rts several main transport protocols, including TCP, UDP, DCCP, and SCTP. TCP is connec- tion oriented, provides for flow and congestion control, and operates in a unicast mode. UDP is connectionless and therefore associated with unreli- able transport. DCCP is one of the new transport protocols; it adds conges- tion control while still being associated with unreliable transport. SCTP is a new transport protocol that provides multistream and multihoming support. It supports many link layers-TCP/IP is data link agnostic. IP can work over many link layers, including Ethernet, IP, PPP, MPLS, ATM, optical, and wireless options. Along with being an open standard, TCP/IP also offers support for a wide variety of implementation scenarios. Chapter 8 The Internet and IP Infrastructures IP over Birds? For a taste of the humorous side of telecommunications and standards, you must take a look at the RFCs for "IP over avian carriers." These are classics. RFC 1149 (www.ietf.org/ rfc/rfc1149.txt) was the original; it was updated by RFC 2549 (www.ietf.org/rfc/rfc2549. txt) to include quality of service. Someone actually tried implementing RFC 1149. Amazing! Read about it at www. blug.linux.no/rfc1149. And be sure to check out the log of the real ping session, at www. blug.linux.no/rfc1149/pinglogg.txt. Figure 8.8 illustrates the main aspects of an ISP's network architecture. The customers at the edge access the ISP's point of presence (POP) via an edge router that is part of the intra-POP fabric, the internal workings of the ISP's POP. (This information is covered in more detail later in this chapter, in the section "The Evo- lution of the POP Architecture.") As you can see in Figure 8.8, the ISP backbone is composed of a series of core routers and the transmission links between them. Remember that the backbone may also contain ATM switches; a large percentage of ISP backbones include ATM Subscribers Routers IP Backbone Intra-POP Network Routers Edge Routers Intra-POP Network Subscribers Figure 8.8 An ISP's network architecture 