41902 Lecture Notes - Lecture 10: Internet Control Message Protocol, Border Gateway Protocol, Routing Protocol
Network Fundamentals
Chapter 5.4: Routing among the ISPs: BGP
Internet inter-AS routing: BGP
BGP (Border Gateway Protocol): is the de facto inter-domain routing protocol
BGP is like the glue that holds the iteret together. It proes eah As a means to:
- eBGP: obtain subnet reachability information from neighbouring AS
- iBGP: propagate reachability information to all AS-internal routers
- Deterie good routes to other etorks ased o reahailit iforatio ad
policy and allows subnet to adertise it’s eistee to the rest of the Iteret
BGP session: to BGP routers peers ehage BGP essages oer a sei-permanent
TCP connection: advertising paths to differet destiatio etork prefies BGP is a path
etor protool.
Path attributes and BGP routes:
- Advertised prefix includes BGP attributes (prefix + attributes = route). Two important
attributes include:
- AS-PATH: list of AS through which prefix advertisement has passed
- NEXT-HOP: indicates specific internal-AS router to next hop AS
- Policy-based routing: gateway receiving route advertisement uses import policy to
accept/decline a path (e.g. never route through a specific AS). AS policy also
determines whether to advertise a path to other neighbouring AS.
BGP messages: exchanged between peers over TCP connection. Include:
- OPEN: opens TCP connection to remote BGP peer and authenticates sending BGP peer
- UPDATE: advertises new path (or withdraws old)
- KEEPALIVE: keeps connection alive in absence of UPDATES; also ACKs OPEN request
- NOTIFICATION: reports errors in previous message; also used to close tonnection
BGP route selection: router may learn about more than one route to destination AS, then
selects a route based on the following:
- Local preference value attribute: policy decision
- Shortest AS-PATH (i.e. least AS hops??)
- Closest NEXT-HOP router: hot potato routing – chooses the local gateway that has least
intra-domain cost (not inter-domain cost, does’t atter if there are ore A“ hops)
- Additional criteria
Chapter 5.6: ICMP: The Internet Control Message Protocol
ICMP is used by hosts and routers to communicate network-level information such as error
reporting (unreachable host/network/port/protocol) and echo request/reply (used by ping).
Netork laer aoe IP: ICMP essages are carried in IP datagrams
find more resources at oneclass.com
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Document Summary
Bgp (border gateway protocol): is the de facto inter-domain routing protocol. Bgp is like the (cid:862)glue that holds the i(cid:374)ter(cid:374)et together(cid:863). Ebgp: obtain subnet reachability information from neighbouring as. Deter(cid:373)i(cid:374)e (cid:862)good(cid:863) routes to other (cid:374)et(cid:449)orks (cid:271)ased o(cid:374) rea(cid:272)ha(cid:271)ilit(cid:455) i(cid:374)for(cid:373)atio(cid:374) a(cid:374)d ibgp: propagate reachability information to all as-internal routers policy and allows subnet to ad(cid:448)ertise it"s e(cid:454)iste(cid:374)(cid:272)e to the rest of the i(cid:374)ter(cid:374)et. Bgp session: t(cid:449)o bgp routers (cid:894)(cid:862)peers(cid:863)(cid:895) e(cid:454)(cid:272)ha(cid:374)ge bgp (cid:373)essages o(cid:448)er a se(cid:373)i-permanent. Tcp connection: advertising paths to differe(cid:374)t desti(cid:374)atio(cid:374) (cid:374)et(cid:449)ork prefi(cid:454)es (cid:894)bgp is a (cid:862)path (cid:448)e(cid:272)tor(cid:863) proto(cid:272)ol(cid:895). Advertised prefix includes bgp attributes (prefix + attributes = route). As-path: list of as through which prefix advertisement has passed. Next-hop: indicates specific internal-as router to next hop as. Policy-based routing: gateway receiving route advertisement uses import policy to accept/decline a path (e. g. never route through a specific as). As policy also determines whether to advertise a path to other neighbouring as. Bgp messages: exchanged between peers over tcp connection.
