booth toll booth (aka link) ten-car caravan (aka 10-bit packet) 100 km 100 km toll booth toll booth (aka link) toll booth Introduction: 1-17
- 18. “Real” Internet delaysand routes ▪what do “real” Internet delay & loss look like? ▪traceroute program: provides delay measurement from source to router along end-end Internet path towards destination. For all i: 3 probes 3 probes 3 probes • sends three packets that will reach router i on path towards destination (with time-to-live field value of i) • router i will return packets to sender • sender measures time interval between transmission and reply Introduction: 1-18
- 19. Real Internet delaysand routes 1 cs-gw (128.119.240.254) 1 ms 1 ms 2 ms 2 border1-rt-fa5-1-0.gw.umass.edu (128.119.3.145) 1 ms 1 ms 2 ms 3 cht-vbns.gw.umass.edu (128.119.3.130) 6 ms 5 ms 5 ms 4 jn1-at1-0-0-19.wor.vbns.net (204.147.132.129) 16 ms 11 ms 13 ms 5 jn1-so7-0-0-0.wae.vbns.net (204.147.136.136) 21 ms 18 ms 18 ms 6 abilene-vbns.abilene.ucaid.edu (198.32.11.9) 22 ms 18 ms 22 ms 7 nycm-wash.abilene.ucaid.edu (198.32.8.46) 22 ms 22 ms 22 ms 8 62.40.103.253 (62.40.103.253) 104 ms 109 ms 106 ms 9 de2-1.de1.de.geant.net (62.40.96.129) 109 ms 102 ms 104 ms 10 de.fr1.fr.geant.net (62.40.96.50) 113 ms 121 ms 114 ms 11 renater-gw.fr1.fr.geant.net (62.40.103.54) 112 ms 114 ms 112 ms 12 nio-n2.cssi.renater.fr (193.51.206.13) 111 ms 114 ms 116 ms 13 nice.cssi.renater.fr (195.220.98.102) 123 ms 125 ms 124 ms 14 r3t2-nice.cssi.renater.fr (195.220.98.110) 126 ms 126 ms 124 ms 15 eurecom-valbonne.r3t2.ft.net (193.48.50.54) 135 ms 128 ms 133 ms 16 194.214.211.25 (194.214.211.25) 126 ms 128 ms 126 ms 17 * * * 18 * * * 19 fantasia.eurecom.fr (193.55.113.142) 132 ms 128 ms 136 ms traceroute: gaia.cs.umass.edu to www.eurecom.fr * Do some traceroutes from exotic countries at www.traceroute.org * means no response (probe lost, router not replying) 3 delay measurements from gaia.cs.umass.edu to cs-gw.cs.umass.edu 3 delay measurements to border1-rt-fa5-1-0.gw.umass.edu looks like delays decrease! Why? trans-oceanic link Introduction: 1-19
- 20. Packet loss ▪queue (akabuffer) preceding link in buffer has finite capacity A B packet being transmitted buffer (waiting area) * Check out the Java applet for an interactive animation (on publisher’s website) of queuing and loss packet arriving to full buffer is lost ▪packet arriving to full queue dropped (aka lost) ▪lost packet may be retransmitted by previous node, by source end system, or not at all Introduction: 1-20
- 21. Throughput ▪throughput: rate (bits/timeunit) at which bits are being sent from sender to receiver • instantaneous: rate at given point in time • average: rate over longer period of time server, with file of F bits to send to client link capacity Rs bits/sec link capacity Rc bits/sec server sends bits (fluid) into pipe pipe that can carry fluid at rate (Rs bits/sec) pipe that can carry fluid at rate (Rc bits/sec) Introduction: 1-21
- 22. Throughput Rs < RcWhat is average end-end throughput? Rs bits/sec Rc bits/sec Rs > Rc What is average end-end throughput? link on end-end path that constrains end-end throughput bottleneck link Rs bits/sec Rc bits/sec Introduction: 1-22
- 23. Throughput: network scenario 10connections (fairly) share backbone bottleneck link Rbits/sec Rs Rs Rs Rc Rc Rc R ▪ per-connection end-end throughput: min(Rc,Rs,R/10) ▪ in practice: Rc or Rs is often bottleneck * Check out the online interactive exercises for more examples: http://gaia.cs.umass.edu/kurose_ross/ Introduction: 1-23
- 24. Chapter 1: roadmap Introduction:1-24 • What is the Internet? • What is a protocol? • Network edge: hosts, access network, physical media • Network core: packet/circuit switching, internet structure • Performance: loss, delay, throughput • Security • Protocol layers, service models • History
- 25. Protocol “layers” andreference models Networks are complex, with many “pieces”: ▪hosts ▪routers ▪links of various media ▪applications ▪protocols ▪hardware, software Question: is there any hope of organizing structure of network? ▪and/or our discussion of networks? Introduction: 1-25
- 26. Example: organization ofair travel ▪ a series of steps, involving many services ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing airplane routing airplane routing How would you define/discuss the system of airline travel? end-to-end transfer of person plus baggage Introduction: 1-26
- 27. Example: organization ofair travel ticket (purchase) baggage (check) gates (load) runway takeoff airplane routing ticket (complain) baggage (claim) gates (unload) runway landing airplane routing airplane routing ticketing service baggage service gate service runway service routing service layers: each layer implements a service ▪via its own internal-layer actions ▪relying on services provided by layer below Introduction: 1-27
- 28. Why layering? Approach todesigning/discussing complex systems: ▪ explicit structure allows identification, relationship of system’s pieces • layered reference model for discussion ▪ modularization eases maintenance, updating of system • change in layer's service implementation: transparent to rest of system • e.g., change in gate procedure doesn’t affect rest of system Introduction: 1-28
- 29. Layered Internet protocolstack ▪application: supporting network applications • HTTP, IMAP, SMTP, DNS ▪transport: process-process data transfer • TCP, UDP ▪network: routing of datagrams from source to destination • IP, routing protocols ▪link: data transfer between neighboring network elements • Ethernet, 802.11 (WiFi), PPP ▪physical: bits “on the wire” link application network transport physical application transport network link physical Introduction: 1-29
- 30. Services, Layering andEncapsulation source ▪ transport-layer protocol encapsulates application-layer message, M, with transport layer-layer header Ht to create a transport-layer segment • Ht used by transport layer protocol to implement its service application transport network link physical destination application transport network link physical Transport-layer protocol transfers M (e.g., reliably) from one process to another, using services of network layer H t M Application exchanges messages to implement some application service using services of transport layer M Introduction: 1-30
- 31. Services, Layering andEncapsulation source Transport-layer protocol transfers M (e.g., reliably) from one process to another, using services of network layer H t M ▪ network-layer protocol encapsulates transport-layer segment [Ht | M] with network layer-layer header Hn to create a network-layer datagram • Hn used by network layer protocol to implement its service application transport network link physical destination M application transport network link physical M H t H n Network-layer protocol transfers transport-layer segment [Ht | M] from one host to another, using link layer services Introduction: 1-31
- 32. Services, Layering andEncapsulation source H t M ▪ link-layer protocol encapsulates network datagram [Hn| [Ht |M], with link-layer header Hl to create a link-layer frame application transport network link physical destination M application transport network link physical M H t H n Link-layer protocol transfers datagram [Hn| [Ht |M] from host to neighboring host, using network-layer services M H t H n H l Network-layer protocol transfers transport-layer segment [Ht | M] from one host to another, using link layer services Introduction: 1-32
- 33. Services, Layering andEncapsulation source application transport network link physical destination application transport network link physical H t M M M H t H n M H t H n H l M H t H n H t M M message segment datagram frame M H t H n H l Introduction: 1-33
- 34. network link physical application transport network link physical application transport network link physical Encapsulation: an end-end view source H t H n M segment H t datagram destinatio n H t H n H l M H t H n M H t M M H t H n H l M H t H n M H t H n M H t H n H l M router switch messageM H t M H n frame link physical Introduction: 1-34
- 35. Wireshark Introduction: 1-35 Transport (TCP/UDP) Network(IP) Link (Ethernet) Physical application (www browser, email client) application OS packet capture (pcap) packet analyzer copy of all Ethernet frames sent/received
- 36. Assignment 1-Wireshark Lab1 -GettingStarted 2-Assignment 1
Editor's Notes
- #7 In reality,, in any given region, there may be a regional ISP to which the access ISPs in the region connect. Each regional ISP then connects to tier-1 ISPs. tier-1 ISPs, which actually do exist, do not have a presence in every city in the world. There are approximately a dozen tier-1 ISPs. not only are there multiple competing tier-1 ISPs, there may be multiple competing regional ISPs in a region. each access ISP pays the regional ISP to which it connects, and each regional ISP pays the tier-1 ISP to which it connects. (An access ISP can also connect directly to a tier-1 ISP, in which case it pays the tier-1 ISP). Note that the tier-1 ISPs do not pay anyone as they are at the top of the hierarchy.
- #8 today’s Internet we must add points of presence (PoPs), multi-homing, peering, and Internet exchange points (IXPs) PoP is simply a group of one or more routers (at the same location) in the provider’s network where customer ISPs can connect into the provider ISP. For a customer network to connect to a provider’s PoP, it can lease a high-speed link from a third-party telecommunications provider to directly connect one of its routers to a router at the PoP. Any ISP (except for tier-1 ISPs) may choose to multi-home, that is, to connect to two or more provider ISPs. So, for example, an access ISP may multi-home with two regional ISPs, or it may multihome with two regional ISPs and also with a tier-1 ISP. When an ISP multi-homes, it can continue to send and receive packets into the Internet even if one of its providers has a failure. The amount that a customer ISP pays a provider ISP reflects the amount of traffic it exchanges with the provider. To reduce these costs, a pair of nearby ISPs at the same level of the hierarchy can peer, that is, they can directly connect their networks together so that all the traffic between them passes over the direct connection rather than through upstream intermediaries. When two ISPs peer, it is typically settlement-free, that is, neither ISP pays the other. As noted earlier, tier-1 ISPs also peer with one another, settlement-free. a third-party company can create an Internet Exchange Point (IXP), which is a meeting point where multiple ISPs can peer together. An IXP is typically in a stand-alone building with its own switches [Ager 2012]. There are over 600 IXPs in the Internet today [PeeringDB 2020].
- #10 by adding content-provider networks. Google is currently one of the leading examples of such a content-provider network. As of this writing, it Google has 19 major data centers distributed across . Additionally, Google has smaller data centers, each with a few hundred servers; these smaller data centers are often located within IXPs. The Google data centers are all interconnected via Google’s private TCP/IP network, however, the Google private network only carries traffic to/from Google servers. content provider not only reduces its payments to upper-tier ISPs, but also has greater control of how its services are ultimately delivered to end users
- #11 today’s Internet—a network of networks—is complex, consisting of a dozen or so tier-1 ISPs and hundreds of thousands of lower-tier ISPs. The ISPs are diverse in their coverage, with some spanning multiple continents and oceans, and others limited to narrow geographic regions. The lower-tier ISPs connect to the higher-tier ISPs, and the higher-tier ISPs interconnect with one another. Users and content providers are customers of lower-tier ISPs, and lower-tier ISPs are customers of higher-tier ISPs
- #13 Instead, computer networks necessarily constrain throughput (the amount of data per second that can be transferred) between end systems, introduce delays between end systems, nodal delay at router A is the time between the arrival of a packet at a node and its arrival at the next node
- #16 Processing Delay The time required to examine the packet’s header and determine where to direct the packet is part of the processing delay. The processing delay can also include other factors, such as the time needed to check for bit-level errors in the packet that occurred in transmitting the packet’s bits from the upstream node to router A. Processing delays in high-speed routers are typically on the order of microseconds or less. After this nodal processing, the router directs the packet to the queue that precedes the link to router B. Queuing Delay The length of the queuing delay of a specific packet will depend on the number of earlier-arriving packets that are queued and waiting for transmission onto the link. If the queue is empty and no other packet is currently being transmitted, then our packet’s queuing delay will be zero Transmission Delay packet by L bits, and denote the transmission rate of the link from router A to router B by R bits/sec. For example, for a 10 Mbps Ethernet link, the rate is R = 10 Mbps; for a 100 Mbps Ethernet link, the rate is R = 100 Mbps. The transmission delay is L/R. This is the amount of time required to push (that is, transmit) all of the packet’s bits into the link. Transmission delays are typically on the order of microseconds to milliseconds in practice. Propagation Delay Once a bit is pushed into the link, it needs to propagate to router B. The time required for one bit (last bit) to propagate from the beginning of the link to router B is the propagation delay The propagation speed depends on the physical medium of the link (that is, fiber optics, twisted-pair copper wire, and so on) and is in the range of 2x108 meters/sec to 3x108 meters/sec. The propagation delay is the distance between two routers divided by the propagation speed. That is, the propagation delay is d/s, where d is the distance between router A and router B and s is the propagation speed of the link The contribution of these delay components can vary significantly. For example, dprop can be negligible (for example, a couple of microseconds) for a link connecting two routers on the same university campus; however, dprop is hundreds of milliseconds for two routers interconnected by a geostationary satellite link,
- #21 the number of bits per second actually transmitted through a network is the network througput the number seconds used for the measurement is significant: if the measurement is taken over a very short time interval, we are measuring the instantaneous throughput if the measurement is taken over a long time interval, for example, the transfer of a whole group of files, we are measuring the average throughput average throughput is usually more consistent than instantaneous throughput