Introduction to Internetworking

1. Introduction to Internetworking 3035/GZ01 Networked Systems Kyle Jamieson Lecture 8 Department of Computer Science University College London

2. Building bigger, heterogeneous networks • We’ve seen a few examples of local area networks so far: • Bridged Ethernets • 802.11 • CDMA • But, local area networks have limitations: • Scaling # of networks, efficiently routing and addressing • Link layer heterogeneity: users on one type of network want to communicate with users on other type • So, we want to interconnect large, heterogeneous networks

3. Today From design principles to the actual design of the Internet • Five basic Internet design decisions • Design of IP • Internet addressing • Forwarding in the Internet

4. Five basic Internet design decisions • Datagram packet switching • Best-effort service model • Layering • A single internetworking protocol • The end-to-end principle (and fate-sharing)

5. Datagram packet switching • Divide messages into a sequence of datagrams • Network deals with each packet individually • Each datagram contains enough information to allow any switch to decide how to get it to its destination • What is an alternative to this? • Means that each datagram must contain all relevant network information in its header • Design of protocol closely follows the header syntax • Every packet contains complete destination address • Switch consults forwarding table • Process of building forwarding tables: routing (later)

6. Why datagram packet switching? • Achieve higher levels of utilization • Statistical multiplexing • Why is this more important for the Internet than for the phone network? • Avoid per-flow state inside the network • Plenty of routing state, but no per-flow state • Follows from notion of fate-sharing • Enables robust fail-over if paths fail • Why not virtual circuits? • The notion of “soft state” is midway between DG and VC • Soft state: Connection-related information in a router that is not necessary for correct operation, and is cached and removed at will

7. What is “best effort?” • Network makes no service guarantees • Just gives its best effort (BE) • The network has failure modes: • Packets may be lost • Packets may be corrupted • Packets may be delivered out of order • Packet may be significantlydelayed Internet Destination Source

8. Why best effort (BE)? • BE means the task of the network is simple • No need to do error detection and correction • No need to remember from one packet to next • No need to manage congestion in the network • No need to reserve bandwidth and memory in the network • No need to make packets follow same path • Easier to survive failures • Transient disruptions are okay during failover • Simplifies interconnection between networks • Minimal service promises

9. But What About Applications? • Some applications want more, for example: • Bulk file transfer: File Transfer Protocol (FTP) • Requires all the data, with no losses or corruption • Order that data is delivered doesn’t matter • Telephone conversation: Skype, RTP • Requires minimal and predictable delays • Losses and corruption don’t matter (to a point) • Perhaps the most important issue in design, which the Internet got right

10. Other layers address failure modes • Packets may be lost or arbitrarily delayed • Sender can send the packets again, or not • No network congestion control (beyond “drop”) • Sender can slow down in response to loss or delay • Packets may be corrupted • Higher-level protocol can detect/correct errors, or not • Packets may be delivered out-of-order • Receiver can put packets back in order, or not • Packets may be arbitrarily delayed • Receiver can buffer packets for smooth playout, or not

11. What can’t higher layers do? • Higher layers cannot make delay smaller • If applications needs guarantee of low delay, then need to ensure adequate bandwidth • Will keep queuing delay low • No way to help with speed-of-light latency • What applications need guaranteed low-delay? • Can the Internet support phone calls?

12. Review: What is layering? • Modularity partitions functionality into modules • Laying is a particularly simple form of modularity • Modules only deal with layers above and below • Simplifies interactions between modules • Simplifies introduction of new protocols

13. Five basic design decisions • Datagram packet switching • Best-effort service model • Layering • A single internetworking protocol • The end-to-end principle (and fate-sharing)

14. IP: one networking layer protocol • Design goal #1 of the Internet: Connect existing heterogeneous networks together • Unifies the architecture • As long as applications can run over IP-based protocols, they can run on any network • As long as networks support IP, they can run any application

15. The Internet hourglass • Only one network-layer protocol: Internet Protocol (IP) • The “narrow waist” facilitates interoperability Application FTP HTTP DNS TFTP TCP Transport UDP Network IP Link Ethernet PPP WiFi Physical Copper Radio

16. Alternatives to universal IP? • What would happen if we had more than one network layer protocol? • Are there disadvantages to having only one network layer protocol? • Some loss of flexibility, but the gain in interoperability more than makes up for this • Because IP is embedded in applications and in interdomain routing, it is very hard to change • Having IP be universal made this mistake easier to make, but it didn’t cause this problem

17. Five basic design decisions • Datagram packet switching • Best-effort service model • Layering • A single internetworking protocol • The end-to-end principle (and fate-sharing)

18. Review: the end-to-end principle • Basic observation: some types of network functionality can only be correctly implemented end-to-end • Because of this, end hosts: • Can satisfy the requirement without network’s help • Will/must do so, since can’t rely on network’s help • Therefore, don’t go out of your way to implement them in the network

19. Related notion of fate-sharing • Fate-sharing is a technique for dealing with failure • Only way that failure can cause loss of the critical state is if the entity that cares about it also fails ... • … in which case it doesn’t matter • Idea: when storing state in a distributed system, keep it co-located with the entities that ultimately rely on the state • Often argues for keeping network state at end hosts rather than inside routers • In keeping with end-to-end principle • e.g., packet-switching rather than circuit-switching • e.g., NFS file handles, HTTP “cookies”

20. Today From design principles to the actual design of the Internet • Five basic Internet design decisions • Design of IP • Internet addressing • Forwarding in the Internet

21. Designing IP • What does it mean to “design” a protocol? • Answer: specify the syntax of its messages and their meaning (semantics). • Syntax: elements in packet header, their types and layout; representation • Semantics: interpretation of elements; information • What semantics should the IP header support?

22. IP functionality (1/2) • Getting the packet there: • Where is the packet going? • Which protocol will process packet on host? • Network handling of packet: • How should the packet be forwarded (e.g., priority) • Where does header and packet end? • Coping with problems: • Has the header been corrupted? (Why not payload?) • Has the packet been fragmented? If so, provide information needed to reconstruct • Is packet caught in a loop? If so, drop packet

23. IP functionality (2/2) • Extensibility: How can we let IP change? • Which IP version and options are expected? • Miscellaneous: • Where did the packet come from? (Why is this needed?)

24. From semantics to syntax • The past two slides discussed the kinds of information the header must provide • Will now show the syntax (layout) of the header, and discuss the semantics in more detail

25. The IP packet header • Version (four bits) • Indicates the version of the IP protocol • Needed to know what other fields to expect • Typically “4” (IPv4), else “6” (IPv6) • Hlen (four bits) • Number of 32-bit words in the header • Typically “5” (for a 20-byte IPv4 header) • Can be more if IP options are used • TOS (one byte) • Type of service • Allows packets to be treated differently based on needs • e.g., low delay for audio, high bandwidth for bulk transfer bit:

26. The IP packet header • Length (16 bits) • Number of bytes in the packet • Maximum size is 65,535 bytes (216−1) though underlying links may impose smaller limits • Ident (16 bits), Flags (three bits), Offset (13 bits) • Support IP fragmentation bit:

27. Coping with different MTUs: the problem • Key to addressing heterogeneity in the Internet • Each link layer has a maximum datagram size or maximum transmission unit(MTU) • Goal: How to ensure datagrams’ size to be equal to the minimum MTU over all link layers along the path they happen to take (path MTU)? • This would minimize header overheads • Don’t want to send all datagrams lowest MTU of any link layer: inefficient, unknown, and always changing depending on route

28. IP’s datagram fragmentation • Basic idea: routers to break datagrams into smaller fragments • Each fragment is its own self-contained IP datagram • Ident (16 bits): used to tell which fragments belong together • Flags (three bits): • More (M): set to “1” if this fragment is not the last one, else “0” • Don’t Fragment (D): instruct routers to not fragment packet even if it won’t fit • Instead, they drop the packet and send back a “Too Large” ICMP control message • Forms the basis for “Path MTU Discovery”, covered later • Reserved (R): unused bit • Offset (13 bits): what part of the original datagram this fragment covers in eight-byte units

29. 500 500 Where should reassembly happen? • Answer #1: within the network, with no help from end-host B (receiver) MTU=1000B MTU=1000B Host A MTU=500B Host B R1 R2 1000 1000

30. 500 500 Where should reassembly happen? • Answer #1: within the network, with no help from end-host B (receiver) • Answer #2: at end-host B (receiver) with no help from the network MTU=1000B MTU=1000B Host A MTU=500B Host B R1 R2 1000

31. Where should reassembly happen? • Answer #1: within the network, with no help from end-host B (receiver) ✗ • Answer #2: at end-host B (receiver) with no help from the network ✔ • Fragments can travel across different paths! R3 MTU=1000B MTU=1000B Host A MTU=500B Host B R1 R2 500 500 1000

32. Fragmentation example M; offset=0 M; offset=64 Offset=128 Ethernet MTU: 1492 bytes FDDI MTU: 4500 bytes PPP MTU: 532 bytes

33. Fragmentation considered harmful • Fragmentation causes inefficient use of resources • Loss of fragments leads to degraded performance • Loss of any fragment requires retransmit of entire datagram • Efficient reassembly is hard • Burden is on gateways to buffer out-of-order fragments • Reordering of different datagrams’ fragments may increase buffering requirements, thus forcing datagram drops! Path MTU

34. Path MTU discovery • Source initially sets path MTU (PMTU) estimate = MTU of first hop • Send datagrams with Don’t Fragment (DF) bit set in Flags field • If any datagrams are too big to be forwarded • Intermediate router will discard them and send an ICMP “Destination Unreachable” message with “datagram too big” flag set • Source reduces its PMTU estimate

35. The time-to-live field • TTL (8 bits) • Potentially catastrophic problem • Forwarding loops can cause datagrams to cycle forever • As these accumulate, eventually consume all capacity • Solution: Routers decrement TTL field at each hop, packet is discarded if TTL reaches zero • ICMP “time exceeded” message sent back to the source bit:

36. Protocol demultiplexing • Protocol (8 bits) • Identifies the higher-layer protocol • e.g.“6” for Transmission Control Protocol (TCP) • e.g. “17” for User Datagram Protocol (UDP) • Important for demultiplexing at the end host • Indicates what kind of header to expect next bit: Protocol=6 Protocol=17 UDP header TCP header UDP payload TCP payload

37. IP checksum • Checksum (16 bits) • Recall: Complement of the one’s complement sum of all 16-bit words in the IP packetheader • If verification fails, router should discard the packet • So it doesn’t act on bogus information • Recalculated at each hop • Why? • Why include the TTL field in the checksum? • Why only over the header? bit:

38. IP checksum (notes) • Checksum (16 bits) • Recall: Complement of the one’s complement sum of all 16-bit words in the IP packetheader • If verification fails, router should discard the packet • So it doesn’t act on bogus information • Recalculated at each hop • Why? Because the TTL field is decremented on each hop. • Why include the TTL field in the checksum? Ensures loop detection works correctly in presence of router bugs. • Why only over the header? e2e argument: if higher layers need reliability, they will implement it; errors can be introduced between layers as well. bit:

39. IP addresses • SourceAddr (32 bits) • Unique identifier for the sending host • Recipient can decide whether to accept packet • Routers can decide whether to forward packet • Enables recipient to reply • DestinationAddr (32 bits) • Unique identifier for the receiving host • Allows each router to make forwarding decisions bit:

40. Today From design principles to the actual design of the Internet • Five basic Internet design decisions • Design of IP • Internet addressing • Forwarding in the Internet

41. Designing IP’s addresses • Question #1: what should an address be associated with? • e.g., a telephone number is associated not with a person, but with a handset • Question #2: what structure should addresses have? • What are the implications of different types of structure? • Question #3: who determines the particular addresses used in the global Internet? • What are the implications of how this is done?

42. IPv4 addresses • A unique 32-bit number • Uniquely identifies and associated with an interface (on a host, on a router, &c.) • Represented in dotted-quad notation • a.b.c.dwhere each component is an eight-bit decimal number between zero and 255 • e.g. 12.34.158.5 12 34 158 5 00001100 00100010 10011110 00000101

43. What are IP addresses used for? • Network uses addresses to figure out where to forward packets • Routers are the network devices that forward packets based on IP addresses over a wide-area network (WAN) • What do “switches” do? • Route on layer-2 addresses (e.g., MAC addresses)

44. Routers • A router consists of • Set of input interfaces where packets arrive • Set of output interfaces from which packets depart • Some form of interconnect connecting inputs to outputs • A router implements • Forward packet to corresponding output interface • Manage bandwidth and buffer space resources ... ... host host host host host host LAN 2 LAN 1 router router router WAN WAN Router

45. Scalability challenge • Suppose hosts had arbitrary addresses • Then every router would need a lot of information to know how to direct packets toward the host 1.2.3.4 5.6.7.8 2.4.6.8 1.2.3.5 5.6.7.9 2.4.6.9 ... ... host host host host host host LAN 2 LAN 1 router router router WAN WAN 1.2.3.4 1.2.3.5 2.4.6.8 ... ... forwarding table

46. Hierarchical addressing in mail • Addressing in the UK mail system • Post code: WC1E 7JG • Street: Malet Place • Building on street: MPEB • Name of occupant: Kyle Jamieson • Forwarding in the UK mail system • Deliver letter to delivery office with initial part of postcode (WC1E) • Deliver mail to recipient from delivery office with final part of postcode (7JG) • Drop letter into mailbox for the building/room • Give letter to the appropriate person ??? Does anyone in the UK mail system know where every house is?

47. Hierarchical addressing • Universal trick in complex systems: When you need more scalability, impose a hierarchical structure • The Internet is an “inter-network” that connects networks together, not hosts • Natural two-level hierarchy: WAN delivers to right LAN; LAN delivers to right host • Key idea: Separate routing tables at each level of hierarchy, each of manageable scale ... ... host host host host host host LAN 2 LAN 1 router router router WAN WAN

48. Hierarchical addressing • Prefix is network address: suffix is host address • “Slash notation” describes prefixes • e.g. 12.34.158.0/23 is a 23-bit prefix with 29 addresses • Terminology: “slash twenty-three” 12 34 158 5 Network (23 bits) Host (nine bits) 00001100 00100010 10011110 00000101

49. Scalability improved • Number related hosts with same prefix • 1.2.3.0/24 on the left LAN • 5.6.7.0/24 on the right LAN 1.2.3.4 1.2.3.5 1.2.3.156 5.6.7.8 5.6.7.9 5.6.7.123 ... ... host host host host host host LAN 2 LAN 1 router router router WAN WAN 1.2.3.0/24 5.6.7.0/24 forwarding table

50. Easy to add new hosts • No need to update the routers • e.g. adding a new host 5.6.7.124 on the right • Doesn’t require adding a new forwarding entry 1.2.3.4 1.2.3.5 1.2.3.156 5.6.7.8 5.6.7.9 5.6.7.123 ... ... host host host host host host LAN 2 LAN 1 router router router host WAN WAN 5.6.7.124 1.2.3.0/24 5.6.7.0/24 forwarding table