Outline

BGP

• The current version of BGP is version 4.
• When configuring BGP, the administrator of each AS picks at least one node to be a BGP speaker, this is essentially the spokesperson for the entire AS.
• The BGP speaker establishes BGP sessions to other BGP speakers in other AS's.
• BGP runs on top of TCP on port 179, and counts on the fact that TCP provides reliable transport of data.
• A BGP speaker's neighbors are manually configured.
• A BGP speaker can be viewed as belonging to more than one AS: It has its home AS as well as the AS of the BGP speakers it can talk with. These might not be the same. It has to belong to the latter so that TCP can be used between speakers without a chicken (use to BGP to route) and the egg (BGP needs route to work) problem .
• Speakers send keep-alive messages every 60 seconds or so to keep the connections to their neighbors.
• BGP sessions are used to exchange reachability information among AS's
• When a speaker comes alive it sends to all its BGP neighbors the networks it knows of; thereafter, it only sends updates.
• In addition to BGP speakers, the AS has one or more border gateways, these are routers through which packets enter or leave the AS.
• Unlike distance-vector or link-state, BGP advertises complete paths as an enumerated list of AS's to reach a particular network.
• Using paths for routing, is sometimes referred to as path-vector routing.
• The complete path is needed to enable checking against a given AS's policies.
• It also makes it easy to check for routing loops.

BGP Example

• Assume in the above that providers are transit networks, customers are stubs.
• A BGP speaker for the AS of provider A (AS 2) would be able to advertise reachability information for each of the network numbers assigned to customers P and Q.
• These advertisements would be sent in so-called UPDATE messages.
• So it would say: "128.96, 192.4.153, 192.4.32, and 192.4.3 are directly reachable from AS2."
• The Backbone network AS 1 can advertise something like: "128.96, 192.4.153, 192.4.32, and 192.4.3 can be reached along (AS1, AS2)."
• It can also advertise : "192.12.69, 192.4.54, and 192.4.23 can be reached along (AS1, AS3)."
• In terms of the BGP message format the path (AS1, AS3), is referred to as the AS-PATH
• The network portion of the message is referred to as the network prefix being advertised.
• The advertisement also includes a NEXT HOP attribute which is the border router to use to start the path towards the given AS. So the advertisement received with AS1 from AS2 would include the NEXT HOP border router to get one to AS2.
• The whole advertisement is referred to as Network Layer Reachability Information (NLRI).

More on BGP

• One important job of BGP is to prevent loops.
• Suppose AS1 learns that it can reach 10.0.1 through AS 2, so it advertises this to AS 3, who in turn advertises this back to AS 2.
• AS 2 might decide that AS 3 was the place to send packets destined for 10.0.1. AS 3 would then send them to AS 1 who would send these packets back to AS 2, letting them loop forever.
• This does not happen because advertisements carry the complete AS path. So AS 2 would realize that the route come back to itself because the route would be advertised as (AS 3, AS 1, AS 2).
• For this to work of course AS numbers carried in advertisements must be unique.
• These numbers called (ASNs - AS Numbers) are 16 bits long and are assigned by a central authority (ICANN) to assure uniqueness.
• Although 65000 numbers is not that many, since AS stub addresses don't need to be unique, this is more than enough. RFC4893 actually extends ASN numbers to 32 bits.

Integrating Interdomain and Intradomain Routing

• Let's look now at how Interdomain and Intradomain Routing interact.
• In the case of a stub AS, the border router is the only choice for routes out of the AS.
• Such a router can inject a default route in the intradomain routing protocol, so that any network not explicitly advertised in the intradomain protocol is reachable through the border router.
• A border router of a provider AS might learn either through BGP or through configuration that some networks are in a given customer AS.
• It could then inject a route to that prefix into the routing protocol running inside the provider AS.
• A border router in the backbone does not use injection, as given the number of prefixes this would be too costly.
• Instead a variant of BGP called interior BGP (iBGP) is used (in contrast to the exterior or eBGP we have been discussing). iBGP is essentially BGP run with peers all within the same AS (in the case we're considering the AS is the backbone).
• In iBGP we only advertise routes that come from outside the current AS we do not advertise routes from other peers with data from only within the current AS.
• iBGP allows any backbone router to learn the best border router to use when sending a packet to any address.

More Integrating Interdomain and Intradomain Routing

• If a packet comes into the backbone AS, it will be directed to the nearest speaker router, via intra-AS routing. This can be done because each of these speakers says in intra-AS land that it can handle all traffic external to the backbone. This means for the backbone AS the link state packets exchanged will not grow with each new incoming network.
• Once at a speaker, the packet can be directed using the tables collected from BGP messages to the correct border router.
• In a simple set-up, iBGP requires a full mesh of speakers -- every speaker would need to maintain connections to every other speaker.
• To make this scalable central point routers called route reflectors are typically peered with several BGP routers to reduce the number of connections. Confederations, which are sets of AS's grouped into one, are also used.
• To reduce the number of routes that need to be advertised, advertisements on this backbone might aggregate networks by decreasing the n in addr/n and only making a single advertisement for what was several advertisements from other AS's.
• Still, the so-called global routing tables in the backbone as of 2011 have about 300,000ish entries.
• A conventional intradomain routing protocol such as Distance Vector or OSPF is used to allow a router to know how to forward to that particular border router.

Route Selection BGP

• If a BGP speaker has a choice of several different routes to a destination, it will choose the best amongst these according to its own local policies.
• If, after these policies decisions, there still exists more than one route, then the route with the shortest AS-PATH is chosen.
• If, after this, there is still more than one route, the one with the closest next hop is selected (where closeness is is determined by the intra-AS algorithm).
• Finally, if more than one route still remains, the route that uses the smallest BGP Identifier is used.
• A BGP speaker is under no obligation to advertise a route to a destination even if it has one.
• BGP also supports negative advertisements if a route needs to be withdrawn.

Routing Areas

• One thing to note: AS don't correspond to necessarily to OSPF areas.
• An internetwork using OSPF might all be one AS, so we would then have the hierarchy: area - AS - Internet
• The book abbreviates routers that border areas as ABRs (area border routers).

IPv6

• Despite using subnetting and CIDR the number of available IPv4 addresses is was exhausted in 2011.
• To address this issue in 1991, the IETF began work on IPng (IP - the next generation), which later became IPv6.
• This project also accreted additional problems that it would try to solve, for instance, real-time services, security support, autoconfiguration, and mobile host support.
• Although many of these things were absent from IPv4 when IPv6 was first designed there were added to it later.

IPv6 Addressing and Routing and Allocation

• IPv6 addresses are 8 two-byte numbers.
• So the address space is 128 bits as opposed to the 32 bits of IPv4.
• Like IPv4, IPv6 addresses are classless, but the address space is still subdivided in various ways based on the leading bits.
• These leading bits specify the kind of use of that particular IPv6 address:
  • all 0's -- unspecified
  • 127 0's 1 -- loopback
  • 1111 1111 -- multicast address
  • 1111 1110 10 -- link local unicast (unique to the given network)
  • 1111 1110 11 -- site local unicast (unique to the given private internetwork)
  • Everything else

IPv6 Addressing Notation

• The standard representation of an IPv6 address is as: x:x:x:x:x:x:x:x where each x represent a 4-digit hexadecimal number.
• For example, 47cd:1234:4422:AC02:0022:1234:A456:0124
• Often lead 0's are not written so 0022 becomes 22.
• Since the address space is so huge, often many of these 4-digit numbers are 0. In which, case, it is common to not write that field. For example,

  47cd:1234:5432:AC02:0000:0000:A456:0124
  

  would be written as

  47cd:1234:5432:AC02:::A456:0124
  

  you don't need to repeat colons, so even more simply:

  47cd:1234:5432:AC02::A456:0124
  

  This is allowed provided you don't have multiple blocks of 0's
• There are two different ways of having IPv4 compatible IPv6 addresses: (a) zero extend a 32 bit address to 128 bits; (b) ::FFFF:IP address. For example, ::FFFF:128.96.33.81
• The former indicates the node is capable of understanding IPv6 the latter indicates the node only understands IPv4.