1. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 1 Switching and Forwarding Network Layer Part I Switching and Forwarding –Generic Router Architecture –Forwarding Tables: Bridges/Layer 2 Switches; VLAN Routers and Layer 3 Switches Forwarding in Layer 3 (Network Layer) –Network Layer Functions –Network Service Models: VC vs. Datagram ATM and IP Datagram Forwarding –IP Addressing Network vs. host: address blocks, longest prefix matching Address allocation and DHCP –IP Datagram Forwarding Model and ARP Protocol –IP and ICMP Protocols, IP Fragmentation and Re-assembly Readings: Textbook: Chapter 4: Section 4.1;

2. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 2 Routing & Forwarding: Logical View of a Router A E D CB F 2 2 1 3 1 1 2 5 3 5

3. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 3 IP Addressing: Basics Globally unique (for “public” IP addresses) IP address: 32-bit identifier for host, router interface Interface: connection between host/router and physical link –router’s typically have multiple interfaces –host may have multiple interfaces –IP addresses associated with each interface Dot notation (for ease of human reading) 223.1.1.1 = 11011111 00000001 00000001 00000001 223 111

4. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 4 IP Addressing: Network vs. Host Two-level hierarchy –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 223.1.1.1 223.1.1.3 223.1.1.4 223.1.2.2 223.1.2.1 223.1.2.6 223.1.3.2 223.1.3.1 223.1.3.27 223.1.1.2 223.1.7.0 223.1.7.1 223.1.8.0223.1.8.1 223.1.9.1 223.1.9.2 multi-access LAN point-to-point link

5. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 5 “Classful” IP Addressing 32 bits 0 network host 10 network host 110 networkhost 1110 multicast address A B C D class 1.0.0.0 to 127.255.255.255 128.0.0.0 to 191.255.255.255 192.0.0.0 to 223.255.255.255 224.0.0.0 to 239.255.255.255 7 7152331 Disadvantage: 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

6. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 6 Classless Addressing: CIDR CIDR: Classless InterDomain Routing Network portion of address is of arbitrary length Addresses allocated in contiguous blocks –Number of addresses assigned always power of 2 Address format: a.b.c.d/x –x is number of bits in network portion of address 11001000 00010111 00010000 00000000 network part host part 200.23.16.0/23

7. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 7 Representation of Address Blocks “Human Readable” address format: a.b.c.d/x –x is number of bits in network portion of address, the network portion is also called the network prefix machine representation of a network (addr block): using a combination of –first IP of address blocks of the network –network mask ( x “1”’s followed by 32-x “0”’s 11001000 00010111 00010000 00000000 network mask: network w/ address block: 200.23.16.0/23 11111111 11111111 11111110 00000000 first IP address of address block:

8. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 8 More Examples Three Address Blocks: First IP address: 11001000 00010111 00010000 00000000 Network mask: 11111111 11111111 11111000 00000000 First IP address: 11001000 00010111 00011000 00000000 Last IP address: 11001000 00010111 00011000 11111111 what is the network prefix? 11001000 00010111 00011000 First IP address : 11001000 00010111 00011001 00000000 Last IP address: 11001000 00010111 00011111 11111111 what is the network prefix? 11001000 00010111 00011 Given an IP address, which network (or address block) does it belong to? Example 2: 11001000 00010111 00011000 10101010 Example 1: 11001000 00010111 00010110 10100001 Use longest prefix matching!

9. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 9 Another Example Consider a datagram network using 32-bit host addresses, suppose a router has four links, numbered 0 through 3, and packets are to be forwarded to the link interfaces as follows: Destination Addr RangeLink Interface 11100000 00000000 00000000 00000000 through 0 11100000 11111111 11111111 11111111 11100001 00000000 00000000 00000000 through 1 11100001 00000000 11111111 11111111 11100001 00000001 00000000 00000000 through 2 11100001 11111111 11111111 11111111 O.W. 3 Provide the forwarding table – a table containing the network prefix and the outgoing interface.

10. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 10 IP Addresses: How to Get One? Q: How does host get IP address? “static” assigned: i.e., hard-coded in a file –Wintel: control-panel->network->configuration->tcp/ip- >properties –UNIX: /etc/rc.config Dynamically assigned: using DHCP (Dynamic Host Configuration Protocol) – dynamically get address from a server –“plug-and-play”

11. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 11 DHCP: Dynamic Host Configuration Protocol Goal: allow host to dynamically obtain its IP address from network DHCP server when it joins network Can renew its lease on address in use Allows reuse of addresses (only hold address while connected as “on”) Support for mobile users who want to join network (more shortly) DHCP overview: –host broadcasts “DHCP discover” msg –DHCP server responds with “DHCP offer” msg –host requests IP address: “DHCP request” msg –DHCP server sends address: “DHCP ack” msg

12. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 12 DHCP Client-Server Scenario 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 A B E DHCP server arriving DHCP client needs address in this network

13. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 13 DHCP Client-Server Scenario DHCP server: 223.1.2.5 arriving client time DHCP discover src : 0.0.0.0, 68 dest.: 255.255.255.255,67 yiaddr: 0.0.0.0 transaction ID: 654 DHCP offer src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 654 Lifetime: 3600 secs DHCP request src: 0.0.0.0, 68 dest:: 255.255.255.255, 67 yiaddrr: 223.1.2.4 transaction ID: 655 Lifetime: 3600 secs DHCP ACK src: 223.1.2.5, 67 dest: 255.255.255.255, 68 yiaddrr: 223.1.2.4 transaction ID: 655 Lifetime: 3600 secs

14. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 14 IP Addresses: How to Get One? … Q: How does a network get network part of IP addr? A: gets an allocated portion of its provider ISP’s address space ISP's block 11001000 00010111 00010000 00000000 200.23.16.0/20 Organization 0 11001000 00010111 00010000 00000000 200.23.16.0/23 Organization 1 11001000 00010111 00010010 00000000 200.23.18.0/23 Organization 2 11001000 00010111 00010100 00000000 200.23.20.0/23... ….. …. …. Organization 7 11001000 00010111 00011110 00000000 200.23.30.0/23

15. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 15 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

16. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 16 NAT: Network Address Translation 10.0.0.1 10.0.0.2 10.0.0.3 10.0.0.4 138.76.29.7 local network (e.g., home network) 10.0.0/24 rest of Internet Datagrams with source or destination in this network have 10.0.0/24 address for source, destination (as usual) All datagrams leaving local network have same single source NAT IP address: 138.76.29.7, different source port numbers 10.0.0.0/8 has been reserved for private networks!

17. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 17 NAT: Network Address Translation Motivation: local network uses just one IP address as far as outside world is concerned: –no need to be allocated range of addresses from ISP: - just one IP address is used for all devices –can change addresses of devices in local network without notifying outside world –can change ISP without changing addresses of devices in local network –devices inside local net not explicitly addressable, visible by outside world (a security plus).

18. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 18 NAT: Network Address Translation Implementation: NAT router must: –outgoing datagrams: replace (source IP address, port #) of every outgoing datagram to (NAT IP address, new port #)... remote clients/servers will respond using (NAT IP address, new port #) as destination addr. –remember (in NAT translation table) every (source IP address, port #) to (NAT IP address, new port #) translation pair –incoming datagrams: replace (NAT IP address, new port #) in dest fields of every incoming datagram with corresponding (source IP address, port #) stored in NAT table

19. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 19 NAT: Network Address Translation 10.0.0.1 10.0.0.2 10.0.0.3 S: 10.0.0.1, 3345 D: 128.119.40.186, 80 1 10.0.0.4 138.76.29.7 1: host 10.0.0.1 sends datagram to 128.119.40, 80 NAT translation table WAN side addr LAN side addr 138.76.29.7, 5001 10.0.0.1, 3345 …… S: 128.119.40.186, 80 D: 10.0.0.1, 3345 4 S: 138.76.29.7, 5001 D: 128.119.40.186, 80 2 2: NAT router changes datagram source addr from 10.0.0.1, 3345 to 138.76.29.7, 5001, updates table S: 128.119.40.186, 80 D: 138.76.29.7, 5001 3 3: Reply arrives dest. address: 138.76.29.7, 5001 4: NAT router changes datagram dest addr from 138.76.29.7, 5001 to 10.0.0.1, 3345

20. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 20 NAT: Network Address Translation 16-bit port-number field: –60,000 simultaneous connections with a single LAN-side address! NAT is controversial: –routers should only process up to layer 3 –violates end-to-end argument NAT possibility must be taken into account by app designers, eg, P2P applications –address shortage should instead be solved by IPv6

21. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 21 IP Forwarding & IP/ICMP Protocol Network layer routing table Routing protocols path selection RIP, OSPF, BGP IP protocol addressing conventions packet handling conventions ICMP protocol error reporting router “signaling” Transport layer: TCP, UDP Data Link layer (Ethernet, WiFi, PPP, …) Physical Layer (SONET, …)

22. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 22 IP Service Model and Datagram Forwarding Connectionless (datagram-based) –Each datagram carries source and destination Best-effort delivery (unreliable service) –packets may be lost –packets can be delivered out of order –duplicate copies of a packet may be delivered –packets can be delayed for a long time Forwarding and IP address –forwarding based on network id Delivers packet to the appropriate network Once on destination network, direct delivery using host id IP destination-based next-hop forwarding paradigm –Each host/router has IP forwarding table Entries like

23. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 23 IP Datagram Format ver length 32 bits data (variable length, typically a TCP or UDP segment) 16-bit identifier Internet checksum time to live 32 bit source IP address IP protocol version number header length (32-bit words) max number remaining hops (decremented at each router) for fragmentation/ reassembly total datagram length (bytes) upper layer protocol to deliver payload to head. len type of service “type” of data flgs fragment offset upper layer 32 bit destination IP address Options (if any) E.g. timestamp, record route taken, specify list of routers to visit. how much overhead with TCP? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer overhead

24. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 24 IP Datagram Forwarding Model IP datagram: misc fields source IP addr dest IP addr data datagram remains unchanged, as it travels source to destination addr fields of interest here 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 A B E Dest. Net. next router Nhops 223.1.1 1 223.1.2 223.1.1.4 2 223.1.3 223.1.1.4 2 forwarding table in A

25. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 25 IP Forwarding Table 4 billion possible entries! (in reality, far less, but can still have millions of “routes”) forwarding table entry format destination network next-hop (IP address) link interface (1 st IP address, network mask ) 11001000 00010111 00010000 00000000, 200.23.16.1 0 11111111 11111111 11111000 00000000 11001000 00010111 00011000 00000000, - (direct) 1 11111111 11111111 11111111 00000000 11001000 00010111 00011001 00000000, 200.23.25.6 2 11111111 11111111 11111000 00000000 otherwise 128.30.0.1 3

26. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 26 Forwarding Table Lookup using Longest Prefix Matching Prefix Match Next Hop Link Interface 11001000 00010111 00010 200.23.16.1 0 11001000 00010111 00011000 - 1 11001000 00010111 00011 200.23.25.6 2 otherwise 128.30.0.1 3 DA: 11001000 00010111 00011000 10101010 Examples DA: 11001000 00010111 00010110 10100001 Which interface?

27. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 27 IP Forwarding: Destination in Same Net Starting at A, send IP datagram addressed to B: look up net. address of B in forwarding table find B is on same net. as A link layer will send datagram directly to B inside link-layer frame –B and A are directly connected misc fields 223.1.1.1223.1.1.3 data Dest. Net. next router Nhops 223.1.1 1 223.1.2 223.1.1.4 2 223.1.3 223.1.1.4 2 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 A B E forwarding table in A

28. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 28 IP Datagram Forwarding on Same LAN: Interaction of IP and data link layers 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 A B E Starting at A, given IP datagram addressed to B: look up net. address of B, find B on same net. as A link layer send datagram to B inside link-layer frame B’s MAC addr A’s MAC addr A’s IP addr B’s IP addr IP payload datagram frame frame source, dest address datagram source, dest address

29. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 29 MAC (Physical) Addresses -- Revisited used to get frames from one interface to another physically- connected interface (same physical network, i.e., p2p or LAN) 48 bit MAC address (for most LANs) –fixed for each adaptor, burned in the adapter ROM –MAC address allocation administered by IEEE 1 st bit: 0 unicast, 1 multicast. all 1’s : broadcast MAC flat address -> portability –can move LAN card from one LAN to another MAC addressing operations on a LAN: – each adaptor on the LAN “sees” all frames – accept a frame if dest. MAC address matches its own MAC address – accept all broadcast (MAC= all1’s) frames – accept all frames if set in “promiscuous” mode – can configure to accept certain multicast addresses (first bit = 1)

30. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 30 MAC vs. IP Addresses 32-bit IP address: network-layer address, logical – i.e., not bound to any physical device, can be re-assigned IP hierarchical address NOT portable –depends on IP network to which an interface is attached –when move to another IP network, IP address re-assigned used to get IP packets to destination IP network –Recall how IP datagram forwarding is performed IP network is “virtual,” actually packet delivery done by the underlying physical networks –from source host to destination host, hop-by-hop via IP routers –over each link, different link layer protocol used, with its own frame headers, and source and destination MAC addresses Underlying physical networks do not understand IP protocol and datagram format!

31. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 31 ARP: Address Resolution Protocol Each IP node (host, router) on LAN has ARP table ARP Table: IP/MAC address mappings for some LAN nodes – timer: time after which address mapping will be forgotten (typically 15 min) Question: how to determine MAC address of B knowing B’s IP address?

32. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 32 ARP Protocol A wants to send datagram to B, and A knows B’s IP address. A looks up B’s MAC address in its ARP table Suppose B’s MAC address is not in A’s ARP table. A broadcasts (why?) ARP query packet, containing B's IP address –all machines on LAN receive ARP query B receives ARP packet, replies to A with its (B's) MAC address –frame sent to A’s MAC address (unicast) A caches (saves) IP-to-MAC address pair in its ARP table until information becomes old (times out) –soft state: information that times out (goes away) unless refreshed ARP is “plug-and-play”: –nodes create their ARP tables without intervention from net administrator

33. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 33 ARP Messages Hardware Address Type: e.g., Ethernet Protocol address Type: e.g., IP Operation: ARP request or ARP response

34. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 34 ARP Request & Response Processing The requester broadcasts ARP request The target node unicasts (why?) ARP reply to requester –With its physical address –Adds the requester into its ARP table (why?) On receiving the response, requester –updates its table, sets timer Other nodes upon receiving the ARP request –Refresh the requester entry if already there –No action otherwise (why?) Some questions to think about: –Shall requester buffer IP datagram while performing ARP? –What shall requester do if never receive any ARP response?

35. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 35 ARP Operation Illustration

36. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 36 IP Forwarding: Destination in Diff. Net Starting at A, dest. E: look up network address of E in forwarding table E on different network –A, E not directly attached routing table: next hop router to E is 223.1.1.4 link layer sends datagram to router 223.1.1.4 inside link- layer frame datagram arrives at 223.1.1.4 continued….. misc fields 223.1.1.1223.1.2.3 data Dest. Net. next router Nhops 223.1.1 1 223.1.2 223.1.1.4 2 223.1.3 223.1.1.4 2 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 A B E forwarding table in A

37. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 37 IP Forwarding: Destination in Diff. Net … Arriving at 223.1.4, destined for 223.1.2.2 look up network address of E in router’s forwarding table E on same network as router’s interface 223.1.2.9 –router, E directly attached link layer sends datagram to 223.1.2.2 inside link-layer frame via interface 223.1.2.9 datagram arrives at 223.1.2.2!!! (hooray!) misc fields 223.1.1.1223.1.2.3 data Dest. Net router Nhops interface 223.1.1 - 1 223.1.1.4 223.1.2 - 1 223.1.2.9 223.1.3 - 1 223.1.3.27 223.1.1.1 223.1.1.2 223.1.1.3 223.1.1.4 223.1.2.9 223.1.2.2 223.1.2.1 223.1.3.2 223.1.3.1 223.1.3.27 A B E forwarding table in router

38. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 38 Forwarding to Another LAN: Interaction of IP and Data Link Layer walkthrough: send datagram from A to B via R assume A knows B IP address Two ARP tables in router R, one for each IP network (LAN) In routing table at source host, find router 111.111.111.110 In ARP table at source, find MAC address E6-E9-00-17-BB-4B, etc A R B

39. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 39 A creates datagram with source A, destination B A uses ARP to get R’s MAC address for 111.111.111.110 A creates link-layer frame with R's MAC address as dest, frame contains A-to-B IP datagram A’s data link layer sends frame R’s data link layer receives frame R removes IP datagram from Ethernet frame, sees its destined to B R uses ARP to get B’s physical layer address R creates frame containing A-to-B IP datagram sends to B AR B

40. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 40 IP Datagram Format Again ver length 32 bits data (variable length, typically a TCP or UDP segment) 16-bit identifier Internet checksum time to live 32 bit source IP address IP protocol version number header length (bytes) max number remaining hops (decremented at each router) for fragmentation/ reassembly total datagram length (bytes) upper layer protocol to deliver payload to head. len type of service “type” of data flgs fragment offset upper layer 32 bit destination IP address Options (if any) E.g. timestamp, record route taken, specify list of routers to visit. how much overhead with TCP? 20 bytes of TCP 20 bytes of IP = 40 bytes + app layer overhead

41. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 41 Fields in IP Datagram IP protocol version: current version is 4, IPv4, new: IPv6 Header length: number of 32-bit words in the header Type of Service: – 3-bit priority,e.g, delay, throughput, reliability bits, … Total length: including header (maximum 65535 bytes) Identification: all fragments of a packet have same identification Flags: don’t fragment, more fragments Fragment offset: where in the original packet (count in 8 byte units) Time to live: maximum life time of a packet Protocol Type: e.g., ICMP, TCP, UDP etc IP Option: non-default processing, e.g., IP source routing option, etc.

42. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 42 IP Fragmentation & Reassembly: Why network links have MTU (maximum transmission unit) - largest possible data gram. –different link types, different MTUs large IP datagram divided (“fragmented”) within net –one datagram becomes several datagrams –“reassembled” only at final destination –IP header bits used to identify, order related fragments fragmentation: in: one large datagram out: 3 smaller datagrams reassembly

43. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 43 IP Fragmentation & Reassembly: How An IP datagram is chopped by a router into smaller pieces if –datagram size is greater than network MTU –Don’t fragment option is not set Each datagram has unique datagram identification –Generated by source hosts –All fragments of a packet carry original datagram id All fragments except the last have more flag set –Fragment offset and Length fields are modified appropriately Fragments of IP packet can be further fragmented by other routers along the way to destination ! Reassembly only done at destination host (why?) –Use IP datagram id, fragment offset, fragment flags. Length

44. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 44 IP Fragmentation and Reassembly: Exp ID =x offset =0 fragflag =0 length =4000 ID =x offset =0 fragflag =1 length =1500 ID =x offset =185 fragflag =1 length =1500 ID =x offset =370 fragflag =0 length =1040 One large datagram becomes several smaller datagrams Example 4000 byte datagram MTU = 1500 bytes

45. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 45 ICMP: Internet Control Message Protocol used by hosts, routers, gateways to communicate network-level information –error reporting: unreachable host, network, port, protocol –echo request/reply (used by ping) network-layer “above” IP: –ICMP msgs carried in IP datagrams ICMP message: type, code plus first 8 bytes of IP datagram causing error Type Code description 0 0 echo reply (ping) 3 0 dest. network unreachable 3 1 dest host unreachable 3 2 dest protocol unreachable 3 3 dest port unreachable 3 6 dest network unknown 3 7 dest host unknown 4 0 source quench (congestion control - not used) 8 0 echo request (ping) 9 0 route advertisement 10 0 router discovery 11 0 TTL expired 12 0 bad IP header

46. Csci 183/183W/232: Computer Networks Forwarding and Routing Network Layer Part I 46 ICMP Message Transport & Usage ICMP messages carried in IP datagrams Treated like any other datagrams –But no error message sent if ICMP message causes error Message sent to the source –8 bytes of the original header included ICMP Usage (non-error, informational): Examples –Testing reachability: ICMP echo request/reply ping –Tracing route to a destination: Time-to-live field traceroute –Path MTU discovery Don’t fragment bit – IP direct (for hosts only): inform hosts of better routes