Windows Embedded CE TCP/IP implements the Nagle algorithm described in RFC 896. The purpose of this algorithm is to reduce the number of very small segments sent, especially on high-delay (remote) links. The Nagle algorithm allows small segments to be sent only if the TCP connection has no unacknowledged and outstanding data. If more small segments are generated while awaiting the ACK for the first one, then these segments are coalesced into one larger segment. Any full-sized segment is always transmitted immediately, assuming there is a sufficient receive window available. The Nagle algorithm is effective in reducing the number of packets sent by interactive applications, such as Telnet, especially over slow links.

The Nagle algorithm can be observed in the following trace captured by Microsoft Network Monitor. The trace was captured by using PPP to dial up an Internet service provider at 9600 BPS. A Telnet (character mode) session was established, then the Y key was held down on the device. At all times, one segment was sent, and further Y characters were held by the stack until an acknowledgment was received for the previous segment. In this example, three to four Y characters were buffered each time and sent together in one segment. The Nagle algorithm resulted in a huge savings in the number of packets sent — the number of packets was reduced by a factor of about three.

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Time Source IP Dest IP Prot Description 0.644 204.182.66.83 199.181.164.4 TELNET To Server Port = 1901 0.144 199.181.164.4 204.182.66.83 TELNET To Client Port = 1901 0.000 204.182.66.83 199.181.164.4 TELNET To Server Port = 1901 0.145 199.181.164.4 204.182.66.83 TELNET To Client Port = 1901 0.000 204.182.66.83 199.181.164.4 TELNET To Server Port = 1901 0.144 199.181.164.4 204.182.66.83 TELNET To Client Port = 1901 . . .

Each segment contained several of the Y characters. The first segment is shown more fully parsed below, and the data portion is pointed out in the hexadecimal value at the bottom.

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*********************************************************************** Time Source IP Dest IP Prot Description 0.644 204.182.66.83 199.181.164.4 TELNET To Server Port = 1901 + FRAME: Base frame properties + ETHERNET: ETYPE = 0x0800 : Protocol = IP: DOD Internet Protocol + IP: ID = 0xEA83; Proto = TCP; Len: 43 + TCP: .AP..., len: 3, seq:1032660278, ack: 353339017, win: 7766, src: 1901 dst: 23 (TELNET) TELNET: To Server From Port = 1901 TELNET: Telnet Data D2 41 53 48 00 00 52 41 53 48 00 00 08 00 45 00 .ASH..RASH....E. 00 2B EA 83 40 00 20 06 F5 85 CC B6 42 53 C7 B5 .+..@. .....BS.. A4 04 07 6D 00 17 3D 8D 25 36 15 0F 86 89 50 18 ...m..=.%6....P. 1E 56 1E 56 00 00 79 79 79 .V.V..yyy ^^^ data

Windows Sockets applications can disable the Nagle algorithm for their connections by setting the TCP_NODELAY socket option. However, this practice should be used wisely and only when needed because this practice increases network traffic. Usually, applications that need to disable the Nagle algorithm are applications that need to send small packets and require that the peer application receive them immediately. Some network applications may not perform well if their design does not take into account the effects of transmitting large numbers of small packets and the Nagle algorithm.

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