Warning

This is an unpolished draft of the second edition of this ebook. If you find any error or have suggestions to improve the text, please create an issue via https://github.com/obonaventure/cnp3/issues/new

# Interdomain routing¶

As explained earlier, the Internet is composed of more than 45,000 different networks [1] called domains. Each domain is composed of a group of routers and hosts that are managed by the same organisation. Example domains include belnet, sprint, level3, geant, abilene, cisco or google ...

Each domain contains a set of routers. From a routing point of view, these domains can be divided into two classes : the transit and the stub domains. A stub domain sends and receives packets whose source or destination are one of its own hosts. A transit domain is a domain that provides a transit service for other domains, i.e. the routers in this domain forward packets whose source and destination do not belong to the transit domain. As of this writing, about 85% of the domains in the Internet are stub domains [11]. A stub domain that is connected to a single transit domain is called a single-homed stub. A multihomed stub is a stub domain connected to two or more transit providers.

Transit and stub domains

The stub domains can be further classified by considering whether they mainly send or receive packets. An access-rich stub domain is a domain that contains hosts that mainly receive packets. Typical examples include small ADSL- or cable modem-based Internet Service Providers or enterprise networks. On the other hand, a content-rich stub domain is a domain that mainly produces packets. Examples of content-rich stub domains include google, yahoo, microsoft, facebook or content distribution networks such as akamai or limelight For the last few years, we have seen a rapid growth of these content-rich stub domains. Recent measurements [ATLAS2009] indicate that a growing fraction of all the packets exchanged on the Internet are produced in the data centers managed by these content providers.

Domains need to be interconnected to allow a host inside a domain to exchange IP packets with hosts located in other domains. From a physical perspective, domains can be interconnected in two different ways. The first solution is to directly connect a router belonging to the first domain with a router inside the second domain. Such links between domains are called private interdomain links or private peering links. In practice, for redundancy or performance reasons, distinct physical links are usually established between different routers in the two domains that are interconnected.

Interconnection of two domains via a private peering link

Such private peering links are useful when, for example, an enterprise or university network needs to be connected to its Internet Service Provider. However, some domains are connected to hundreds of other domains [2] . For some of these domains, using only private peering links would be too costly. A better solution to allow many domains to interconnect cheaply are the Internet eXchange Points (IXP). An IXP is usually some space in a data center that hosts routers belonging to different domains. A domain willing to exchange packets with other domains present at the IXP installs one of its routers on the IXP and connects it to other routers inside its own network. The IXP contains a Local Area Network to which all the participating routers are connected. When two domains that are present at the IXP wish [3] to exchange packets, they simply use the Local Area Network. IXPs are very popular in Europe and many Internet Service Providers and Content providers are present in these IXPs.

Interconnection of two domains at an Internet eXchange Point

In the early days of the Internet, domains would simply exchange all the routes they know to allow a host inside one domain to reach any host in the global Internet. However, in today’s highly commercial Internet, this is no longer true as interdomain routing mainly needs to take into account the economical relationships between the domains. Furthermore, while intradomain routing usually prefers some routes over others based on their technical merits (e.g. prefer route with the minimum number of hops, prefer route with the minimum delay, prefer high bandwidth routes over low bandwidth ones, etc) interdomain routing mainly deals with economical issues. For interdomain routing, the cost of using a route is often more important than the quality of the route measured by its delay or bandwidth.

There are different types of economical relationships that can exist between domains. Interdomain routing converts these relationships into peering relationships between domains that are connected via peering links.

The first category of peering relationship is the customer->provider relationship. Such a relationship is used when a customer domain pays an Internet Service Provider to be able to exchange packets with the global Internet over an interdomain link. A similar relationship is used when a small Internet Service Provider pays a larger Internet Service Provider to exchange packets with the global Internet.

A simple Internet with peering relationships

To understand the customer->provider relationship, let us consider the simple internetwork shown in the figure above. In this internetwork, AS7 is a stub domain that is connected to one provider : AS4. The contract between AS4 and AS7 allows a host inside AS7 to exchange packets with any host in the internetwork. To enable this exchange of packets, AS7 must know a route towards any domain and all the domains of the internetwork must know a route via AS4 that allows them to reach hosts inside AS7. From a routing perspective, the commercial contract between AS7 and AS4 leads to the following routes being exchanged :

• over a customer->provider relationship, the customer domain advertises to its provider all its routes and all the routes that it has learned from its own customers.
• over a provider->customer relationship, the provider advertises all the routes that it knows to its customer.

The second rule ensures that the customer domain receives a route towards all destinations that are reachable via its provider. The first rule allows the routes of the customer domain to be distributed throughout the Internet.

Coming back to the figure above, AS4 advertises to its two providers AS1 and AS2 its own routes and the routes learned from its customer, AS7. On the other hand, AS4 advertises to AS7 all the routes that it knows.

The second type of peering relationship is the shared-cost peering relationship. Such a relationship usually does not involve a payment from one domain to the other in contrast with the customer->provider relationship. A shared-cost peering relationship is usually established between domains having a similar size and geographic coverage. For example, consider the figure above. If AS3 and AS4 exchange many packets via AS1, they both need to pay AS1. A cheaper alternative for AS3 and AS4 would be to establish a shared-cost peering. Such a peering can be established at IXPs where both AS3 and AS4 are present or by using private peering links. This shared-cost peering should be used to exchange packets between hosts inside AS3 and hosts inside AS4. However, AS3 does not want to receive on the AS3-AS4 shared-cost peering links packets whose destination belongs to AS1 as AS3 would have to pay to send these packets to AS1.

From a routing perspective, over a shared-cost peering relationship a domain only advertises its internal routes and the routes that it has learned from its customers. This restriction ensures that only packets destined to the local domain or one of its customers is received over the shared-cost peering relationship. This implies that the routes that have been learned from a provider or from another shared-cost peer is not advertised over a shared-cost peering relationship. This is motivated by economical reasons. If a domain were to advertise the routes that it learned from a provider over a shared-cost peering relationship that does not bring revenue, it would have allowed its shared-cost peer to use the link with its provider without any payment. If a domain were to advertise the routes it learned over a shared cost peering over another shared-cost peering relationship, it would have allowed these shared-cost peers to use its own network (which may span one or more continents) freely to exchange packets.

Finally, the last type of peering relationship is the sibling. Such a relationship is used when two domains exchange all their routes in both directions. In practice, such a relationship is only used between domains that belong to the same company.

These different types of relationships are implemented in the interdomain routing policies defined by each domain. The interdomain routing policy of a domain is composed of three main parts :

• the import filter that specifies, for each peering relationship, the routes that can be accepted from the neighbouring domain (the non-acceptable routes are ignored and the domain never uses them to forward packets)
• the export filter that specifies, for each peering relationship, the routes that can be advertised to the neighbouring domain
• the ranking algorithm that is used to select the best route among all the routes that the domain has received towards the same destination prefix

A domain’s import and export filters can be defined by using the Route Policy Specification Language (RPSL) specified in RFC 2622 [GAVE1999] . Some Internet Service Providers, notably in Europe, use RPSL to document [4] their import and export policies. Several tools help to easily convert a RPSL policy into router commands.

The figure below provides a simple example of import and export filters for two domains in a simple internetwork. In RPSL, the keyword ANY is used to replace any route from any domain. It is typically used by a provider to indicate that it announces all its routes to a customer over a provider->customer relationship. This is the case for AS4‘s export policy. The example below clearly shows the difference between a provider->customer and a shared-cost peering relationship. AS4‘s export filter indicates that it announces only its internal routes (AS4) and the routes learned from its clients (AS7) over its shared-cost peering with AS3, while it advertises all the routes that it uses (including the routes learned from AS3) to AS7.

Import and export policies

## The Border Gateway Protocol¶

The Internet uses a single interdomain routing protocol : the Border Gateway Protocol (BGP). The current version of BGP is defined in RFC 4271. BGP differs from the intradomain routing protocols that we have already discussed in several ways. First, BGP is a path-vector protocol. When a BGP router advertises a route towards a prefix, it announces the IP prefix and the interdomain path used to reach this prefix. From BGP’s point of view, each domain is identified by a unique Autonomous System (AS) number [5] and the interdomain path contains the AS numbers of the transit domains that are used to reach the associated prefix. This interdomain path is called the AS Path. Thanks to these AS-Paths, BGP does not suffer from the count-to-infinity problems that affect distance vector routing protocols. Furthermore, the AS-Path can be used to implement some routing policies. Another difference between BGP and the intradomain routing protocols is that a BGP router does not send the entire contents of its routing table to its neighbours regularly. Given the size of the global Internet, routers would be overloaded by the number of BGP messages that they would need to process. BGP uses incremental updates, i.e. it only announces the routes that have changed to its neighbors.

The figure below shows a simple example of the BGP routes that are exchanged between domains. In this example, prefix 2001:db8:1234/48 is announced by AS1. AS1 advertises a BGP route towards this prefix to AS2. The AS-Path of this route indicates that AS1 is the originator of the prefix. When AS4 receives the BGP route from AS1, it re-announces it to AS2 and adds its AS number to the AS-Path. AS2 has learned two routes towards prefix 2001:db8:1234/48. It compares the two routes and prefers the route learned from AS4 based on its own ranking algorithm. AS2 advertises to AS5 a route towards 2001:db8:1234/48 with its AS-Path set to AS2:AS4:AS1. Thanks to the AS-Path, AS5 knows that if it sends a packet towards 2001:db8:1234/48 the packet first passes through AS2, then through AS4 before reaching its destination inside AS1.

Simple exchange of BGP routes

BGP routers exchange routes over BGP sessions. A BGP session is established between two routers belonging to two different domains that are directly connected. As explained earlier, the physical connection between the two routers can be implemented as a private peering link or over an Internet eXchange Point. A BGP session between two adjacent routers runs above a TCP connection (the default BGP port is 179). In contrast with intradomain routing protocols that exchange IP packets or UDP segments, BGP runs above TCP because TCP ensures a reliable delivery of the BGP messages sent by each router without forcing the routers to implement acknowledgements, checksums, etc. Furthermore, the two routers consider the peering link to be up as long as the BGP session and the underlying TCP connection remain up [6]. The two endpoints of a BGP session are called BGP peers.

A BGP peering session between two directly connected routers

In practice, to establish a BGP session between routers R1 and R2 in the figure above, the network administrator of AS3 must first configure on R1 the IP address of R2 on the R1-R2 link and the AS number of R2. Router R1 then regularly tries to establish the BGP session with R2. R2 only agrees to establish the BGP session with R1 once it has been configured with the IP address of R1 and its AS number. For security reasons, a router never establishes a BGP session that has not been manually configured on the router.

The BGP protocol RFC 4271 defines several types of messages that can be exchanged over a BGP session :

• OPEN : this message is sent as soon as the TCP connection between the two routers has been established. It initialises the BGP session and allows the negotiation of some options. Details about this message may be found in RFC 4271
• NOTIFICATION : this message is used to terminate a BGP session, usually because an error has been detected by the BGP peer. A router that sends or receives a NOTIFICATION message immediately shutdowns the corresponding BGP session.
• UPDATE: this message is used to advertise new or modified routes or to withdraw previously advertised routes.
• KEEPALIVE : this message is used to ensure a regular exchange of messages on the BGP session, even when no route changes. When a BGP router has not sent an UPDATE message during the last 30 seconds, it shall send a KEEPALIVE message to confirm to the other peer that it is still up. If a peer does not receive any BGP message during a period of 90 seconds [7], the BGP session is considered to be down and all the routes learned over this session are withdrawn.

As explained earlier, BGP relies on incremental updates. This implies that when a BGP session starts, each router first sends BGP UPDATE messages to advertise to the other peer all the exportable routes that it knows. Once all these routes have been advertised, the BGP router only sends BGP UPDATE messages about a prefix if the route is new, one of its attributes has changed or the route became unreachable and must be withdrawn. The BGP UPDATE message allows BGP routers to efficiently exchange such information while minimising the number of bytes exchanged. Each UPDATE message contains :

• a list of IP prefixes that are withdrawn
• a list of IP prefixes that are (re-)advertised
• the set of attributes (e.g. AS-Path) associated to the advertised prefixes

In the remainder of this chapter, and although all routing information is exchanged using BGP UPDATE messages, we assume for simplicity that a BGP message contains only information about one prefix and we use the words :

• Withdraw message to indicate a BGP UPDATE message containing one route that is withdrawn
• Update message to indicate a BGP UPDATE containing a new or updated route towards one destination prefix with its attributes

From a conceptual point of view, a BGP router connected to N BGP peers, can be described as being composed of four parts as shown in the figure below.

Organisation of a BGP router

In this figure, the router receives BGP messages on the left part of the figure, processes these messages and possibly sends BGP messages on the right part of the figure. A BGP router contains three important data structures :

• the Adj-RIB-In contains the BGP routes that have been received from each BGP peer. The routes in the Adj-RIB-In are filtered by the import filter before being placed in the BGP-Loc-RIB. There is one import filter per BGP peer.
• the Local Routing Information Base (Loc-RIB) contains all the routes that are considered as acceptable by the router. The Loc-RIB may contain several routes, learned from different BGP peers, towards the same destination prefix.
• the Forwarding Information Base (FIB) is used by the dataplane to forward packets towards their destination. The FIB contains, for each destination, the best route that has been selected by the BGP decision process. This decision process is an algorithm that selects, for each destination prefix, the best route according to the router’s ranking algorithm that is part of its policy.
• the Adj-RIB-Out contains the BGP routes that have been advertised to each BGP peer. The Adj-RIB-Out for a given peer is built by applying the peers export filter on the routes that have been installed in the FIB. There is one export filter per BGP peer. For this reason, the Adj-RIB-Out of a peer may contain different routes than the Adj-RIB-Out of another peer.

When a BGP session starts, the routers first exchange OPEN messages to negotiate the options that apply throughout the entire session. Then, each router extracts from its FIB the routes to be advertised to the peer. It is important to note that, for each known destination prefix, a BGP router can only advertise to a peer the route that it has itself installed inside its FIB. The routes that are advertised to a peer must pass the peer’s export filter. The export filter is a set of rules that define which routes can be advertised over the corresponding session, possibly after having modified some of its attributes. One export filter is associated to each BGP session. For example, on a shared-cost peering, the export filter only selects the internal routes and the routes that have been learned from a customer. The pseudo-code below shows the initialisation of a BGP session.

def initialize_BGP_session( RemoteAS, RemoteIP):
# Initialize and start BGP session
# Send BGP OPEN Message to RemoteIP on port 179
# advertise local routes and routes learned from peers*/
for d in BGPLocRIB :
B=build_BGP_Update(d)
S=Apply_Export_Filter(RemoteAS,B)
if (S != None) :
send_Update(S,RemoteAS,RemoteIP)
# entire RIB has been sent
# new Updates will be sent to reflect local or distant
# changes in routers


In the above pseudo-code, the build_BGP_UPDATE(d) procedure extracts from the BGP Loc-RIB the best path towards destination d (i.e. the route installed in the FIB) and prepares the corresponding BGP UPDATE message. This message is then passed to the export filter that returns NULL if the route cannot be advertised to the peer or the (possibly modified) BGP UPDATE message to be advertised. BGP routers allow network administrators to specify very complex export filters, see e.g. [WMS2004]. A simple export filter that implements the equivalent of split horizon is shown below.

def apply_export_filter(RemoteAS, BGPMsg) :
if RemoteAS is BGPMsg.ASPath :
BGPMsg=None
# Many additional export policies can be configured :
# Accept or refuse the BGPMsg
# Modify selected attributes inside BGPMsg
return BGPMsg


At this point, the remote router has received all the exportable BGP routes. After this initial exchange, the router only sends BGP UPDATE messages when there is a change (addition of a route, removal of a route or change in the attributes of a route) in one of these exportable routes. Such a change can happen when the router receives a BGP message. The pseudo-code below summarizes the processing of these BGP messages.

def Recvd_BGPMsg(Msg, RemoteAS) :
B=apply_import_filter(Msg,RemoteAS)
if (B== None): # Msg not acceptable
return
if IsUPDATE(Msg):
Old_Route=BestRoute(Msg.prefix)
Insert_in_RIB(Msg)
Run_Decision_Process(RIB)
if (BestRoute(Msg.prefix) != Old_Route) :
# best route changed
B=build_BGP_Message(Msg.prefix);
S=apply_export_filter(RemoteAS,B);
if (S!=None) : # announce best route
send_UPDATE(S,RemoteAS,RemoteIP);
else if (Old_Route != None) :
send_WITHDRAW(Msg.prefix,RemoteAS, RemoteIP)
else : # Msg is WITHDRAW
Old_Route=BestRoute(Msg.prefix)
Remove_from_RIB(Msg)
Run_Decision_Process(RIB)
if (Best_Route(Msg.prefix) !=Old_Route):
# best route changed
B=build_BGP_Message(Msg.prefix)
S=apply_export_filter(RemoteAS,B)
if (S != None) : # still one best route towards Msg.prefix
send_UPDATE(S,RemoteAS, RemoteIP);
else if(Old_Route != None) : # No best route anymore
send_WITHDRAW(Msg.prefix,RemoteAS,RemoteIP);


When a BGP message is received, the router first applies the peer’s import filter to verify whether the message is acceptable or not. If the message is not acceptable, the processing stops. The pseudo-code below shows a simple import filter. This import filter accepts all routes, except those that already contain the local AS in their AS-Path. If such a route was used, it would cause a routing loop. Another example of an import filter would be a filter used by an Internet Service Provider on a session with a customer to only accept routes towards the IP prefixes assigned to the customer by the provider. On real routers, import filters can be much more complex and some import filters modify the attributes of the received BGP UPDATE [WMS2004] .

def apply_import_filter(RemoteAS, BGPMsg):
if MysAS in BGPMsg.ASPath :
BGPMsg=None
# Many additional import policies can be configured :
# Accept or refuse the BGPMsg
# Modify selected attributes inside BGPMsg
return BGPMsg


Note

The bogon filters

Another example of frequently used import filters are the filters that Internet Service Providers use to ignore bogon routes. In the ISP community, a bogon route is a route that should not be advertised on the global Internet. Typical examples include the documentation IPv6 prefix (2001:db8::/32 used for most examples in this book), the loopback address (::1/128) or the IPv6 prefixes that have not yet been allocated by IANA. A well managed BGP router should ensure that it never advertises bogons on the global Internet. Detailed information about these bogons may be found in [IMHM2013].

If the import filter accepts the BGP message, the pseudo-code distinguishes two cases. If this is an Update message for prefix p, this can be a new route for this prefix or a modification of the route’s attributes. The router first retrieves from its RIB the best route towards prefix p. Then, the new route is inserted in the RIB and the BGP decision process is run to find whether the best route towards destination p changes. A BGP message only needs to be sent to the router’s peers if the best route has changed. For each peer, the router applies the export filter to verify whether the route can be advertised. If yes, the filtered BGP message is sent. Otherwise, a Withdraw message is sent. When the router receives a Withdraw message, it also verifies whether the removal of the route from its RIB caused its best route towards this prefix to change. It should be noted that, depending on the content of the RIB and the export filters, a BGP router may need to send a Withdraw message to a peer after having received an Update message from another peer and conversely.

Let us now discuss in more detail the operation of BGP in an IPv6 network. For this, let us consider the simple network composed of three routers located in three different ASes and shown in the figure below.

Utilisation of the BGP nexthop attribute

This network contains three routers : R1, R2 and R3. Each router is attached to a local IPv6 subnet that it advertises using BGP. There are two BGP sessions, one between R1 and R2 and the second between R2 and R3. A /127 subnet is used on each interdomain link (2001:db8::4/127 on R1-R2 and 2001:db8::0/127 on R2-R3) in conformance with the latest recommendation RFC 6164. The BGP sessions run above TCP connections established between the neighboring routers (e.g. 2001:db8::5 - 2001:db8::6 for the R1-R2 session).

Let us assume that the R1-R2 BGP session is the first to be established. A BGP Update message sent on such a session contains three fields :

• the BGP nexthop
• the attributes including the AS-Path

We use the notation U(prefix, nexthop, attributes) to represent such a BGP Update message in this section. Similarly, W(prefix) represents a BGP withdraw for the specified prefix. Once the R1-R2 session has been established, R1 sends U(2001:db8:1234::/48,2001:db8::5,AS10) to R2 and R2 sends U(2001:db8:5678:/48,2001:db8::6,AS20). At this point, R1 can reach 2001:db8:5678::/48 via 2001:db8::6 and R2 can reach 2001:db8:1234::/48 via 2001:db8::5.

Once the R2-R3 has been established, R3 sends U(2001:db8:acbd::/48,2001:db8::2,AS30). R2 announces on the R2-R3 session all the routes inside its RIB. It thus sends to R3 : U(2001:db8:1234::/48,2001:db8::1,AS20:AS10) and U(2001:db8:5678::/48,2001:db8::1,AS20). Note that when R2 advertises the route that it learned from R1, it updates the BGP nexthop and adds its AS number to the AS-Path. R2 also sends U(2001:db8:abcd::48,2001:db8::6,AS20:AS30) to R1 on the R1-R3 session. At this point, all BGP routes have been exchanged and all routers can reach 2001:db8::1234/48, 2001:db8:5678::/48 and 2001:db8:abcd::/48.

If the link between R2 and R3 fails, R3 detects the failure as it did not receive KEEPALIVE messages recently from R2. At this time, R3 removes from its RIB all the routes learned over the R2-R3 BGP session. R2 also removes from its RIB the routes learned from R3. R2 also sends W(2001:db8:acbd::/48) to R1 over the R1-R3 BGP session since it does not have a route anymore towards this prefix.

Note

Origin of the routes advertised by a BGP router

A frequent practical question about the operation of BGP is how a BGP router decides to originate or advertise a route for the first time. In practice, this occurs in two situations :

• the router has been manually configured by the network operator to always advertise one or several routes on a BGP session. For example, on the BGP session between UCLouvain and its provider, belnet , UCLouvain’s router always advertises the 2001:6a8:3080/48 IPv6 prefix assigned to the campus network
• the router has been configured by the network operator to advertise over its BGP session some of the routes that it learns with its intradomain routing protocol. For example, an enterprise router may advertise over a BGP session with its provider the routes to remote sites when these routes are reachable and advertised by the intradomain routing protocol

The first solution is the most frequent. Advertising routes learned from an intradomain routing protocol is not recommended, this is because if the route flaps [8], this would cause a large number of BGP messages being exchanged in the global Internet.

### The BGP decision process¶

Besides the import and export filters, a key difference between BGP and the intradomain routing protocols is that each domain can define is own ranking algorithm to determine which route is chosen to forward packets when several routes have been learned towards the same prefix. This ranking depends on several BGP attributes that can be attached to a BGP route.

The first BGP attribute that is used to rank BGP routes is the local-preference (local-pref) attribute. This attribute is an unsigned integer that is attached to each BGP route received over an eBGP session by the associated import filter.

When comparing routes towards the same destination prefix, a BGP router always prefers the routes with the highest local-pref. If the BGP router knows several routes with the same local-pref, it prefers among the routes having this local-pref the ones with the shortest AS-Path.

The local-pref attribute is often used to prefer some routes over others.

A common utilisation of local-pref is to support backup links. Consider the situation depicted in the figure below. AS1 would always like to use the high bandwidth link to send and receive packets via AS2 and only use the backup link upon failure of the primary one.

How to create a backup link with BGP ?

As BGP routers always prefer the routes with the highest local-pref attribute, this policy can be implemented using the following import filter on R1

import: from  AS2 RA at R1 set localpref=100;
from  AS2 RB at R1 set localpref=200;
accept ANY


With this import filter, all the BGP routes learned from RB over the high bandwidth links are preferred over the routes learned over the backup link. If the primary link fails, the corresponding routes are removed from R1‘s RIB and R1 uses the route learned from RA. R1 reuses the routes via RB as soon as they are advertised by RB once the R1-RB link comes back.

The import filter above modifies the selection of the BGP routes inside AS1. Thus, it influences the route followed by the packets forwarded by AS1. In addition to using the primary link to send packets, AS1 would like to receive its packets via the high bandwidth link. For this, AS2 also needs to set the local-pref attribute in its import filter.

import: from  AS1 R1 at RA set localpref=100;
from  AS1 R1 at RB set localpref=200;
accept AS1


Sometimes, the local-pref attribute is used to prefer a cheap link compared to a more expensive one. For example, in the network below, AS1 could wish to send and receive packets mainly via its interdomain link with AS4.

How to prefer a cheap link over an more expensive one ?

AS1 can install the following import filter on R1 to ensure that it always sends packets via R2 when it has learned a route via AS2 and another via AS4.

import: from  AS2 RA at R1 set localpref=100;
from  AS4 R2 at R1 set localpref=200;
accept ANY


However, this import filter does not influence how AS3 , for example, prefers some routes over others. If the link between AS3 and AS2 is less expensive than the link between AS3 and AS4, AS3 could send all its packets via AS2 and AS1 would receive packets over its expensive link. An important point to remember about local-pref is that it can be used to prefer some routes over others to send packets, but it has no influence on the routes followed by received packets.

Another important utilisation of the local-pref attribute is to support the customer->provider and shared-cost peering relationships. From an economic point of view, there is an important difference between these three types of peering relationships. A domain usually earns money when it sends packets over a provider->customer relationship. On the other hand, it must pay its provider when it sends packets over a customer->provider relationship. Using a shared-cost peering to send packets is usually neutral from an economic perspective. To take into account these economic issues, domains usually configure the import filters on their routers as follows :

• insert a high local-pref attribute in the routes learned from a customer
• insert a medium local-pref attribute in the routes learned over a shared-cost peering
• insert a low local-pref attribute in the routes learned from a provider

With such an import filter, the routers of a domain always prefer to reach destinations via their customers whenever such a route exists. Otherwise, they prefer to use shared-cost peering relationships and they only send packets via their providers when they do not know any alternate route. A consequence of setting the local-pref attribute like this is that Internet paths are often asymmetrical. Consider for example the internetwork shown in the figure below.

Asymmetry of Internet paths

Consider in this internetwork the routes available inside AS1 to reach AS5. AS1 learns the AS4:AS6:AS7:AS5 path from AS4, the AS3:AS8:AS5 path from AS3 and the AS2:AS5 path from AS2. The first path is chosen since it was learned from a customer. AS5 on the other hand receives three paths towards AS1 via its providers. It may select any of these paths to reach AS1 , depending on how it prefers one provider over the others.

### BGP convergence¶

In the previous sections, we have explained the operation of BGP routers. Compared to intradomain routing protocols, a key feature of BGP is its ability to support interdomain routing policies that are defined by each domain as its import and export filters and ranking process. A domain can define its own routing policies and router vendors have implemented many configuration tweaks to support complex routing policies. However, the routing policy chosen by a domain may interfere with the routing policy chosen by another domain. To understand this issue, let us first consider the simple internetwork shown below.

The disagree internetwork

In this internetwork, we focus on the route towards 2001:db8::1234/48 which is advertised by AS1. Let us also assume that AS3 (resp. AS4) prefers, e.g. for economic reasons, a route learned from AS4 (AS3) over a route learned from AS1. When AS1 sends U(2001:db8::1234/48,AS1) to AS3 and AS4, three sequences of exchanges of BGP messages are possible :

1. AS3 sends first U(2001:db8:1234/48,AS3:AS1) to AS4. AS4 has learned two routes towards 2001:db8:1234/48. It runs its BGP decision process and selects the route via AS3 and does not advertise a route to AS3
2. AS4 first sends U(2001:db8:1234/48,AS3:AS1) to AS3. AS3 has learned two routes towards 2001:db8:1234/48. It runs its BGP decision process and selects the route via AS4 and does not advertise a route to AS4
3. AS3 sends U(2001:db8:1234/48,AS3:AS1) to AS4 and, at the same time, AS4 sends U(2001:db8:1234/48,AS4:AS1). AS3 prefers the route via AS4 and thus sends W(2001:db8:1234/48) to AS4. In the mean time, AS4 prefers the route via AS3 and thus sends W(2001:db8:1234/48) to AS3. Upon reception of the BGP Withdraws, AS3 and AS4 only know the direct route towards 2001:db8:1234/48. AS3 (resp. AS4) sends U(2001:db8:1234/48,AS3:AS1) (resp. U(2001:db8:1234/48,AS4:AS1)) to AS4 (resp. AS3). AS3 and AS4 could in theory continue to exchange BGP messages for ever. In practice, one of them sends one message faster than the other and BGP converges.

The example above has shown that the routes selected by BGP routers may sometimes depend on the ordering of the BGP messages that are exchanged. Other similar scenarios may be found in RFC 4264.

From an operational perspective, the above configuration is annoying since the network operators cannot easily predict which paths are chosen. Unfortunately, there are even more annoying BGP configurations. For example, let us consider the configuration below which is often named Bad Gadget [GW1999]

In this internetwork, there are four ASes. AS0 advertises one route towards one prefix and we only analyse the routes towards this prefix. The routing preferences of AS1, AS3 and AS4 are the following :

• AS1 prefers the path AS3:AS0 over all other paths
• AS3 prefers the path AS4:AS0 over all other paths
• AS4 prefers the path AS1:AS0 over all other paths

AS0 sends U(p,AS0) to AS1, AS3 and AS4. As this is the only route known by AS1, AS3 and AS4 towards p, they all select the direct path. Let us now consider one possible exchange of BGP messages :

1. AS1 sends U(p, AS1:AS0) to AS3 and AS4. AS4 selects the path via AS1 since this is its preferred path. AS3 still uses the direct path.
2. AS4 advertises U(p,AS4:AS1:AS0) to AS3.
3. AS3 sends U(p, AS3:AS0) to AS1 and AS4. AS1 selects the path via AS3 since this is its preferred path. AS4 still uses the path via AS1.
4. As AS1 has changed its path, it sends U(p,AS1:AS3:AS0) to AS4 and W(p) to AS3 since its new path is via AS3. AS4 switches back to the direct path.
5. AS4 sends U(p,AS4:AS0) to AS1 and AS3. AS3 prefers the path via AS4.
6. AS3 sends U(p,AS3:AS4:AS0) to AS1 and W(p) to AS4. AS1 switches back to the direct path and we are back at the first step.

This example shows that the convergence of BGP is unfortunately not always guaranteed as some interdomain routing policies may interfere with each other in complex ways. [GW1999] have shown that checking for global convergence is either NP-complete or NP-hard. See [GSW2002] for a more detailed discussion.

Fortunately, there are some operational guidelines [GR2001] [GGR2001] that can guarantee BGP convergence in the global Internet. To ensure that BGP will converge, these guidelines consider that there are two types of peering relationships : customer->provider and shared-cost. In this case, BGP convergence is guaranteed provided that the following conditions are fulfilled :

1. The topology composed of all the directed customer->provider peering links is an acyclic graph
2. An AS always prefers a route received from a customer over a route received from a shared-cost peer or a provider.

The first guideline implies that the provider of the provider of ASx cannot be a customer of ASx. Such a relationship would not make sense from an economic perspective as it would imply circular payments. Furthermore, providers are usually larger than customers.

The second guideline also corresponds to economic preferences. Since a provider earns money when sending packets to one of its customers, it makes sense to prefer such customer learned routes over routes learned from providers. [GR2001] also shows that BGP convergence is guaranteed even if an AS associates the same preference to routes learned from a shared-cost peer and routes learned from a customer.

From a theoretical perspective, these guidelines should be verified automatically to ensure that BGP will always converge in the global Internet. However, such a verification cannot be performed in practice because this would force all domains to disclose their routing policies (and few are willing to do so) and furthermore the problem is known to be NP-hard [GW1999].

In practice, researchers and operators expect that these guidelines are verified [9] in most domains. Thanks to the large amount of BGP data that has been collected by operators and researchers [10], several studies have analysed the AS-level topology of the Internet. [SARK2002] is one of the first analysis. More recent studies include [COZ2008] and [DKF+2007]

Based on these studies and [ATLAS2009], the AS-level Internet topology can be summarised as shown in the figure below.

The layered structure of the global Internet

The domains on the Internet can be divided in about four categories according to their role and their position in the AS-level topology.

• the core of the Internet is composed of a dozen-twenty Tier-1 ISPs. A Tier-1 is a domain that has no provider. Such an ISP has shared-cost peering relationships with all other Tier-1 ISPs and provider->customer relationships with smaller ISPs. Examples of Tier-1 ISPs include sprint, level3 or opentransit
• the Tier-2 ISPs are national or continental ISPs that are customers of Tier-1 ISPs. These Tier-2 ISPs have smaller customers and shared-cost peering relationships with other Tier-2 ISPs. Example of Tier-2 ISPs include France Telecom, Belgacom, British Telecom, ...
• the Tier-3 networks are either stub domains such as enterprise or campus networks networks and smaller ISPs. They are customers of Tier-1 and Tier-2 ISPs and have sometimes shared-cost peering relationships
• the large content providers that are managing large datacenters. These content providers are producing a growing fraction of the packets exchanged on the global Internet [ATLAS2009]. Some of these content providers are customers of Tier-1 or Tier-2 ISPs, but they often try to establish shared-cost peering relationships, e.g. at IXPs, with many Tier-1 and Tier-2 ISPs.

Due to this organisation of the Internet and due to the BGP decision process, most AS-level paths on the Internet have a length of 3-5 AS hops.

Footnotes

 [1] An analysis of the evolution of the number of domains on the global Internet during the last ten years may be found in http://www.potaroo.net/tools/asn32/
 [2] See http://as-rank.caida.org/ for an analysis of the interconnections between domains based on measurements collected in the global Internet
 [3] Two routers that are attached to the same IXP only exchange packets when the owners of their domains have an economical incentive to exchange packets on this IXP. Usually, a router on an IXP is only able to exchange packets with a small fraction of the routers that are present on the same IXP.
 [4] See ftp://ftp.ripe.net/ripe/dbase for the RIPE database that contains the import and export policies of many European ISPs
 [5] In this text, we consider Autonomous System and domain as synonyms. In practice, a domain may be divided into several Autonomous Systems, but we ignore this detail.
 [6] The BGP sessions and the underlying TCP connection are typically established by the routers when they boot based on information found in their configuration. The BGP sessions are rarely released, except if the corresponding peering link fails or one of the endpoints crashes or needs to be rebooted.
 [7] 90 seconds is the default delay recommended by RFC 4271. However, two BGP peers can negotiate a different timer during the establishment of their BGP session. Using a too small interval to detect BGP session failures is not recommended. BFD [KW2009] can be used to replace BGP’s KEEPALIVE mechanism if fast detection of interdomain link failures is required.
 [8] A link is said to be flapping if it switches several between an operational state and a disabled state within a short period of time. A router attached to such a link would need to frequently send routing messages.
 [9] Researchers such as [MUF+2007] have shown that modelling the Internet topology at the AS-level requires more than the shared-cost and customer->provider peering relationships. However, there is no publicly available model that goes beyond these classical peering relationships.
 [10] BGP data is often collected by establishing BGP sessions between Unix hosts running a BGP daemon and BGP routers in different ASes. The Unix hosts stores all BGP messages received and regular dumps of its BGP routing table. See http://www.routeviews.org, http://www.ripe.net/ris, http://bgp.potaroo.net or http://irl.cs.ucla.edu/topology/
 [11] Several web sites collect and analyse data about the evolution of BGP in the global Internet. http://bgp.potaroo.net provides lots of statistics and analyses that are updated daily.