A real layered system End system Application web
A real layered system End system Application (web browser) HTML on HTTP Application (web server) HTML on HTTP TCP User-space Socket layer (socket interface) IP Ethernet (Card and NIC driver) TCP IP Kernel-space (operating-system) Physical connector Ethernet (Card and NIC driver) Physical connector NIC: Network Interface Card
A real layered system End system Application (web browser) HTML on HTTP Application (web server) HTML on HTTP TCP IP IP Ethernet (Card and NIC driver) Physical connector Ethernet (data link-layer) Physical connector NIC: Network Interface Card Physical connector
A real layered system End system Application (web browser) HTML on HTTP Application (web server) HTML on HTTP Transport TCP (4) Network IP (3) Ethernet (Card and NIC driver) Physical connector Ethernet (data link-layer) Data-Link (2) Physical connector Physical (1) Physical connector Physical(1) NIC: Network Interface Card Physical connector
Line Coding Examples where Baud=bit-rate Non-Return-to-Zero (NRZ) 0 1 0 1 1 0 1 Non-Return-to-Zero-Mark (NRZM) 1 = transition 0 = no transition 0 1 1 0 0 Non-Return-to-Zero Inverted (NRZI) (note transitions on the 1) 0 1 1 0 0
Line Coding Examples - II Non-Return-to-Zero (NRZ) (Baud = bit-rate) 0 1 0 0 1 1 1 Clock Manchester example (Baud = 2 x bit-rate) 0 1 0 0 Clock Quad-level code (2 x Baud = bit-rate) 0 1 0 0
Line Coding Examples - III Data to send 0 1 0 0 1 1 Line-(Wire) representation 0 Name 0 1 2 3 4 5 6 7 8 9 A B C D E F 1 4 b 0000 0001 0010 0011 0100 0101 0110 0111 1000 1001 1010 1011 1100 1101 1110 1111 5 b 11110 01001 10100 101010 01011 01110 01111 10010 10011 10110 10111 11010 11011 11100 11101 0 Description hex data 0 hex data 1 hex data 2 hex data 3 hex data 4 hex data 5 hex data 6 hex data 7 hex data 8 hex data 9 hex data A hex data B hex data C hex data D hex data E hex data F Name Q I J K T R H 4 b -NONE-NONE-NONE-NONE- 5 b 00000 11111 110001 01101 00111 00100 Description Quiet Idle SSD #1 SSD #2 ESD #1 ESD #2 Halt Block coding transfers data with a fixed overhead: 20% less information per Baud in the case of 4 B/5 B So to send data at 100 Mbps; the line rate (the Baud rate) must be 125 Mbps. 1 Gbps uses an 8 b/10 b codec; encoding entire bytes at a time but with 25% overhead
Line Coding Examples - IV Scrambling Sequence Message XOR Sequence Scrambling Sequence Communications Channel Message XOR Sequence
Line Coding Examples - IV Scrambling Sequence Message XOR Sequence Scrambling Sequence Communications Channel Message XOR Sequence e. g. (Self-synchronizing) scrambler δ δ δ
Line Coding Examples – V (Hybrid) … 100111101101010001011001110100010110101001001110100… Inserted bits marking “start of frame/block/sequence” Scramble / Transmit / Unscramble δ δ δ δ δ … 0100010110011101000101101010010011101001001011101111000… Identify (and remove) “start of frame/block/sequence” This gives you the Byte-delineations for free 64 b/66 b combines a scrambler and a framer. The start of frame is a pair of bits 01 or 10: 01 means “this frame is data” 10 means “this frame contains data and control” – control could be configuration information, length of encoded data or simply “this line is idle” (no data at all)
Single Flow ARQ in operation Only W packets may be outstanding Rule for adjusting W – If an ACK is received: W ← W+1/W – If a packet is lost: W ← W/2
Application (7) Presentation (6) Session (5) Transport (4) Network (3) Data-Link (2) Physical (1)
Physical layer interconnect (a connector to the rest of us , in this case it connects male connector to male connector) Application (7) Presentation (6) Session (5) Transport (4) Network (3) Data-Link (2) Physical (1)
Physical device – media convertor speaks fluent Physical-layer But only a bit of the Data-Link (Ethernet) Application (7) Presentation (6) Session (5) Transport (4) Network (3) Data-Link (2) Physical (1) Physical (1)
Data-Link device aka an Ethernet switch speaks fluent Ethernet and often the extra/new Ethernet-related standards (802. 1 q for VLAN and 802. 1 x for Network Access Control) Application (7) Presentation (6) Session (5) Transport (4) Network (3) Data-Link (2) Physical (1)
Data-Link device aka an Ethernet switch speaks fluent Ethernet (two standards: wire and wireless!) Another example with media conversion Application (7) Presentation (6) Session (5) Transport (4) Network (3) Data-Link (2) Physical (1)
Network device aka an IP Router speaks fluent IP and can convert between any data-link and any physical standard to any other. Application (7) Presentation (6) Session (5) Transport (4) Network (3) Data-Link (2) Physical (1)
Transport-layer mapping (pretty uncommon) speaks fluent TCP and ? ? ? ? Application (7) Presentation (6) Session (5) Transport (4) Network (3) Data-Link (2) Physical (1)
Application device aka a Web-load balancer speaks fluent HTTP Application (7) Presentation (6) Session (5) Transport (4) Network (3) Data-Link (2) Physical (1)
ARP Recall from the primary slide-stack (slide 8 -22) we need a mechanism for higher-layer entities to know what address to utilize in the lower entities. ARP (address resolution protocol) provides a mechanism for establishing a host’s link-layer (e. g. Ethernet) address using only a network-address (e. g. IP address) ARP is not limited to either Ethernet or IP; although that is the most common current use. A linux machine’s ARP table can be shown using the command arp -a
Consider the following situation: Application (7) Presentation (6) Session (5) Transport (4) Network (3) Data-Link (2) Physical (1)
An Ethernet (switch) example ARP D ff. ff D fe. 34. 56. 32. d 5. 29 DSP 128. 187. 174. 10 128. 187. 171. 2 SP 128. 187. 174. 10 DSH 44. fe. 34. 56. 32. d 5. 29 SH 44. fe. 34. 56. 32. d 5 S DP 128. 187. 171. 2 128. 187. 174. 10 DP 128. 187. 171. 2 S DH fe. 34. 56. 32. d 5. 29 0. 0 DH fe. 34. 56. 32. d 5. 29 H 1 H 2 D ff. ff SP 128. 187. 171. 2 SH fe. 34. 56. 32. d 5. 29 DP 128. 187. 174. 10 DH 0. 0 H 3 H 7 H 8 H 9 Switch H 4 H 5 H 6 D ff. ff SP 128. 187. 171. 2 SH fe. 34. 56. 32. d 5. 29 DP 128. 187. 174. 10 DH 0. 0 H 11 H 12 D ff. ff D fe. 34. 56. 32. d 5. 29 D 128. 187. 174. 10 SP 128. 187. 171. 2 SP 128. 187. 174. 10 D 44. fe. 34. 56. 32. d 5 SH fe. 34. 56. 32. d 5. 29 SH 44. fe. 34. 56. 32. d 5 S 128. 187. 171. 2 DP 128. 187. 174. 10 DP 128. 187. 171. 2 S fe. 34. 56. 32. d 5. 29 DH 0. 0 DH fe. 34. 56. 32. d 5. 29 H 10= IP 128. 187. 174. 10, Ethernet 44. fe. 34. 56. 32. d 5
Notes • ARP table entries timeout in about 10 minutes ARP table rules: • update table with source when you are the target • update table if you already have an entry • do not refresh table entries upon reference
CIDR and Longest Prefix Matches v v The IP address space is broken into line segments. Each line segment is described by a prefix. A prefix is of the form x/y where x indicates the prefix of all addresses in the line segment, and y indicates the length of the segment. e. g. The prefix 128. 9/16 represents the line segment containing addresses in the range: 128. 9. 0. 0 … 128. 9. 255. 128. 9. 0. 0 65/8 0 128. 9. 16. 14 142. 12/19 128. 9/16 232 -1
Classless Interdomain Routing (CIDR) 128. 9. 19/24 128. 9. 25/24 128. 9. 16/20 128. 9. 176/20 128. 9/16 0 232 -1 128. 9. 16. 14 Most specific route = “longest matching prefix”
Topology of Routers Video Server HD Display
Subnet Configuration . 1. 1. 1. 2 . 4. 1 . 7. 1 . 10. 1 . 13. 1 . 16. 1 . 4. 2 . 7. 2 . 10. 2 . 13. 2 . 16. 2 . 3. 1. 2. 1 . 30. 2 . 6. 2. 3. 2 . 5. 1 . 30. 1 . 6. 1 . 28. 2. 28. 1 Video Server . 8. 1 . 26. 1. 27. 2 . 29. 1 . 9. 2. 9. 1 . 11. 1 . 12. 1 . 23. 1. 24. 2 . 27. 1 . 12. 2 . 25. 2. 25. 1 . 22. 2. 22. 1 Shortest Path . 14. 1 . 18. 2 . 21. 2. 24. 1 . 15. 2 . 20. 1. 21. 1 . 19. 2. 19. 1 Video Client . 15. 1 . 18. 1 . 17. 1
Step 1 – Observe the Routing Tables The router is already configured and running on your machines The routing table has converged to the routing decisions with minimum number of hops Next, break a link …
Step 2 - Dynamic Re-routing Break the link between video server and video client . 1. 1. 1. 2 . 7. 1 . 10. 1 . 4. 2 . 7. 2 . 10. 2 . 3. 1. 2. 1 Routers re-route traffic around the broken link and video continues playing . 4. 1 . 30. 2 . 6. 2. 3. 2 . 5. 1 . 30. 1. 29. 1 . 9. 2 . 6. 1 . 8. 1 . 28. 2. 28. 1 . 12. 1 . 23. 1 . 25. 2. 25. 1 . 24. 1 . 14. 1 . 22. 2. 22. 1 . 20. 1. 21. 1 . 16. 2. 15. 2 . 18. 2. 21. 2 . 24. 2. 27. 1 . 11. 1 . 16. 1 . 13. 2. 12. 2 . 9. 1 . 26. 1. 27. 2 . 13. 1 . 19. 2. 19. 1 . 15. 1 . 18. 1 . 17. 1
Working IP Router – Observe PW-OSPF re-routing traffic around a failure – Video is temporarily disrupted then resumes playing as TCP recovers from packet-loss
CSMA/CD Ethernet (10 Mbps circa 1980) Collision detection can take as long as 2τ A B Packet starts at time 0 A B Packet arrives at B at τ-ε A B Packet starts from B at time τ Collision shortly thereafter A B Noise (the collision) arrives at A at time 2τ
User A B C D E In pure Aloha, frames are transmitted at arbitrary (and uncoordinated) times Time
- Slides: 31