Other Developments: CIDR

Transcription

1 Other Developments: CIDR CIDR (classless Inter domain routing) Too many small networks requiring multiple class C addresses Running out of class B addresses, not enough nets in class A Assign contiguous block of class C addresses Use CIDR address mask to aggregate Ex , , Send it to R3

2 CIDR Original addressing schemes (class-based): 32 bits divided into 2 parts: 0 Class A Class B Class C network host network 8 host 110 network host Class C address has max of 254 hosts Not enough for many organizations Too many class C addresses huge routing tables Classless Inter domain routing (CIDR) CIDR introduced to solve 2 problems: exhaustion of IP address space size and growth rate of routing table ~2 million nets 256 hosts

3 Supernetting Example: an organization needs 500 addresses. A single class C address not enough (256 hosts). Instead a class B address is allocated. (~64K hosts). a huge waste. CIDR allows multiple Class C addresses to be assigned to an organization but still occupy one entry in the routing table < ,2> this is used to specify that 2 network addresses and is allocated to an organization Typically the starting address with a CIDR mask that indicates the common most significant bits for the ranges is used to specify the block of addresses /notation / and are assigned

4 Reducing Routing Table Size Without CIDR: service provider Routing table With CIDR: service provider /16 Routing table

5 CIDR: Classless Inter-Domain Routing Address format <IP address/prefix P>. The prefix denotes the upper P bits of the IP address. Can be used to specify arbitrary blocks of addresses Say an ISP has , 192,5.49.0, ,192, 51.0 then the IP address advertised will be /22 An ISP can obtain a block of addresses and partition this further to its customers Say an ISP has /24 address (256 addresses) He has another customer who needs only 4 addresses from then that block can be specified as /30

6 CIDR Class C prefix CIDR Block Prefix /27 /26 /25 /24 /23 /22 /21 # Equivalent Class C 1/8th of a Class C 1/4th of a Class C 1/2 of a Class C 1 Class C 2 Class C 4 Class C 8 Class C # of Host Addresses 32 hosts 64 hosts 128 hosts 256 hosts 512 hosts 1,024 hosts 2,048 hosts

7 CIDR allocation Address allocation Consider the block of 2048 class C network numbers beginning with CIDR mask would be Range of addresses would be to 192.x.y (what is x and y?) Assume this service provider connects three clients "C1" requiring fewer than 2048 addresses (8 class C networks) "C2" requiring fewer than 4096 addresses (16 class C networks) "C3" requiring fewer than 1024 addresses (4 class C networks) The service provider allocates its address space as follows: C1: allocate through ? What is the CIDR mask C2: allocate through ? What is the CIDR mask C3: allocate through ?. What is the CIDR mask

8 BGP protocol (Chapter 4.5.2) BGP uses TCP as its transport protocol, on port 179. On connection start, BGP peers exchange complete copies of their routing tables, which can be quite large. However, only changes (deltas) are then exchanged, which makes long running BGP sessions more efficient than shorter ones. Four Basic messages: Open: Establishes BGP session (uses TCP port #179) Notification: Report unusual conditions Update: Inform neighbor of new routes that become active Inform neighbor of old routes that become inactive Keepalive: Inform neighbor that connection is still viable

9 OPEN Message During session establishment, two BGP speakers exchange their AS numbers BGP identifiers (usually one of the router s IP addresses) A BGP speaker has option to refuse a session Select the value of the hold timer: maximum time to wait to hear something from other end before assuming session is down. authentication information (optional)

10 NOTIFICATION and KEEPALIVE Messages NOTIFICATION Indicates an error terminates the TCP session gives receiver an indication of why BGP session terminated Examples: header errors, hold timer expiry, bad peer AS, bad BGP identifier, malformed attribute list, missing required attribute, AS routing loop, etc. KEEPALIVE protocol requires some data to be sent periodically. If no UPDATE to send within the specified time period, then send KEEPALIVE message to assure partner that connection still alive

11 UPDATE Message used to either advertise and/or withdraw prefixes path attributes: list of attributes that pertain to ALL the prefixes in the Reachability Info field FORMAT: Withdrawn routes length (2 octets) Withdrawn routes (variable length) Total path attributes length (2 octets) Path Attributes (variable length) Reachability Information (variable length)

12 Advertising a prefix When a router advertises a prefix to one of its BGP neighbors: information is valid until first router explicitly advertises that the information is no longer valid BGP does not require routing information to be refreshed if node A advertises a path for a prefix to node B, then node B can be sure node A is using that path itself to reach the destination.

13 PATH ATTRIBUTES ORIGIN(TYPE CODE=1): Who originated the announcement? Where was a prefix injected into BGP? Manually configured, directly connected, by other intra-routing protocols AS-PATH (TYPE CODE =2) a list of AS s through which the announcement for a prefix has passed each AS prepends its AS # to the AS-PATH attribute when forwarding an announcement useful to detect and prevent loops AS length can be used to select among routes unless a LOCAL PREF attribute overrides AS3, AS2, AS1 AS3 AS2, AS1 AS /16 AS1 AS2

14 Attribute: Local Preference (type code = 5) Used to indicate preference among multiple paths for the same prefix anywhere in the internet. The higher the value the more it is preferred AS2 Default value is 100 Local to the AS Often used to select a specific exit point for a particular destination Used when AS path lengths are Prefix, as-path same AS1 AS4 BGP table at AS4: localpref /20 AS3

15 Use of local pref Local-pref= /8 OC3 T /8

16 Attribute: Multi-Exit Discriminator (MED) (code=4) when AS s interconnected via 2 or more links AS path length are same AS announcing prefix sets MED enables AS2 to indicate its preference (lower MED is better) AS receiving prefix uses MED to select link a way to specify how close a prefix is to the link it is announced on Link B AS1 Link A MED=500 MED=100 AS2 AS3 AS4

17 Use of MED /8 MED = /12 MED = /8 MED = /12 MED = 50 L1 L2 Router C