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IP datagram format
IP protocol version number header length (bytes) "type" of data max number remaining hops (decremented at each router) upper layer protocol to deliver payload to 32 bits ver head. type of len service length fragment 16-bit identifier flgs offset time to upper Internet live layer checksum 32 bit source IP address 32 bit destination IP address Options (if any) E.g. timestamp, record route taken, pecify list of routers to visit. total datagram length (bytes) for fragmentation/ reassembly
data (variable length, typically a TCP or UDP segment)
4: Network Layer
4b-1
IP Fragmentation & Reassembly
network links have MTU (max.transfer size) - largest possible link-level frame. r different link types, different MTUs large IP datagram divided ("fragmented") within net r one datagram becomes several datagrams r "reassembled" only at final destination r IP header bits used to identify, order related fragments
fragmentation: in: one large datagram out: 3 smaller datagrams
reassembly
4: Network Layer
4b-2
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IP Fragmentation and Reassembly
length ID fragflag offset =4000 =x =0 =0 One large datagram becomes several smaller datagrams length ID fragflag offset =1500 =x =1 =0 length ID fragflag offset =1500 =x =1 =1480 length ID fragflag offset =1040 =x =0 =2960
4: Network Layer
4b-3
ICMP: Internet Control Message Protocol
used by hosts, routers, gateways to communication network-level information r error reporting: unreachable host, network, port, protocol r echo request/reply (used by ping) network-layer "above" IP: r ICMP msgs carried in IP datagrams ICMP message: type, code plus first 8 bytes of IP datagram causing error
Type 0 3 3 3 3 3 3 4 8 9 10 11 12
Code 0 0 1 2 3 6 7 0 0 0 0 0 0
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
4: Network Layer
4b-4
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Routing in the Internet
The Global Internet consists of Autonomous Systems (AS) interconnected with each other:
r r r
Stub AS: small corporation Multihomed AS: large corporation (no transit) Transit AS: provider
Two-level routing:
r r
Intra-AS: administrator is responsible for choice Inter-AS: unique standard
4: Network Layer
4b-5
Internet AS Hierarchy
Intra-AS border (exterior gateway) routers
Inter-AS interior (gateway) routers
4: Network Layer 4b-6
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Intra-AS Routing
Also known as Interior Gateway Protocols (IGP) Most common IGPs:
r r r
RIP: Routing Information Protocol OSPF: Open Shortest Path First IGRP: Interior Gateway Routing Protocol (Cisco propr.)
4: Network Layer
4b-7
RIP ( Routing Information Protocol)
Distance vector algorithm Included in BSD-UNIX Distribution in 1982 Distance metric: # of hops (max = 15 hops)
r
Can you guess why?
Distance vectors: exchanged every 30 sec via Response Message (also called advertisement) Each advertisement: route to up to 25 destination nets
4: Network Layer
4b-8
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RIP (Routing Information Protocol)
z w A x D C
Destination Network
B
y
w y z x
Next Router
....
A B B -....
Num. of hops to dest.
2 2 7 1
....
Routing table in D
4: Network Layer 4b-9
Figure 13-3
Example of updating a routing table
RIP message from C Net2 4 Net3 8 Net6 4 Net8 3 Net9 5 Old routing table Net1 Net2 Net6 Net8 Net9 7 2 8 4 4 A C F E F
RIP message from C after increment Net2 5 Net3 9 Net6 5 Net8 4 Net9 6
Updating algorithm
New routing table Net1 7 Net2 5 Net3 9 Net6 5 Net8 4 Net9 4
A C C C E F
Rules
Net1: No news, don't change Net2: Same next hop, replace Net3: A new router, add Net6: Different next hop, new hop count smaller, replace Net8: Different next hop, new hop count the same, don't change Net9: Different next hop, new hop count larger, don't change
4: Network Layer 4b-10
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Figure 13-4
Initial routing table in a small autonomous system
14 1 -- 55 1 -- 14 1 -- 23 1 -- 78 1 -- A Net: 78 F Net: 23 Net: 14
B Net: 55 55 1 -- 66 1 -- C 78 1 -- 92 1 -- Net: 92
Net: 66
E 08 1 -- 23 1 --
Net: 08
08 1 -- 66 1 --
D
4: Network Layer 4b-11
Figure 13-5
Final routing tables for the previous figure
08 14 23 55 66 78 92
2 E 1 -- 1 -- 2 B 3 E 1 -- 2 F A
Net: 14
08 14 23 55 66 78 92
3 1 2 1 2 2 3
A -- A -- C A A
B Net: 55
08 14 23 55 66 78 92
2 2 3 1 1 3 4 C
D B D -- -- B B
Net: 78 F Net: 23
08 14 23 55 66 78 92
3 2 2 3 4 1 1
A A A A A -- --
Net: 92
Net: 66
E 08 14 23 55 66 78 92 1 2 1 3 2 2 3 -- A -- A D A A
Net: 08 08 14 23 55 66 78 92 1 3 2 2 1 3 4
D -- E E C -- E E
4: Network Layer 4b-12
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RIP: Link Failure and Recovery
If no advertisement heard after 180 sec --> neighbor/link declared dead r routes via neighbor invalidated r new advertisements sent to neighbors r neighbors in turn send out new advertisements (if tables changed) r link failure info quickly propagates to entire net r poison reverse used to prevent ping-pong loops (infinite distance = 16 hops)
4: Network Layer 4b-13
RIP Table processing
RIP routing tables managed by application-level process called route-d (daemon) advertisements sent in UDP packets, periodically repeated
4: Network Layer 4b-14
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RIP Table example (continued)
Router: giroflee.eurocom.fr
Destination -------------------127.0.0.1 192.168.2. 193.55.114. 192.168.3. 224.0.0.0 default Gateway Flags Ref Use Interface -------------------- ----- ----- ------ --------127.0.0.1 UH 0 26492 lo0 192.168.2.5 U 2 13 fa0 193.55.114.6 U 3 58503 le0 192.168.3.5 U 2 25 qaa0 193.55.114.6 U 3 0 le0 193.55.114.129 UG 0 143454
Three attached class C networks (LANs) Router only knows routes to attached LANs Default router used to "go up" Route multicast address: 224.0.0.0 Loopback interface (for debugging)
4: Network Layer 4b-15
Figure 13-10
Slow convergence
0 seconds
15 seconds
n 15 seconds
...
R1 Net1 Net2 R2
...
Rn Net n + 1
4: Network Layer 4b-16
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Figure 13-11
Hop counts
R1 Net1 Net2
R2
Total hop count should be less than 16
...
Rn Net n + 1
4: Network Layer 4b-17
OSPF (Open Shortest Path First)
"open": publicly available Uses Link State algorithm
r r r
LS packet dissemination
Topology map at each node Route computation using Dijkstra' algorithm s
OSPF advertisement carries one entry per neighbor router Advertisements disseminated to entire AS (via flooding)
4: Network Layer 4b-18
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OSPF "advanced" features (not in RIP)
Security: all OSPF messages authenticated (to prevent malicious intrusion); TCP connections used Multiple same-cost paths allowed (only one path RIP) in For each link, multiple cost metrics for different TOS (eg, satellite link cost set "low" for best effort; high for real time) Integrated uni- and multicast support:
r
Hierarchical OSPF in large domains.
Multicast OSPF (MOSPF) uses same topology data base as OSPF
4: Network Layer 4b-19
Hierarchical OSPF
4: Network Layer 4b-20
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Hierarchical OSPF
Two-level hierarchy: local area, backbone. r Link-state advertisements only in area r each nodes has detailed area topology; only know direction (shortest path) to nets in other areas. Area border routers: "summarize" distances to nets in own area, advertise to other Area Border routers. Backbone routers: run OSPF routing limited to backbone. Boundary routers: connect to other ASs.
4: Network Layer 4b-21
IGRP (Interior Gateway Routing Protocol)
CISCO proprietary; successor of RIP (mid 80s) Distance Vector, like RIP several cost metrics (delay, bandwidth, reliability, load etc) uses TCP to exchange routing updates Loop-free routing via Distributed Updating Alg. (DUAL) based on diffused computation
4: Network Layer 4b-22
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Inter-AS routing
4: Network Layer 4b-23
Internet inter-AS routing: BGP
BGP (Border Gateway Protocol): the de facto standard Path Vector protocol: r similar to Distance Vector protocol r each Border Gateway broadcast to neighbors (peers) entire path (I.e, sequence of ASs) to destination r E.g., Gateway X may send its path to dest. Z:
Path (X,Z) = X,Y1,Y2,Y3,... ,Z
4: Network Layer 4b-24
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Internet inter-AS routing: BGP
Suppose: gateway X send its path to peer gateway W W may or may not select path offered by X r cost, policy (don' route via competitors AS), loop t prevention reasons. If W selects path advertised by X, then: Path (W,Z) = w, Path (X,Z) Note: X can control incoming traffic by controling it route advertisements to peers: r e.g., don' want to route traffic to Z -> don' t t advertise any routes to Z
4: Network Layer 4b-25
Figure 13-44
Path vector packets
AS1
N1
AS2
R2 R1
N1 R1 AS1 N1 R2 AS2, AS1
N1 R3
AS3, AS2, AS1
R3
R4 AS4 AS3
4: Network Layer 4b-26
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Internet inter-AS routing: BGP
BGP messages exchanged using TCP. BGP messages: r OPEN: opens TCP connection to peer and authenticates sender r UPDATE: advertises new path (or withdraws old) r KEEPALIVE keeps connection alive in absence of UPDATES; also ACKs OPEN request r NOTIFICATION: reports errors in previous msg; also used to close connection
4: Network Layer 4b-27
Why different Intra- and Inter-AS routing ? Policy:
Inter-AS: admin wants control over how its traffic routed, who routes through its net. Intra-AS: single admin, so no policy decisions needed
Scale:
hierarchical routing saves table size, reduced update traffic Performance: Intra-AS: can focus on performance Inter-AS: policy may dominate over performance
4: Network Layer 4b-28
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Router Architecture Overview
Two key router functions:
run routing algorithms/protocol (RIP, OSPF, BGP) switching datagrams from incoming to outgoing link
4: Network Layer 4b-29
Input Port Functions
Physical layer: bit-level reception Data link layer: e.g., Ethernet see chapter 5
Decentralized switching:
given datagram dest., lookup output port
using routing table in input port memory goal: complete input port processing at ` line speed' queuing: if datagrams arrive faster than forwarding rate into switch fabric
4: Network Layer 4b-30
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Input Port Queuing
Fabric slower that input ports combined -> queueing may occur at input queues Head-of-the-Line (HOL) blocking: queued datagram at front of queue prevents others in queue from moving forward queueing delay and loss due to input buffer overflow!
4: Network Layer 4b-31
Three types of switching fabrics
4: Network Layer 4b-32
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Switching Via Memory
First generation routers: packet copied by system' (single) CPU s speed limited by memory bandwidth (2 bus crossings per datagram)
Input Port Memor...