Chapter 19. Protocol architecture. Summary. Key ideas. Copyright 1998, David G. Messerschmitt. All rights reserved. by David G.

Size: px

Start display at page:

Philip McDowell
5 years ago
Views:

1 Chapter 19 Protocol architecture Summary Network architecture Internet protocols Some network security issues 3 Key ideas Packet encapsulation: one packet can be encapsulated in another Packet fragmentation: one packet can be split into two or more packets and encapsulated Protocol layering: one service can be layered on another fragmentation and encapsulation 4 New header Original packet Packet encapsulated in another packet Header Payload Original packet Original packet (including its header) is payload of new packet Encapsulated again Encapsulated packet Divide into packet fragments New packets Original packet Reassembled original packet 5 6 1

2 Layer n+1 Layer n+1 Peer-to-peer communication Layer n Layer n protocol header Layer n+1 protocol header 7 Layer n Encapsulated layer n+1 packet, including header Layered protocol modularity Layer n+1 protocol is logically peer-to-peer Layer n+1 protocol depends on services of layer n Layer n+1 packets are encapsulated in layer n packets Layer n+1 never sees layer n packet headers Layer n ignores layer n+1 packet headers (part of payload) 8 Key ideas (again) IETF/OMG Layers Packet encapsulation: one packet can be encapsulated in another Packet fragmentation: one packet can be split into two or more packets and encapsulated Protocol layering: one service can be layered on another fragmentation and encapsulation 9 Layering Physical Logical Application Application ORB ORB IIOP IIOP TCP or UDP TCP or UDP IP IP IP Network 1 N 1 N 2 Network 2 A Switch B 10 Internet: logical view Internet: physical view Application TCP or UDP A Peer-to-peer 11 Application TCP or UDP B The constituent networks are not visible to the application; TCP/UDP does not reside in packet switches IP IP IP Network 1 N 1 N 2 Network 2 A Switch B IP serves to connect heterogeneous networks into an internetwork 12 2

3 What IP does do Allow packets to traverse multiple networks Deliver packet to specified destination host Best effort: deliver as reliably and as soon as it can What IP Doesn t Do Guarantee latency for packets that are delivered Guarantee delivery, or notify source host if packet is not delivered Guarantee order of delivery Guarantee integrity of packet payload Maintain conversational context (each packet is independent) Specify what process that should receive the packet at destination host IP header Version Priority FlowLabel PayloadLen NextHeader HopLimit SourceAddress DestinationAddress Transport services: UDP and TCP Direct packet to a particular process UDP adds: Payload integrity for packets delivered TCP adds: Reliable delivery of testream session 32 bits Comparison of services IP: host-tohost -toprocess IP: Best-effort datagram TCP UDP UDP: best-effort datagram with payload integrity te te te te te te TCP: reliable bi-directional testream 17 SourcePort UDP/TCP ports (publish/subscribe) Port IP: host-tohost DestinationPort (rest of UDP or TCP header) 32 bits 18 Port Encapsulated in IP packet 3

4 HTTP Service Client can make requests GET (pull) POST (push) (some others) Server responds HTTP headers HTML document or JPEG, or GIF, or 19 URL Structure <scheme>://<host>:<port>/<path> Scheme HTTP, FTP, GOPHER, MAILTO,... An IP address or D name Port TCP port number Optional (defaults to 80 for http) 20 HTTP example When a browser fetches says to use HTTP protocol Resolve in D Make TCP connection , port 80 Send the following text string GET /~presnick/ 21 Server sends back HTTP/ OK Date: Mon, 22 Dec :12:32 GMT Server: Apache/1.2.4 Last-Modified: Thu, 04 Dec :26:10 GMT ETag: "5f2f2-33fd-3486d9a2" Content-Length: Accept-Ranges: tes Connection: close Content-Type: text/html <HTML>. 22 HTML <H1> Paul Resnick</H1> <IMG SRC="RESNICK.gif" ALT="[PHOTO]" HSPACE=10 ALIGN=LEFT> <BR>Associate Professor <BR>University of Michigan <BR>School of Information <BR>314 West Hall <BR>550 East University Avenue <BR>Ann Arbor, MI <BR>presnick@umich.edu 23 What Browsers Send to Servers Your IP address The browser type The refer link What URL you last looked at Cookies (persistent client state for a URL) Server response can include a set-cookie header Browser saves the cookie Browser resends to server next time 24 4

5 Aggregating te stream Original tes are aggregated and. te te te te te te How TCP works.encapsulated in TCP packets, with a sequence number included in the TCP header te te te te te te te te te te The TCP packets are encapsulated in IP packets 26 TCP connections TCP establishes a session with ordered and bidirectional reliable delivery of tes Establishment: Inform receiving port of connection Initialize packet sequence number Congestion and flow control state Disestablishment By either peer Free state and resources 27 HTTP uses TCP Often have to request another page e.g., image HTTP/1.0 requires a new TCP session for each Overhead of session establishment HTTP/1.1 permits reuse of one TCP session for multiple requests 28 Reliable packet delivery: acknowledgement and resending Source Destination Source Destination Source Destination IP is used to send TCP packets and return Timeout Source Destination IP loses packets Task Concurrent tasks for higher throughput Packets can be reordered using sequence number

6 TCP Congestion TCP Congestion Control If link is congested Switch queue for that link fills up Drops packets Source resends non- ed packets Makes congestion worse 31 Voluntary source-imposed policy Source controls the number of non- ed packets that have been sent Controls the number of concurrent sends, and hence packet throughput Slow start, slowly increase rate Monitor non- s and delay of s to estimate congestion Quickly decrease if congestion detected 32 TCP congestion control flaws Fairness criterion Maybe equal division of resources is not what is wanted Estimating congestion retransmission is flawed for wireless links Depends on accurate implementation -- cheating possible Application can avoid congestion control using UDP 33 TCP Flow Control Recall that this is to avoid recipient from being overwhelmed Recipient must control source Recipient explicitly requests lower send rate MaxUnackedPackets is a parameter of s 34 Multicasting So far, we ve assumed node A sends to B Multicasting: node A sends same message to B, C, and D Could set up A-B, A-C, and A-D connections But A becomes a bottleneck Handling S and resends And it s inefficient Some intermediate nodes may receive the message several times Multicast protocols try to have A send only once Intermediate nodes do more work B A C D Domain Name System 35 6

7 root eecs.berkeley.edu s Delegate name search to local name server tj.watson.ibm.com berkeley.edu root info.sims.berkeley.edu berkeley.edu sims.berkeley.edu watson.ibm.com sims.berkeley.edu sims.berkeley.edu Local server caches recent search results berkeley.edu Supplements Network security Network security Some things to worry about: Sniffing Spoofing Security flaws in public servers 41 Improving security Security tools covered earlier Firewalls: a place where security policies can be enforced Who gains access What servers (ports) can be accessed What hosts can be accessed What protocols can pass Other security policies can be enforced 42 7

8 Public hosts Internal hosts Protected enclave Firewall 43 Global Internet Bastion hosts Second firewall Problems with firewalls Benign internal users assumption is naïve Obstacle to deployment of innovative applications and services Increasingly organizations want to extend extranet to suppliers and customers Solution: resource-based rather than enclave-based security Analogy: border patrol not enough, need secure buildings and vehicles, guards, police, etc. 44 Where to use encryption Packet structure Per link (wireless) Firewall-to-firewall (extranet) -to-host (IPsec) -to-process (TCP-SSL) Application 45 What are some strengths and weaknesses in these approaches? Header: Information for switches Serves as protocol message Packet length limited network policy Payload: Data for application Ignored network and protocol (Qualification: may also be encapsulated packet) 46 Protocol endpoints IP: host-tohost -toprocess Internetworking layer focuses on getting datagrams from one host to another 47 TCP UDP Transport layer focuses on process-to-process communication services Addresses vs. names 128 bits (Network,host) info.sims.berkeley.edu 48 Address specifies topological location of host to the network Name is easy to remember or construct and reflects administrative boundaries 8

9 Issues in congestion control Social issue: how do we divide limited network resources among users/applications? Approaches: Voluntary (e.g. UDP) Bad citizen is rewarded Policy driven (e.g. TCP) Incentivized (e.g. pricing) 49 Advantages of pricing-based congestion control Policies can never take into account the importance of traffic Users and applications are forced to consider the common resource implications of their actions Users and applications can choose the most important traffic for periods of congestion Shift other traffic to off-peak times Source of revenue to expand capacity 50 Technical approaches to congestion control Source-driven throttle algorithm Voluntary, policy, or incentive driven Network-driven Must use fairness criteria Network-to-source flow control Network access enforcement (policing) Traffic priorities allow source to control what traffic is discarded 51 Undesirability of fixed pricing per unit of capacity p Willingness to pay for one more unit of capacity D(c) c In reducing capacity from c to c Revenue gained Revenue lost c c max 52 Capacity Downsides of pricing Infrastructure for Usage monitoring Congestion monitoring QoS configuration Billing Operational costs How do costs compare to the benefits? 53 9

QoS attributes of a packet. Quality of service (QoS) Packet latency affects transport service QoS. Transport services.

QoS attributes of a packet. Quality of service (QoS) Packet latency affects transport service QoS. Transport services. University of California at Berkeley School of Information Management and Systems Information Systems 206 Distributed Computing Applications and Infrastructure Quality of service (QoS) QoS attributes of