Reference Notes on TCP/IP

1. Reference Notes on TCP/IP

2. Internetworking • Interconnection of 2 or more networks forming an internetwork, or internet. • LANs, MANs, and WANs. • Different networks man different protocols. • TCP/IP, IBM’s SNA, DEC’s DECnet, ATM, Novell and AppleTalk (for LANs). • Also, satellite and cellular networks.

3. Example Internet LAN-WAN- LAN 802.5 LAN R 802.3 LAN 802.3 LAN 802.4 LAN X.25 WAN B R R LAN- WAN LAN-LAN R Gateway: device connecting 2 or more different networks. SNA WAN

4. Gateways • Repeaters: operate at physical layer (bits); amplify/regenerate signal. • Bridges: store-and-forward frames; data link layer devices. • Routers: operate at network layer. • Transport gateways: connect networks at the transport layer. • Application gateways: connect 2 parts of an application at application layer.

5. How do networks differ? • Service offered: connection-oriented versus connection-less. • Protocols: IP, IPX, AppleTalk, DECnet. • Addressing: flat (802) versus hierarchical (IP). • Maximum packet size. • Quality of service. • Error control: reliable, ordered, unordered delivery. • Flow control: sliding window versus rate-based. • Congestion control: leaky bucket, choke packets. • Security: privacy rules, encryption. • Parameters: different timeouts.

6. Types of Internetworks • Connection-oriented concatenation of VC subnets. • VC between source and router closest to destination network. • Router builds V to gateway to other subnet. • Gateway keeps state about that VC. • Builds VC to router in the next subnet, etc. • Every packet traverses same path. • Ordered delivery. • Routers convert between packet formats.

7. Connection-oriented concatenation • VC between source and router closest to destination network. • Router builds VC to gateway to other subnet. Gateway keeps state about VC. • Gateway builds VC to router in the next subnet, etc. • Every packet traverses same path. • Ordered delivery. • Routers convert between packet formats.

8. Connectionless Internetworking • Datagram model. • Different packets may take different routes. • Separate routing decision for each packet. • No ordered delivery guarantees.

9. Datagram versus VC Internets • VC: • Plus’s: resources reserved in advance, ordered delivery, short headers. • Minus’s: vulnerability to failures, less adaptive, hard if involving datagram subnet. • Datagram: • Plus’s: more robust and adaptive, can be used over datagram subnets (many LANs, mobile networks). • Minus’s: Longer headers, unordered delivery.

10. Tunneling • Interconnecting through a “foreign” subnet. Tunnel Ethernet 2 Ethernet 1 G G WAN IP IP IP Ethernet frame Ethernet frame IP packet inside payload field of WAN packet.

11. Internetwork Routing 1 • 2-level hierarchy: • Routing within each network: interior gateway protocol. • Routing between networks: exterior gateway protocol. • Within each network, different routing algorithms can be used. • Each network is autonomously managed and independent of others: autonomous system (AS).

12. Internetwork Routing 2 • Typically, packet starts in its LAN. Gateway receives it (broadcast on LAN to “unknown” destination). • Gateway sends packet to gateway on the destination network using its routing table. If it can use the packet’s native protocol, sends packet directly. Otherwise, tunnels it.

13. Fragmentation 1 • Network-specific maximum packet size. • Width of TDM slot. • OS buffer limitations. • Protocol (number of bits in packet length field). • Maximum payloads range from 48 bytes (ATM cells) to 64Kbytes (IP packets).

14. Fragmentation 2 • What happens when large packet wants to travel through network with smaller maximum packet size? Fragmentation. • Gateways break packets into fragments; each sent as separate packet. • Gateway on the other side have to reassemble fragments into original packet. • 2 kinds of fragmentation: transparent and non-transparent.

15. Transparent Fragmentation • Small-packet network transparent to other subsequent networks. • Fragments of a packet addressed to the same exit gateway, where packet is reassembled. • OK for concatenated VC internetworking. • Subsequent networks are not aware fragmentation occurred. • ATM networks (through special hardware) provide transparent fragmentation: segmentation.

16. Problems with Transparent Fragmentation • Exit gateway must know when it received all the pieces. • Fragment counter or “end of packet” bit. • Some performance penalty but requiring all fragments to go through same gateway. • May have to repeatedly fragment and reassemble through series of small-packet networks.

17. Non-Transparent Fragmentation • Only reassemble at destination host. • Each fragment becomes a separate packet. • Thus routed independently. • Problems: • Hosts must reassemble. • Every fragment must carry header until it reaches destination host.

18. Keeping Track of Fragments 1 • Fragments must be numbered so that original data stream can be reconstructed. • Tree-structured numbering scheme: • Packet 0 generates fragments 0.0, 0.1, 0.2, … • If these fragments need to be fragmented later on, then 0.0.0, 0.0.1, …, 0.1.0, 0.1.1, … • But, too much overhead in terms of number of fields needed. • Also, if fragments are lost, retransmissions can take alternate routes and get fragmented differently.

19. Keeping Track of Fragments 2 • Another way is to define elementary fragment size that can pass through every network. • When packet fragmented, all pieces equal to elementary fragment size, except last one (may be smaller). • Packet may contain several fragments.

20. Keeping Track of Fragments 3 • Header contains packet number, number of first fragment in the packet, and last-fragment bit. 1 byte Last-fragment bit 27 0 1 A B C D E F G H I J (a) Original packet with 10 data bytes. Number of first fragment Packet number 27 0 0 A B C D E F G H 27 8 1 I J (b) Fragments after passing through network with maximum packet size = 8 bytes.

21. The Internet Network Layer • The Internet as a collection on networks or autonomous systems (ASs). • Hierarchical structure. Transcontinental US backbone Transcontinental links links European backbone Regional network National network

22. IP (Internet Protocol) • Glues Internet together. • Common network-layer protocol spoken by all Internet participating networks. • Best effort datagram service: • No reliability guarantees. • No ordering guarantees.

23. IP • Transport layer breaks data streams into datagrams; fragments transmitted over Internet, possibly being fragmented. • When all packet fragments arrive at destination, reassembled by network layer and delivered to transport layer at destination host.

24. IP Versions • IPv4: IP version 4. • Current, predominant version. • 32-bit long addresses. • IPv6: IP version 6 (aka, IPng). • Evolution of IPv4. • Longer addresses (16-byte long).

25. IP Datagram Format • IP datagram consists of header and data (or payload). • Header: • 20-byte fixed (mandatory) part. • Variable length optional part.

26. IP Header 32 bits Header length Type of service Version Total length U D M Identification Fragment offset TTL Protocol Header checksum Source address Destination address Options

27. IP Header Fields 1 • Version: which IP version datagram uses. • Header length: how long (in 32-bit words) is header; minimum=5; maximum=15 (options=40 bytes). • Type of service: precedence (priority), 3 flags (delay, throughput, reliability). In practice, routers ignore type of service. • Total length: length of total datagram, i.e., header + data (max = 64Kbytes).

28. IP Header Fields 2 • Identification: which datagram fragment belongs to. • U: unused bit. • D: don’t fragment. • M: more fragments. • Fragment offset: position of fragment in datagram. • TTL: datagram lifetime.

29. IP Header Fields 3 • Protocol: number of the transport protocol that generated the datagram. • Header checksum: verifies header integrity; computed at each hop. • Source and destination address: IP addresses of source and destination. • Options: way of extending the protocol.

30. Addressing • Required for packet delivery. • Each network may use different addressing scheme. • Addresses must be unique. • Flat addresses: physical addresses (e.g., Ethernet address). • Hierarchical addresses: use hierarchy scheme like postal addresses (e.g., IP).

31. Address Types • Unicast: uniquely distinguishes a single node. • Multicast: shared by a group of nodes. • Broadcast: shared by all nodes.

32. IP Addresses • Every host and router on the Internet must have an IP address. • 2-level hierarchy: • Network number. • Host number. • Notations: • Binary: 10000000 00000110 11110000 00000011 • Dotted decimal: 128.6.240.3

33. IP Address Formats 1 • 4 different classes: Network Host 0XXXXXXX Class A: 128 nets. 16M hosts/net. Class B: 16K nets. 64K hosts/net. Class C: 2M nets. 256 hosts/net. Class D: Multicast. 10XXXXXX XXXXXXXX 110XXXXX XXXXXXXX XXXXXXXX 1110XXXX XXXXXXXX XXXXXXXX XXXXXXXX

34. IP Address Formats 2 • Class A: 1~127. • Class B: 128~191. • Class C: 192~223. • Class D: 224~239.

35. Multi-addresses • A router usually has more than one IP address. • Multi-homed host: host with multiple network interfaces each of which has different IP address. 236.240.128.0 129.98.0.0 236.240.128.3 129.98.95.1 80.0.0.0 80.0.0.8

36. Management and Scalability 1 • Network numbers assigned by single authority: NIC (network information center). • All hosts in a network must have same network number. • What if networks grow?

37. Management and Scalability 2 • Example: company starts with 1 class C LAN, thus can connect up to 256 hosts. • It might grow to more than 256 hosts. • It might get more LANs. • For every new LAN, need new network number from NIC. • Moving machines between LANs needs address change.

38. Subnetting 1 • Split address space into several “internal” subnets. • Still act like single network to outside world. • Example: Class B address. Class B: 16K nets. 64K hosts/net 10XXXXXX XXXXXXXX HHHHHHHH HHHHHHHH 10XXXXXX XXXXXXXX SSSSSSHH HHHHHHHH Class B with subnetting: 62 LANs, 1022 hosts each. 1st. subnet: 130.50.4.1 2nd. subnet: 130.50.8.1

39. Subnetting 2 • Routing: hierarchical. • (network, -) entries: distant networks hosts. • (this network, host) entries: local hosts. • Routers only need to keep track of other networks and local hosts. • With subnetting: • (network, -) entries: distant networks hosts. • (this network, subnet, -). • (this network, this subnet, host). • Adds extra hierarchical level => smaller RTs.

40. Subnet Mask • Used to compute the subnet number; i.e., gets rid of the host number. • Facilitates routing table look-up. • IP address AND subnet mask = subnet # • Example: 10XXXXXX XXXXXXXX SSSSSSHH HHHHHHHH 11111111 11111111 11111100 00000000 Ex: 130.50.15.6 AND subnet mask = 130.50.12.0, which is subnet 3.

41. Internet Control Protocols • IP carries data. • There are other network layer protocols that carry control information. • Example: ICMP, ARP, RARP, BOOTP.

42. ICMP • Internet Control Message Protocol. • Report specific events. • Generated by routers. • Encapsulated in IP packets.

43. ICMP Messages Destination unreachable Packet couldn’t be delivered Time exceeded TTL field hit 0 Parameter problem Invalid header field Source quench Choke packets Redirect Route problem Echo request Check if destination is up Echo reply Destination responds Timestamp request Same as echo request + TS Timestamp reply Same as echo reply + TS

44. Mapping IP to DLL Address • Internet applications refer to hosts by their IP addresses; once packet gets to destination LAN, node needs to figure out the destination DLL address. • One solution is to have configuration file. • Hard to maintain/update. • Address Resolution Protocol (ARP): • Run by every node to map IP to DLL address (RFC 826).

45. ARP • Advantage: • Easy to administer, less human intervention. • Example: 2 hosts on the same Ethernet want to communicate. • Host 1 must figure out host 2’s Ethernet address. • Host 1 broadcasts ARP packet on Ethernet asking for the Ethernet address of host 2. • Host 2 receives the ARP request, and replies with its Ethernet address.

46. ARP Optimizations • Caching of ARP replies. • Entries may have large TTLs. • When sending ARP request, piggyback its own IP-DLL address mapping. • Every machine broadcasts its mapping at boot time. • No response is expected. • Other machines cache that information.

47. Proxy ARP • What if host 1 wants to send data to host 3 on a different LAN? • Router connecting the 2 LANs can be configured to respond to ARP requests for the networks it interconnects: proxy arp. • Another solution is for host 1 to recognize host 3 is on remote network and use default LAN address that handles all remote traffic; that could be the router’s Ethernet address.

48. RARP • Reverse Address Resolution Protocol. • Given LAN address, what’s the IP address? • Usually for booting diskless workstation. • Gets the OS image from remote file server. • Same image for all machines. • Machine broadcasts its LAN address. • Remote RARP server responds with machine’s IP address.

49. BOOTP • RARP broadcasts are not forwarded by routers. • Need RARP server on every network. • BOOTP uses UDP messages that are forwarded by routers. • Also provides additional information such as IP address of file server holding OS image, subnet mask, etc.

50. Internet Routing • IGPs and EGPs • IGPs: routing within ASs. • EGPs: routing between ASs.