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Internetwork Protocols

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1 Internetwork Protocols
Lesson 11 NETS2150/2850 School of Information Technologies

2 Lesson Outcomes The needs for internetworking and not standalone network! Design issues in a connection-less model Understand the intricacies of IP addressing What’s missing in IPv4? The importance of ICMP, the companion protocol of IP

3 Internetworking Terms
Communications Network Facility that provides data transfer service An internet Collection of communications networks interconnected by routers The Internet - note upper case I The global collection of thousands of individual machines and networks An intranet Corporate internet operating within the organization Uses Internet (TCP/IP) technology to deliver documents and resources Can be isolated internet, or can have links to the Internet

4 The Internet/Network layer
Host, router network layer functions: Transport layer: TCP, UDP IP protocol addressing conventions packet format packet handling conventions Routing protocols path selection RIP, OSPF Network layer routing table ICMP protocol error reporting router “signalling” Data link layer physical layer

5 Architectural Approaches
Mode of Operation: Connection oriented Connectionless application transport network data link physical application transport network data link physical

6 1: Bridges and routers are examples of ISs.
Connection Oriented Assume that each network is connection oriented Intermediate System (IS1) connect two or more networks Logical connection set up between ESs Concatenation of logical connections across networks Individual network virtual circuits joined by IS 1: Bridges and routers are examples of ISs.

7 Connectionless Operation
Corresponds to datagram mechanism in packet switched network Each NPDU treated separately Network layer protocol common to all ESs and routers Known generically as the internet protocol Internet Protocol (IP) One such internet protocol was developed for ARPANET (Advanced Research Projects Agency Network ) RFC 791 Lower layer protocol needed to access particular network

8 Connectionless Internetworking
Pros: Flexibility Robust No connection setup overhead Cons: Unreliable, not guaranteed delivery Not guaranteed order of delivery Packets can take different routes Reliability is responsibility of next layer up (e.g. TCP)

9 Ordered Delivery PDUs may arrive out of order
Different paths through network But, PDU order must be maintained Number PDUs sequentially Easy to reorder received PDUs

10 IP Operation

11 IP Design Issues Routing Datagram lifetime
Fragmentation and re-assembly Error control Flow control

12 Routing End systems and routers maintain routing tables Source routing
Indicate next router to which packet should be sent, for all possible destination network Could be static But, may contain alternative routes Or Dynamic Flexible response to congestion and errors Source routing Source specifies route as sequential list of routers to be followed because of: Security Priority

13 Datagram Lifetime Datagram could loop indefinitely
Consumes resources Transport protocol may need upper bound on datagram life Datagram marked with lifetime TTL field in IP Decrement TTL on passing through each router Once lifetime expires, datagram discarded

14 Fragmentation and Reassembly
Network links have MTU (max. transmission unit) - largest possible data link-level frame Different link types, different MTUs ATM  53 octets Ethernet  1518 octets Pros: More efficient error control Smaller retransmission Fairer Prevent monopolising Smaller buffers at rcvr fragmentation: in: one large packet out: 3 smaller packets reassembly

15 Disadvantages of Fragmentation
Make PDUs as large as possible because PDU contains some control information Smaller block, larger overhead to data ratio PDU arrival generates interrupt to CPU Waste CPU time Smaller blocks, more interrupts!

16 Fragmentation and Reassembly
When to re-assemble?? At destination? Results in packets getting smaller as data traverses internet Intermediate re-assembly? Need large buffers at routers Buffers may fill with fragments All fragments must go through same router Inhibits dynamic routing

17 IP Fragmentation (1) IP reassembles at destination only
It uses these fields in header Identifies end system originated packet Source and destination address Protocol layer generating data (e.g. TCP) Identification supplied by IP layer Total Length Length of packet in octets

18 IP Fragmentation (2) Offset More fragment flag
Position of fragment of user data in original packet In multiples of 64-bit (8-octet) units More fragment flag Indicates that this is not the last fragment 0 – last or the only packet 1 – not last

19 IP Fragmentation and Reassembly
ID =x offset =0 Moreflag length =4000 =1 =1500 =185 =370 =1040 One large packet becomes 3 smaller packets Example 4000-octet packet (with 20-octet header) MTU = 1500 octets Data in each is 1480 octets Fragments = 3980/1480 = 3 Offset in 1st fragment = 0, 2nd fragment = (1480/8) = 185 and 3rd fragment = ( ) = 370

20 Dealing with Failure Reassembly may fail if some fragments get lost
So, need to detect failure Reassembly time out Assigned to first fragment to arrive If timeout expires before all fragments arrive, discard partial data Use packet lifetime (time to live) If TTL runs out, kill partial data

21 Error Control IP do NOT guarantee delivery
IP uses checksum for error detection Router should attempt to inform source if packet discarded e.g. for TTL expiring or destination unreachable But, datagram identification needed Handled by ICMP protocol (see later)

22 IP Checksum – 2 steps Add the 16-bit values up. Each time a carry-out (17th bit) is produced, swing that bit around and add it back into the lsb Once all the values are added in this manner, invert all the bits in the result - called its “one's complement”

23 Example: First, we add the 16-bit values 2 at a time: First 16-bit value Second 16-bit value Produced a carry-out, which gets added + \ > 1 back into lsb Third 16-bit value No carry to swing around (**) Fourth 16-bit value Produced a carry-out, which gets added Our sum lsb msb

24 Example (Cont.) Then we have to take the one's complement of the sum.
We do this by simply inverting all the bits in the final result from above: Our sum The "one's complement" So the checksum stored in the header would be

25 Flow Control Allows routers and/or stations to limit rate of incoming data Difficult in connectionless systems Not done here, left to higher layer (i.e. transport)

26 Internet Protocol (IP) Version 4
Part of TCP/IP Specified in RFC 791 Will (eventually) be replaced by IPv6

27 IP Services Send primitive Deliver primitive
Request transmission of data unit onto the network Deliver primitive Notify user of arrival of data unit from the network

28 IPv4 Header

29 Header Fields (1) Version (4 bits) Internet header length (4 bits)
Currently 4 Internet header length (4 bits) In 32-bit units Including options Type of service (before) – Differentiated Service (now) (8 bits) Allows classification of packets Total length (16 bits) in octets Header plus data

30 Header Fields (2) Identification (16 bits) Flags (3 bits)
Sequence number Used with addresses and user protocol to identify packet uniquely Flags (3 bits) More bit (1 bit) Don’t fragment (1 bit) Fragmentation offset (13 bits) Time to live (8 bits) Protocol (8 bits) Next higher layer to receive data field at destination

31 Header Fields (3) Header checksum (16 bits) Source address (32 bits)
Reverified and recomputed at each router Uses 16-bit ones complement sum Source address (32 bits) Destination address (32 bits) Options (variable) See next slide Padding (variable) To fill to multiple of 32 bits long

32 Options Security Label Source routing Route recording
Allows secured handling of packets Source routing A list of router addresses specifies a route to follow Route recording Records route taken by a packet Stream identification Allows special handling of streaming traffic Timestamping Intermediate routers add timestamp on packet

33 Data Field Carries user data from next layer up
Integer multiple of 8 bits long (octet) Max length of packet (header plus data) 65,535 octets

34 IP Addressing: Introduction
IP address: 32-bit identifier for host, router interface interface: connection between host/router and physical link router’s have multiple interfaces IP addresses associated with each interface Dotted-decimal notation Decimal number representing each of the octets, separated by a decimal point = 223 1

35 IP Addressing IP address: 2 parts
network part (high order bits) host part (low order bits) What’s a network ? (from IP address perspective) device interfaces with same network part of IP address can physically reach each other without intervening router LAN

36 IP Addresses given notion of “network”, let’s re-examine IP addresses:
“classful” addressing: class Address range A to network host B to 10 network host to C 110 network host to D 1110 multicast address 32 bits All host ids.

37 Private IP addresses Also called non-routable IP addresses
IP blocks reserved for private networks are: through through through Network Address Translation (NAT) protocol could be used to map private IP addresses to external IP address space (see RFC 1631) Use to hide internal network structure from the outside world (Security measure!)

38 IP addressing: CIDR Classful addressing contributed to:
inefficient use of address space, address space exhaustion e.g., class B net allocated enough addresses for 65K hosts, even if only 2K hosts in that network Solution  classless addressing CIDR: Classless InterDomain Routing network portion of address is of arbitrary length address format: a.b.c.d/x, where x is # bits in network portion of address (Slash notation) network part host /23

39 IP addresses: how to get one?
Q: How does host get IP address? hard-coded by network admin: MS Windows: control-panel->network->configuration->tcp/ip->properties Red-Hat LINUX: /etc/sysconfig DHCP: Dynamic Host Configuration Protocol: dynamically gets address from a server Client “plug-and-play”

40 Who assigns IP addresses?
Q: How does network get network part of IP addr? A: gets allocated portion of its provider ISP’s address space ISP's block /20 Organization /23 Organization /23 Organization /23 … … …. Organization /23

41 Subnets and Subnet Masks
Allow arbitrary complexity of internetworked LANs within organisation Each LAN segment assigned subnet number Host portion of address partitioned into subnet number and host number With the help of subnet mask

42 IP addressing: the last word...
Q: How does an ISP get block of addresses? A: ICANN: Internet Corporation for Assigned Names and Numbers allocates addresses manages DNS assigns domain names, resolves disputes

43 ICMP: Internet Control Message Protocol
Used by hosts, routers to communicate network-level information (RFC 792) error reporting: unreachable host, network, port, protocol query: echo request/reply (used by ping) In network-layer “above” IP: ICMP msgs carried in IP packets ICMP message: type, code plus IP header and first 8 octets of data causing error Type Code description echo reply (ping) dest. network unreachable dest host unreachable dest protocol unreachable dest port unreachable dest network unknown dest host unknown source quench (congestion control - not used) echo request (ping) route advertisement router discovery TTL expired bad IP header IP ICMP

44 IP v6 - Version Number IP v 1-3 defined and replaced
IP v4 - current version IP v5 - non-IP real-time streaming protocol IP v6 - replacement for IP v4 During development it was called IPng Overall spec in RFC 2460

45 Why Change IP? Address space exhaustion
Two level addressing (network and host) wastes space Network addresses used even if not connected to Internet Exponential growth of the Internet Single address per host Requirements for new types of service

46 Ethereal: A Packet Sniffer
Network sniffer or a protocol analyzer: Ethereal Ethereal: A Packet Sniffer A basic tool for observing messages exchanges between protocol entities It captures messages being sent/received from/by your computer Other packet sniffers are tcpdump, Zx Sniffer & AnalogX PacketMon




50 Summary IP enables host-to-host delivery of packets, unreliably
Allows a flexible approach Some assistance by ICMP when error Who looks at process-to-process delivery?? Transport layer (next lesson!) Read Stallings Chapter 18 Download RFC 791, a classic (1981)!

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