CS 3516: Advanced Computer Networks

Transcription

1 Welcome to CS 3516: Advanced Computer Networks Prof. Yanhua Li Time: 9:00am 9:50am M, T, R, and F Location: Fuller 320 Fall 2016 A-term 1 Some slides are originally from the course materials of the textbook Computer Networking: A Top Down Approach, 6th edition, by Jim Kurose, Keith Ross, Addison-Wesley March Copyright J.F Kurose and K.W. Ross, All Rights Reserved.

2 Final Review v (1) CRC v (2) DHCP v (3) CSMA/CD (to be introduced on Monday 10/10) v (4) LS and DV routing v (5) IPv4 classful addressing v (6) Inter-AS routing and BGP v (1) 7 questions v (2) 32 points v (3) Q1-4: Small v (4) Q5-7: bigger questions v TA office hour this afternoon 1-3pm v My office hour 10-10:45AM Link Layer 5-2

3 DHCP client-server scenario / DHCP server arriving DHCP client needs address in this network / /24 Network Layer 4-3

4 DHCP client-server scenario DHCP server: DHCP discover src : , 68 dest.: ,67 yiaddr: transaction ID: 654 Broadcast: is there a DHCP server out there? arriving client DHCP request DHCP offer src: , 68 dest:: , 67 yiaddrr: transaction that IP address! ID: 655 lifetime: 3600 secs Broadcast: OK. I ll take src: , 67 dest: , 68 yiaddrr: transaction ID: 654 lifetime: 3600 secs Broadcast: I m a DHCP server! Here s an IP address you can use DHCP ACK src: , 67 dest: , 68 yiaddrr: transaction ID: 655 lifetime: 3600 secs Broadcast: OK. You ve got that IP address! Network Layer 4-4

5 CSMA/CD (collision detection) spatial layout of nodes B D Link Layer 5-5

6 Ethernet CSMA/CD algorithm 1. NIC receives datagram from network layer, creates frame 2. If NIC senses channel idle, starts frame transmission. If NIC senses channel busy, waits until channel idle, then transmits. 3. If NIC transmits entire frame without detecting an transmission, NIC is done with frame! 4. If NIC detects an transmission while transmitting, aborts and sends jam signal 5. After aborting, NIC enters binary (exponential) backoff: after mth collision, NIC chooses K at random from {0,1,2,, 2 m -1}. NIC waits K 512 bit times, returns to Step 2 longer backoff interval with more collisions Link Layer 5-6

7 Interconnected ASes 3c 3a 3b AS3 1a 1c 1d 1b Intra-AS Routing algorithm AS1 Forwarding table Inter-AS Routing algorithm 2a 2c 2b AS2 v forwarding table configured by both intraand inter-as routing algorithm intra-as sets entries for internal dests inter-as & intra-as sets entries for external dests Network Layer 4-7

8 Inter-AS tasks v suppose router in AS1 receives datagram destined outside of AS1: router should forward packet to gateway router, but which one? 3b 3c AS3 3a 1a AS1 1c 1d x 1b AS1 must: 1. learn which dests are reachable through AS2, which through AS3 2. propagate this reachability info to all routers in AS1 job of inter-as routing! 2a 2c AS2 2b Network Layer 4-8

9 BGP basics: distributing path information v 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 1bto-2a ebgp session v when router learns of new prefix, it creates entry for prefix in its forwarding table. x 3b 3a AS3 1a AS1 1c 1d ebgp session ibgp session 1b 2a 2c AS2 2b Network Layer 4-9

10 Example: setting forwarding table in router 1d v suppose AS1 learns (via inter-as protocol) that subnet x reachable via AS3 (gateway 1c), but not via AS2 inter-as protocol propagates reachability info to all internal routers v router 1d determines from intra-as routing info that its interface e is on the least cost path to 1c installs forwarding table entry (x,e) 3c 3a 3b AS3 1a AS1 1c 1d x 1b 2a 2c AS2 2b Network Layer 4-10

11 Example: choosing among multiple ASes v now suppose AS1 learns from inter-as protocol that subnet x is reachable from AS3 and from AS2. v to configure forwarding table, router 1d must determine which gateway it should forward packets towards for dest x this is also job of inter-as routing protocol! hot potato routing: send packet towards closest of two routers. 3b 3c AS3 3a 1a AS1 1c x 1d? 1b 2a 2c AS2 2b Network Layer 4-11

12 Path attributes and BGP routes v advertised prefix includes BGP attributes prefix + attributes = route v two important attributes: AS-PATH: contains ASs through which prefix advertisement has passed: e.g., AS 67, AS 17; AS numbers are maintained by ICANN; NEXT-HOP: indicates specific internal-as router to nexthop AS. (may be multiple links from current AS to nexthop-as) Network Layer 4-12

13 Questions? Network Layer 4-13