Homework #3 is out! Due Tuesday.

Transcription

1 Homework #3 is out! Due Tuesday.

2 Internet and IP Protocol IP (Internet Protocol) ICMP (Internet Control Message Protocol)

3 IP Protocol Stack IP (Internet Protocol) is a Network Layer Protocol TCP UDP Transport Layer IP ICMP IGMP Network Layer ARP Network Access RARP Link Layer Media IP s current version is Version 4 (IPv4)

4 IP Overview IP is the highest layer protocol that is implemented at both routers and hosts Offers send and deliver primitives Application Application protocol Application TCP TCP protocol TCP IP IP IP protocol IP protocol IP IP protocol IP Network Access Data Link Network Access Network Access Data Link Network Access Network Access Data Link Network Access Host Router Router Host

5 IP Datagrams IP breaks data into Datagrams limited to 64K bytes each Datagrams prevent long flows from monopolizing the network for a long time In future gigabit networks the 64K limit can be increased Example: For how long will a 1M byte datagram tie up a T1 line (1.5Mbps)? How about a 1 Gbps optical fiber? Datagrams can further be fragmented depending on packet size of the data link layer (e.g., Ethernet)

6 IP Send Send { source address, destination address, protocol (e.g., TCP, UDP, ICMP), type of service datagram identification, and (don t) fragment flag, time to live, data length, options, data }

7 IP Datagram Service IP provides an unreliable and connectionless service ( datagram service ). Unreliable: IP does not guarantee that a transmitted packet will be delivered Connectionless: Each packet ( datagram ) is handled independently. IP is not aware that packets between hosts may be sent in a logical sequence Consequences of an unreliable, connectionless service Lost Packets Packets are delivered out-of-sequence Duplicated Packets

8 IP Datagram Format 20 bytes Header Size 2 4 * 32 bit-words = 60 bytes 20 bytes Total Length 2 16 bytes = bytes Total Length (in bytes) (16 bits) Type of Service/TOS (8 bits) header length version (4 bits) Fragment Offset (13 bits) flags (3 bits) Identification (16 bits) Header Checksum (16 bits) Protocol (8 bits) TTL Time-to-Live (8 bits) Source IP address (32 bits) >= five 32-bit words Destination IP address (32 bits) Options (if any, <40 bytes) DATA bit word

9 IP Header Features Version: Coexistence of multiple versions (smooth transition) Identification, Fragment offset: 64K datagrams, fragmentation ToS: Allows prioritized or differentiated services TTL: Loop suppression

10 IP Header (Continued) Protocol: Specifies the higher-layer protocol. Used for demultiplexing to higher layers. Application TCP UDP ICMP IGMP IP Header checksum: verifies correctness of header.

11 IP Header (Continued) Options: Security restrictions Record Route: each router that processes the packet adds its IP address to the header. Timestamp: each router that processes the packet adds its IP address and time to the header. (loose) Source Routing: specifies a list of routers that must be traversed. (strict) Source Routing: specifies a list of the only routers that can be traversed. Padding: ensures that header ends on a 4-byte boundary

12 IP Fragmentation Host A sends a large IP datagram to Host B. Any Problem with that? Ethernet FDDI Ring Host A Router Host B MTUs: FDDI: 4352 Ethernet: 1500 IP router splits the datagram into several fragments (=Fragmentation)

13 What s involved in Fragmentation? The following fields in the IP header are involved: version (4 bits) header length Identification Type of Service/TOS Total Length (in bytes) flags Fragment Offset TTL Time-to-Live (8 bits) Protocol (8 bits) Header Checksum (16 bits)... Identification is the same in all fragments. Flags contains a more fragments bit (There is also a don t fragment bit that can be set) Fragment offset contains the offset of current fragment in the original datagram Total length is changed by fragmentation

14 IP Addresses Each router or host on the Internet has a unique global address, called the IP address An IP address: - is 32 bits long. - encodes a network number and a host number IP addresses are written in a dotted decimal notation: means in 1st Byte in 2nd Byte in 3rd Byte in 4th Byte

15 Internet Address Classes IP distinguishes 5 classes of addresses. Class A 0 network id host 7 bits 24 bits Class B 1 network id 0 host 14 bits 16 bits 1 0 host Class C 1 network id 21 bits 8 bits Class D multicast group id 28 bits Class E (reserved for future use) 27 bits

16 Hierarchical Routing The scheme allows 2-level hierarchical routing Routing table entries fall into two categories (remote network) : forwarding address (my network, host) : forwarding address

17 Trade-off of Address Classes There are a total of 2 32 = 4,294,967,296 IP addresses. The network numbers are assigned by InterNIC (Network Information Center). Class A: Class B: Class C: 7 bits for netid (only 128 Class A networks) each net can have 16 million (2 24 ) hosts. 14 bits for netid (about 16,000 Class B networks) About 65,000 (2 14 ) hosts per network 21 bits for netid (about 2 million Class C networks) Only 255 hosts per network

18 The Sparse Denomination Problem Imagine a currency with denominations $1 (class C), $250 (class B), and $62,000 (class A). Class C is too small, class A is too big, so most use class B, but don t fully utilize it. Internet runs out of IP addresses! One temporary solution: subnets

19 Subnets Networks are split into subnets Routers outside the network do not have to know subnet details Routers inside a network know about its subnets (they must know the length of the subnet field) 10 Network Subnet Host Subnet mask is used to determine subnet field

20 Hierarchical Routing with Subnets The scheme allows a 3-level hierarchy Routing table entries fall into three categories: (remote network) : forwarding address (my network, subnet) : forwarding address (my network, my subnet, host) : forwarding address

21 Classless Inter-Domain Routing (CIDR) Generalizes the subnet architecture Imposes no restrictions on classes Allows hierarchies of arbitrary length Performs longest prefix matching at routers Example: Match ?

22 ICMP The Internet Control Message Protocol TCP UDP IP ICMP IGMP ARP Network Access RARP Media Provides error reporting Transport Layer Network Layer Link Layer

23 ICMP The Internet Control Message Protocol (ICMP) is the protocol used for error and control messages in the Internet ICMP provides an error reporting mechanism of routers to the sources All ICMP packets are encapsulated as IP datagrams The packet format is simple: Type (8 bits) Code (8 bits) Checksum (16 bits) (additional information dependent on Type and Code) bit word

24 Types of ICMP Packets Many ICMP packet types exist, each with its own format. A Selection: Type Field: Message Type: 0 Echo Reply 3 Destination Unreachable 4 Source Quench 5 Redirect (Change Route) 8 Echo Request 11 Time Exceeded 12 Parameter Problem in Datagram 14 Timestamp Request

25 ICMP Message Types ICMP messages are either query messages or error messages. ICMP query messages: Echo request / Echo reply Router advertisement / Router solicitation Timestamp request / Timestamp reply Address mask request / Address mask reply ICMP error messages: Host unreachable Source quench Time Exceeded Parameter Problem

26 The PING program PING (=Packet IntetNet Groper) is a program that utilizes the ICMP echo request and echo reply messages PING is used to verify if a certain hosts is up and running. It is used extensively for fault isolation in IP networks

27 Echo Request and Reply PING s are handled directly by the kernel. Each Ping is translated into an ICMP Echo Request The Ping ed host responds with an ICMP Echo Reply ICMP ECHO REQUEST AIDA MNG ICMP ECHO REPLY

28 Format of Echo Request and Reply Type (=0 or 8) Code (=0) Checksum identifier sequence number optional Identifier is set to process Id of querying process Sequence number is incremented for each returning packet

29 ARP (Address Resolution Protocol) Converts IP addresses to data link addresses TCP UDP Transport Layer IP ICMP IGMP Network Layer ARP Network Access RARP Link Layer Media

30 ARP Converts IP addresses into Data Link (e.g., Ethernet) addresses Protocol: Broadcasts a message Does anyone know the data link address of this IP? Machine with the given IP replies with its address Router can reply with its own address if IP is remote

31 ARP Example 1 Consider machine A sending an IP packet to B over an Ethernet Case 1: A and B on the same Ethernet A broadcasts Does anyone know this IP? B replies with its hardware address A puts that address in the destination field of its Ethernet header and sends packet

32 ARP Example 2 Case 2: A and B on different networks A locally broadcasts Does anyone know this IP? Local router sees that IP is remote; responds with its own hardware address A puts that address in the destination field of its Ethernet header and sends packet (i.e. to router) Router inspects IP address and forwards the packet to the right destination network Destination router locally broadcasts Does anyone know this IP? B replies with its hardware address Router sends packet to B

33 ARP Optimizations ARP can reduce the number of requests sent using some optimizations Caching IP to data link address mappings Machines can announce their hardware addresses when they boot. Others will cache it. Cache entries must expire to allow replacing stale entires

34 RARP Reverse ARP TCP UDP Transport Layer IP ICMP IGMP Network Layer ARP Network Access RARP Link Layer Media Determines the IP given the data link address

35 RARP (Reverse ARP) Gives the IP of a machine given its hardware address Useful for booting diskless workstations Protocol Machine (e.g., one that is booting) locally broadcasts Here s my hardware address. What is my IP? RARP server replies with IP Machines with disks can of course store their IP in configuration files.