[PATCHv2 2.6.18-rc1-mm2 1/3] net: UDP-Lite generic support

[Gerrit Renker <gerrit@erg.abdn.ac.uk>]

Generic support (header files, configuration, and documentation)
for the UDP-Lite protocol (RFC 3828).

This has been tested successfully on AMD, i386/i686, and SMP
architectures (compiles cleanly and works under different configurations).

With regard to the previously submitted patch set, no conceptual
changes (it works), but a lot of tidying up and re-organisation: 

* the number of changes to other files has been reduced to the 
  bare minimum (would greatly value ideas for further reduction),
* ifdefs have been replaced by generic functions (whose implementation
  depends on CONFIG_IP_UDPLITE),
* much of the declarations and concepts shared between v4 and v6 UDP-
  Lite is now concentrated in the header file

I hope that this makes the code a lot more accessible. Both ipv4/udplite.c
and ipv6/udplite.c derive from the respective udp.c variants. 

There is documentation enclosed, as well as a few (v6-enabled) applications
to test the kernel implementation. These should be easy to built - please 
give it a test run if you have a little time.

Previous comments have been a great help in improving the code: I would greatly 
value any further ideas, suggestions, and comments -- in particular ACKS & NAKS
or application. 


Signed-off-by: Gerrit Renker <gerrit@erg.abdn.ac.uk>
Signed-off-by: William Stanislaus <william@erg.abdn.ac.uk>
---

Documentation/networking/udplite.txt |  334 +++++++++++++++++++++++++++++++++++
 include/linux/in.h                   |    1
 include/linux/socket.h               |    1
 include/linux/udplite.h              |   96 ++++++++++
 include/net/udplite.h                |  196 ++++++++++++++++++++
 net/core/sock.c                      |    7
 net/ipv4/Kconfig                     |   21 ++
 7 files changed, 655 insertions(+), 1 deletion(-)



diff -Nurp  a/net/ipv4/Kconfig b/net/ipv4/Kconfig
--- a/net/ipv4/Kconfig	2006-07-14 10:15:27.000000000 +0100
+++ b/net/ipv4/Kconfig	2006-07-14 10:17:50.000000000 +0100
@@ -581,3 +581,24 @@ config TCP_CONG_BIC
 
 source "net/ipv4/ipvs/Kconfig"
 
+config IP_UDPLITE
+	bool "The UDP-Lite Protocol (EXPERIMENTAL)"
+	depends on INET && EXPERIMENTAL
+        default n
+	---help---
+	  The UDP-Lite Protocol            <http://www.ietf.org/rfc/rfc3828.txt>
+
+	  UDP-Lite is  a Standards-Track IETF  transport protocol (RFC 3828). It
+	  features  a  variable-length checksum;  which allows partially damaged
+	  packets  to  be forwarded to media codecs, rather than being discarded
+	  due to invalid (UDP) checksum values. This can have advantages for the
+	  transport of multimedia (e.g. video/audio) over wireless networks.
+
+	  The protocol runs on both IPv4 and IPv6. The socket API resembles that
+	  of UDP. Applications must indicate their wish to utilise the partial
+	  checksum coverage feature by setting a socket option; UDP-Lite will
+	  otherwise run in (compatible) UDP mode.
+
+	  Detailed documentation in <file:Documentation/networking/udplite.txt>.
+
+	  If in doubt, say N.
diff -Nurp  a/include/linux/udplite.h b/include/linux/udplite.h
--- a/include/linux/udplite.h	1970-01-01 01:00:00.000000000 +0100
+++ b/include/linux/udplite.h	2006-07-14 10:17:50.000000000 +0100
@@ -0,0 +1,96 @@
+/*
+ *              Header file for UDP Lite (RFC 3828).
+ *
+ * Version:	see net/ipv4/udplite.c
+ *
+ * Authors:	Gerrit Renker, <gerrit@erg.abdn.ac.uk>
+ *		William Stanislaus, <william@erg.abdn.ac.uk>
+ *
+ * Fixes:
+ * Changes:
+ *
+ *		This program is free software; you can redistribute it and/or
+ *		modify it under the terms of the GNU General Public License
+ *		as published by the Free Software Foundation; either version
+ *		2 of the License, or (at your option) any later version.
+ */
+#ifndef _LINUX_UDPLITE_H
+#define _LINUX_UDPLITE_H
+#include <linux/types.h>
+
+/**
+ *   struct udplitehdr  -  UDP-Lite header re-interpreting UDP (RFC 768) fields
+ *
+ *   @source:   source port number      (as in UDP)
+ *   @dest:     destination port number (as in UDP)
+ *   @checklen: checksum coverage length
+ *   @check:    checksum field          (as in UDP)
+ *
+ *   For the detailed semantics see RFC 3828.
+ */
+struct udplitehdr {
+	__u16 source;
+	__u16 dest;
+	__u16 checklen;
+	__u16 check;
+};
+
+/* UDP-Lite socket options */
+#define UDPLITE_SEND_CSCOV   10
+#define UDPLITE_RECV_CSCOV   11
+
+
+#ifdef __KERNEL__
+#include <linux/config.h>
+#include <net/sock.h>
+#include <linux/ip.h>
+
+/**
+ *   struct udplite_sock  -  unreliable, connection-less UDP-Lite service
+ *
+ *   @inet:       has to be the first member
+ *   @pending:    any pending frames?
+ *   @corkflag:   when cork is required
+ *   @encap_type: is this an encapsulation socket?
+ *   @len:        total  length of pending frames
+ *   @pcslen:     partial checksum coverage length for sending socket
+ *   @pcrlen:     partial checksum coverage length for receiving socket
+ *   @pcflag:     partial checksum coverage flag
+ *
+ *   NOTE: Checksum coverage length has different semantics for sending and
+ *   receiving sockets.
+ *
+ */
+struct udplite_sock {
+	struct inet_sock  inet;
+	int               pending;
+	unsigned int      corkflag;
+	__u16             encap_type;
+	/*  The following members retain the information to create a
+	 *  UDP-Lite header when the socket is uncorked.  */
+	__u16             len;
+	__u16             pcslen;
+	__u16             pcrlen;
+/* checksum coverage set indicators used by pcflag */
+#define UDPLITE_SEND_CC   0x1
+#define UDPLITE_RECV_CC   0x2
+	__u8              pcflag;
+};
+
+static inline struct udplite_sock *udplite_sk(const struct sock *sk)
+{
+	return (struct udplite_sock *)sk;
+}
+
+
+struct udplite6_sock {
+	struct udplite_sock  udpl;
+	/*
+	 * ipv6_pinfo has to be the last member of udplite6_sock,
+	 * see inet6_sk_generic.
+	 */
+	struct ipv6_pinfo    inet6;
+};
+#endif		/*  __KERNEL__      */
+
+#endif		/* _LINUX_UDPLITE_H */
diff -Nurp  a/include/net/udplite.h b/include/net/udplite.h
--- a/include/net/udplite.h	1970-01-01 01:00:00.000000000 +0100
+++ b/include/net/udplite.h	2006-07-14 10:17:50.000000000 +0100
@@ -0,0 +1,196 @@
+/*
+ *		Definitions for the UDP-Lite (RFC 3828) code.
+ *
+ * Version:	see net/ipv4/udplite.c
+ *
+ * Authors:	Gerrit Renker, <gerrit@erg.abdn.ac.uk>
+ *		William Stanislaus, <william@erg.abdn.ac.uk>
+ *
+ * Fixes:
+ * Changes:
+ *
+ *		This program is free software; you can redistribute it and/or
+ *		modify it under the terms of the GNU General Public License
+ *		as published by the Free Software Foundation; either version
+ *		2 of the License, or (at your option) any later version.
+ *
+ *      	NOTE: In UDP-Lite the checksum MUST always be computed, hence there is
+ *		no UDPLITE_CSUM_DEFAULT and no UDPLITE_CSUM_NOXMIT here.
+ */
+
+#ifndef _UDPLITE_H
+#define _UDPLITE_H
+#ifdef CONFIG_IP_UDPLITE
+#include <linux/udplite.h>
+#include <net/snmp.h>
+#include <net/udp.h>       	/* for UDP_HTABLE_SIZE and proc structures */
+/*
+ * 	Global variables
+ */
+extern struct proto 		udplite_prot;
+extern struct hlist_head 	udplite_hash[UDP_HTABLE_SIZE];
+extern rwlock_t 		udplite_hash_lock;
+extern int 			udplite_port_rover;
+
+/**
+ *	struct udp_lite_skb  -  UDP-Lite private variables
+ *
+ *	@cscov:   checksum coverage length
+ *	@partial: flag, if set indicates partial csum coverage
+ *	@header:  private variables used by IPv4/v6 (thanks tcp.h!)
+ */
+struct udplite_skb_cb {
+	union {
+		struct inet_skb_parm	h4;
+#if defined(CONFIG_IPV6) || defined (CONFIG_IPV6_MODULE)
+		struct inet6_skb_parm	h6;
+#endif
+	} header;
+	__u16		cscov;
+	__u8		partial;
+};
+#define UDPLITE_SKB_CB(skb)	((struct udplite_skb_cb *)&((skb)->cb))
+
+
+/*
+ * 	Inlined functions shared between UDP-Litev4 and UDP-Litev6
+ */
+
+/* shared port space between ipv4/udplite.c and ipv6/udplite.c, cf. udp.c */
+static inline int udplite_lport_inuse(u16 num)
+{
+	struct sock        *sk;
+	struct hlist_node  *node;
+
+	sk_for_each(sk, node, &udplite_hash[num & (UDP_HTABLE_SIZE - 1)])
+	    if (inet_sk(sk)->num == num)
+		return 1;
+	return 0;
+}
+
+/*
+ *  Calculate / check variable-length UDP-Lite checksum
+ *  skb->csum holds the sum of the IPv4 or IPv6 pseudo-header.
+ */
+static inline u16  __udplite_checksum_complete(struct sk_buff *skb)
+{
+	if (! UDPLITE_SKB_CB(skb)->partial)
+		return __skb_checksum_complete(skb);
+
+	return  csum_fold(skb_checksum(skb, 0, UDPLITE_SKB_CB(skb)->cscov,
+			  skb->csum));
+}
+
+static inline u16  udplite_checksum_complete(struct sk_buff *skb)
+{
+	return skb->ip_summed != CHECKSUM_UNNECESSARY &&
+		__udplite_checksum_complete(skb);
+}
+
+/*
+ * UDP-Lite checksum computation is all in software, hence simpler getfrag
+ */
+static inline int udplite_getfrag(void *from, char *to, int   offset, int len,
+				  int odd, struct sk_buff *skb)
+{
+	return memcpy_fromiovecend(to, (struct iovec *) from, offset, len);
+}
+
+
+/*
+ *  	net/ipv4/udplite.c
+ */
+extern void 	    udplite4_register(void);
+extern int 	    udplite4_mib_init(void);
+extern unsigned int udplite_poll(struct file *file, struct socket *sock,
+				 poll_table * wait);
+extern int 	    udplite_rcv(struct sk_buff *skb);
+extern void	    udplite_err(struct sk_buff *, u32);
+extern int	    udplite_disconnect(struct sock *sk, int flags);
+extern int	    udplite_ioctl(struct sock *sk, int cmd, unsigned long arg);
+extern int	    udplite_sendmsg(struct kiocb *iocb, struct sock *sk,
+				    struct msghdr *msg, size_t len);
+#ifdef CONFIG_PROC_FS
+extern int	   udplite_proc_register(struct udp_seq_afinfo *afinfo);
+extern void 	   udplite_proc_unregister(struct udp_seq_afinfo *afinfo);
+
+extern int  	   udplite4_proc_init(void);
+extern void	   udplite4_proc_exit(void);
+#endif  /* CONFIG_PROC_FS */
+
+
+/*
+ *	net/ipv6/udplite.c
+ */
+#if defined(CONFIG_IPV6) || defined (CONFIG_IPV6_MODULE)
+extern struct proto udplitev6_prot;
+extern void	    udplitev6_init(void);
+extern void	    udplitev6_exit(void);
+#ifdef CONFIG_PROC_FS
+extern int	    udplite6_proc_init(void);
+extern void	    udplite6_proc_exit(void);
+#endif
+#endif  /* CONFIG_IPV6 */
+
+
+/*
+ *	MIB / runtime statistics for UDP-Litev4 and UDP-Litev6.
+ */
+enum {
+	UDPLITE_MIB_NUM = 0,
+	UDPLITE_MIB_INDATAGRAMS,     /* total received datagramns             */
+	UDPLITE_MIB_IN_PARTIALCOV,   /* rcvd datagrams with partial coverage  */
+	UDPLITE_MIB_NOPORTS,	     /* rcvd datagrams to wrong ports         */
+	UDPLITE_MIB_INERRORS,	     /* total erroneous received datagrams    */
+	UDPLITE_MIB_IN_BAD_COV,	     /* checksum coverage errors              */
+	UDPLITE_MIB_IN_BAD_CSUM,     /* checksum itself did not qualify       */
+	UDPLITE_MIB_OUTDATAGRAMS,    /* total sent datagrams                  */
+	UDPLITE_MIB_OUT_PARTIALCOV,  /* sent datagrams with partial coverage  */
+	__UDPLITE_MIB_MAX
+};
+
+struct udplite_mib {
+	unsigned long	mibs[__UDPLITE_MIB_MAX];
+} __SNMP_MIB_ALIGN__;
+
+
+DECLARE_SNMP_STAT(struct udplite_mib, udplite_statistics);
+#define UDPLITE_INC_STATS(f)	    SNMP_INC_STATS(udplite_statistics, f)
+#define UDPLITE_INC_STATS_BH(f)     SNMP_INC_STATS_BH(udplite_statistics, f)
+#define UDPLITE_INC_STATS_USER(f)   SNMP_INC_STATS_USER(udplite_statistics, f)
+
+#if defined(CONFIG_IPV6) || defined (CONFIG_IPV6_MODULE)
+DECLARE_SNMP_STAT(struct udplite_mib, udplite_stats_in6);
+#define UDPLITE6_INC_STATS(f)	    SNMP_INC_STATS(udplite_stats_in6, f)
+#define UDPLITE6_INC_STATS_BH(f)    SNMP_INC_STATS_BH(udplite_stats_in6, f)
+#define UDPLITE6_INC_STATS_USER(f)  SNMP_INC_STATS_USER(udplite_stats_in6, f)
+#endif  /* CONFIG_IPV6 */
+
+
+
+#else   /* CONFIG_IP_UDPLITE is not enabled */
+
+
+/* empty v4 function definitions: */
+inline void 	    	udplite4_register(void)  { }
+#define                 udplite4_mib_init(void)  (1)
+#ifdef CONFIG_PROC_FS
+#define			udplite4_proc_init(void) (0)
+inline void		udplite4_proc_exit(void) { }
+#endif  /* CONFIG_PROC_FS */
+
+
+/* empty v6 function definitions: */
+#if defined(CONFIG_IPV6) || defined (CONFIG_IPV6_MODULE)
+inline void		udplitev6_init(void)     { }
+inline void		udplitev6_exit(void)     { }
+#ifdef CONFIG_PROC_FS
+#define			udplite6_proc_init(void) (0)
+inline void		udplite6_proc_exit(void) { }
+#endif  /* CONFIG_PROC_FS */
+#endif  /* CONFIG_IPV6  */
+
+#endif  /* CONFIG_IP_UDPLITE */
+
+
+#endif	/* _UDPLITE_H */
diff -Nurp  a/include/linux/socket.h b/include/linux/socket.h
--- a/include/linux/socket.h	2006-07-06 09:08:15.000000000 +0100
+++ b/include/linux/socket.h	2006-07-14 10:17:50.000000000 +0100
@@ -264,6 +264,7 @@ struct ucred {
 #define SOL_IPV6	41
 #define SOL_ICMPV6	58
 #define SOL_SCTP	132
+#define SOL_UDPLITE	136     /* UDP-Lite (RFC 3828) */
 #define SOL_RAW		255
 #define SOL_IPX		256
 #define SOL_AX25	257
diff -Nurp  a/include/linux/in.h b/include/linux/in.h
--- a/include/linux/in.h	2006-06-19 08:45:25.000000000 +0100
+++ b/include/linux/in.h	2006-07-14 10:17:50.000000000 +0100
@@ -44,6 +44,7 @@ enum {
 
   IPPROTO_COMP   = 108,                /* Compression Header protocol */
   IPPROTO_SCTP   = 132,		/* Stream Control Transport Protocol	*/
+  IPPROTO_UDPLITE = 136,	/* UDP-Lite Protocol (RFC 3828)	        */
 
   IPPROTO_RAW	 = 255,		/* Raw IP packets			*/
   IPPROTO_MAX
diff -Nurp  a/net/core/sock.c b/net/core/sock.c
--- a/net/core/sock.c	2006-07-06 09:08:24.000000000 +0100
+++ b/net/core/sock.c	2006-07-14 10:17:50.000000000 +0100
@@ -479,7 +479,12 @@ set_rcvbuf:
 			break;
 
 	 	case SO_NO_CHECK:
-			sk->sk_no_check = valbool;
+			/* UDP-Lite (RFC 3828) mandates checksumming,
+			 * hence user must not enable this option.   */
+			if (sk->sk_protocol == IPPROTO_UDPLITE)
+				ret = -EOPNOTSUPP;
+			else
+ 			    sk->sk_no_check = valbool;
 			break;
 
 		case SO_PRIORITY:
diff -Nurp  a/Documentation/networking/udplite.txt b/Documentation/networking/udplite.txt
--- a/Documentation/networking/udplite.txt	1970-01-01 01:00:00.000000000 +0100
+++ b/Documentation/networking/udplite.txt	2006-07-14 10:17:50.000000000 +0100
@@ -0,0 +1,334 @@
+  ===========================================================================
+                      The UDP-Lite protocol (RFC 3828)
+  ===========================================================================
+  last modified: 	Fri 14th July 2006
+
+
+  UDP-Lite is a Standards-Track IETF transport protocol whose characteristic
+  is a variable-length checksum. This has advantages for transport of multimedia
+  (video, VoIP) over wireless networks, as partly damaged packets can still be
+  fed into the codec instead of being discarded due to a failed checksum test.
+
+  This file briefly describes the existing kernel support and the socket API.
+  For in-depth information, you can consult:
+
+   o The UDP-Lite Homepage: http://www.erg.abdn.ac.uk/users/gerrit/udp-lite/
+       Fom here you can always also pull the latest patch for the stable
+       kernel tree and example application source code.
+
+   o The UDP-Lite HOWTO on
+       http://www.erg.abdn.ac.uk/users/gerrit/udp-lite/files/UDP-Lite-HOWTO.txt
+
+   o The Ethereal UDP-Lite WiKi (with capture files):
+       http://wiki.ethereal.com/Lightweight_User_Datagram_Protocol
+
+   o The Protocol Spec, RFC 3828:    http://www.ietf.org/rfc/rfc3828.txt
+
+
+  I) APPLICATIONS
+
+  Several applications have been ported successfully to UDP-Lite. Ethereal
+  (now called wireshark) has UDP-Litev4/v6 support by default. The tarball on
+
+   http://www.erg.abdn.ac.uk/users/gerrit/udp-lite/files/udplite_linux.tar.gz
+
+  has source code for several v4/v6 client-server and network testing examples.
+
+  Porting applications to UDP-Lite is straightforward: only socket level and
+  IPPROTO need to be changed; senders additionally set the checksum coverage
+  length (default = header length = 8). Details are in the next section.
+  UDP-Lite is not enabled per default, set CONFIG_IP_UDPLITE=y to support it.
+
+
+  II) PROGRAMMING API
+
+  UDP-Lite provides a connectionless, unreliable datagram service and hence
+  uses the same socket type as UDP. In fact, porting from UDP to UDP-Lite is
+  dead easy: simply add `IPPROTO_UDPLITE' as the last argument of the socket(2)
+  call so that the statement looks like:
+
+      s = socket(PF_INET, SOCK_DGRAM, IPPROTO_UDPLITE);
+
+                      or, respectively,
+
+      s = socket(PF_INET6, SOCK_DGRAM, IPPROTO_UDPLITE);
+
+  Since both UDP-Litev4 and UDP-Litev6 are supported, the porting process is the
+  same in both occasions. With just this change you are able to run UDP-Lite
+  services or connect to UDP-Lite servers. The kernel will assume that you are
+  not interested in using partial checksum coverage and so emulate UDP mode.
+
+  To make use of the partial checksum coverage facilities requires setting just
+  one socket option which takes an integer specifying the coverage length:
+
+    * Sender checksum coverage: UDPLITE_SEND_CSCOV
+
+      For example,
+
+        int val = 20;
+        setsockopt(s, SOL_UDPLITE, UDPLITE_SEND_CSCOV, &val, sizeof(int));
+
+      sets the checksum coverage length to 20 bytes (12b data + 8b header).
+      Of each packet only the first 20 bytes (plus the pseudo-header) will be
+      checksummed. This is useful for RTP applications which have a 12-byte
+      base header.
+
+
+    * Receiver checksum coverage: UDPLITE_RECV_CSCOV
+
+      This option is the receiver-side analogue. It is truly optional, i.e. not
+      required to enable traffic with partial checksum coverage. Its function is
+      that of a traffic filter: when enabled, it instructs the kernel to drop
+      all packets which have a coverage _less_ than this value. For example, if
+      RTP and UDP headers are to be protected, a receiver can enforce that only
+      packets with a minimum coverage of 20 are admitted:
+
+        int min = 20;
+        setsockopt(s, SOL_UDPLITE, UDPLITE_RECV_CSCOV, &min, sizeof(int));
+
+  The calls to getsockopt(2) are analogous. Being an extension and not a stand-
+  alone protocol, all socket options known from UDP can be used in exactly the
+  same manner as before, e.g. UDP_CORK or UDP_ENCAP.
+
+  A detailed discussion of UDP-Lite checksum coverage options is in section IV.
+
+
+
+  III) HEADER FILES
+
+  The socket API requires support through header files in /usr/include:
+
+    * /usr/include/netinet/in.h
+        to define IPPROTO_UDPLITE
+
+    * /usr/include/netinet/udplite.h
+        for UDP-Lite header fields and protocol constants
+
+  For testing purposes, the following can serve as a `mini' header file:
+
+    #define IPPROTO_UDPLITE       136
+    #define SOL_UDPLITE           136
+    #define UDPLITE_SEND_CSCOV     10
+    #define UDPLITE_RECV_CSCOV     11
+
+  Ready-made header files for various distros are in the UDP-Lite tarball.
+
+
+
+  IV) KERNEL BEHAVIOUR WITH REGARD TO THE VARIOUS SOCKET OPTIONS
+
+  To enable debugging messages, the log level must be set to 8, as most
+  messages use the KERN_DEBUG level (7).
+
+
+  1) Sender Socket Options
+
+  If the sender specifies a value of 0 as coverage length, the module
+  assumes full coverage, transmits a packet with coverage length of 0
+  and according checksum.  If the sender specifies a coverage < 8 and
+  different from 0, the kernel assumes 8 as default value.  Finally,
+  if the specified coverage length exceeds the packet length, the packet
+  length is used instead as coverage length.
+
+
+  2) Receiver Socket Options
+
+  The receiver specifies the minimum value of the coverage length it
+  is willing to accept.  A value of 0 here indicates that the receiver
+  always wants the whole of the packet covered. In this case, all
+  partially covered packets are dropped and an error is logged.
+
+  It is not possible to specify illegal values (<0 and <8); in these
+  cases the default of 8 is assumed.
+
+  All packets arriving with a coverage value less than the specified
+  threshold are discarded, these events are also logged.
+
+
+  3) Disabling the Checksum Computation
+
+  On both sender and receiver, trying to disable the UDP-Lite checksum
+  (option SO_NO_CHECK) in setsockopt(2) results in an error. Thus
+
+        setsockopt(sockfd, SOL_SOCKET, SO_NO_CHECK,  ... );
+
+  will always result in an error, while
+
+        getsockopt(sockfd, SOL_SOCKET, SO_NO_CHECK, &value, ...);
+
+  will always return a value of 0 (meaning checksum enabled). Packets
+  with a zero checksum field are silently discarded by the receiver.
+
+
+  4) Fragmentation
+
+  The checksum computation respects both buffersize and MTU. The size
+  of UDP-Lite packets is determined by the size of the send buffer. The
+  minimum size of the send buffer is 2048 (defined as SOCK_MIN_SNDBUF
+  in include/net/sock.h), the default value is configurable as
+  net.core.wmem_default or via setting the SO_SNDBUF socket(7)
+  option. The maximum upper bound for the send buffer is determined
+  by net.core.wmem_max.
+
+  Given a payload size larger than the send buffer size, UDP-Lite will
+  split the payload into several individual packets, filling up the
+  send buffer size in each case.
+
+  The precise value also depends on the interface MTU. The interface MTU,
+  in turn, may trigger IP fragmentation. In this case, the generated
+  UDP-Lite packet is split into several IP packets, of which only the
+  first one contains the L4 header.
+
+  The send buffer size has implications on the checksum coverage length.
+  Consider the following example:
+
+  Payload: 1536 bytes          Send Buffer:     1024 bytes
+  MTU:     1500 bytes          Coverage Length:  856 bytes
+
+  UDP-Lite will ship the 1536 bytes in two separate packets:
+
+  Packet 1: 1024 payload + 8 byte header + 20 byte IP header = 1052 bytes
+  Packet 2:  512 payload + 8 byte header + 20 byte IP header =  540 bytes
+
+  The coverage packet covers the UDP-Lite header and 848 bytes of the
+  payload in the first packet, the second packet is fully covered. Note
+  that for the second packet, the coverage length exceeds the packet
+  length. The kernel always re-adjusts the coverage length to the packet
+  length in such cases.
+
+  As an example of what happens when one UDP-Lite packet is split into
+  several tiny fragments, consider the following example.
+
+  Payload: 1024 bytes            Send buffer size: 1024 bytes
+  MTU:      300 bytes            Coverage length:   575 bytes
+
+  +-+-----------+--------------+--------------+--------------+
+  |8|    272    |      280     |     280      |     280      |
+  +-+-----------+--------------+--------------+--------------+
+               280            560            840           1032
+                                    ^
+  *****checksum coverage*************
+
+  The UDP-Lite module generates one 1032 byte packet (1024 + 8 byte
+  header). According to the interface MTU, these are split into 4 IP
+  packets (280 byte IP payload + 20 byte IP header). The kernel module
+  sums the contents of the entire first two packets, plus 15 bytes of
+  the last packet before releasing the fragments to the IP module.
+
+  To see the analogous case for IPv6 fragmentation, consider a link
+  MTU of 1280 bytes and a write buffer of 3356 bytes. If the checksum
+  coverage is less than 1232 bytes (MTU minus IPv6/fragment header
+  lengths), only the first fragment needs to be considered. When using
+  larger checksum coverage lengths, each eligible fragment needs to be
+  checksummed. Suppose we have a checksum coverage of 3062. The buffer
+  of 3356 bytes will be split into the following fragments:
+
+    Fragment 1: 1280 bytes carrying  1232 bytes of UDP-Lite data
+    Fragment 2: 1280 bytes carrying  1232 bytes of UDP-Lite data
+    Fragment 3:  948 bytes carrying   900 bytes of UDP-Lite data
+
+  The first two fragments have to be checksummed in full, of the last
+  fragment only 598 (= 3062 - 2*1232) bytes are checksummed.
+
+  While it is important that such cases are dealt with correctly, they
+  are (annoyingly) rare: UDP-Lite is designed for optimising multimedia
+  performance over wireless (or generally noisy) links and thus smaller
+  coverage lenghts are likely to be expected.
+
+
+  V) UDP-LITE RUNTIME STATISTICS AND THEIR MEANING
+
+  Exceptional and error conditions are logged to syslog at the KERN_DEBUG
+  level.  Live statistics about UDP-Lite are available in /proc/net/snmp
+  and can (with newer versions of netstat) be viewed using
+
+                            netstat -svu
+
+  This displays UDP-Lite statistics variables, whose meaning is as follows.
+
+   InDatagrams:     Total number of received datagrams (as in UDP).
+
+   InPartialCov:    Number of received datagrams with csum coverage < length.
+
+   NoPorts:         Number of packets received to an unknown port (as in UDP).
+                    These cases are counted separately (not as InErrors).
+
+   InErrors:        Number of erroneous UDP-Lite packets. Errors include:
+                      * internal socket queue receive errors
+                      * packet too short (less than 8 bytes or stated
+                        coverage length exceeds received length)
+                      * xfrm4_policy_check() returned with error
+                      * application has specified larger min. coverage
+                        length than that of incoming packet (cf. below)
+                      * checksum coverage violated          (InBadCoverage)
+                      * bad checksum                        (InBadChecksum)
+
+   InBadCoverage:   Datagrams with invalid checksum coverage (also InErrors):
+                      * coverage length is less than the minimum 8
+                      * coverage length exceeds actual datagram length
+
+   InBadChecksum:   Datagrams with wrong checksum (also InErrors).
+
+   OutDatagrams:    Total number of sent datagrams.
+
+   OutPartialCov:   Number of sent datagrams  with csum coverage < length.
+
+  If a receiving application has specified a  minimum coverage length and
+  received a packet with a smaller coverage value than this, or if it has
+  specified full coverage (UDP mode) and received a partially covered packet,
+  this counts as error (under InErrors), and an error message is logged.
+
+  These statistics variables obey the following relations:
+
+     Total_received_datagrams   =  InDatagrams + InErrors + NoPorts
+
+                     InErrors  >=  InBadCoverage + InBadChecksum
+
+  The `>' includes other errors such as socket queue errors (usually 0). For
+  IPv6, the same statistics variables are used, using the `UdpLite6' prefix,
+  and can be viewed using "grep ^UdpLite6  /proc/net/snmp6". Alternatively,
+  you can use the `nstat' utility found in the iproute2 package.
+
+
+
+  VI) OPEN ISSUES
+
+  1) Sharing Code with UDP
+
+  On the mailing list there has been a suggestion to share code between
+  ipv?/udp.c and ipv?/udplite.c. There is indeed a potential for this, but the
+  challenge is not to mess up existing code. A line-by-line comparison between
+  ipv4/udp.c  and ipv4/udplite.c revealed the following similarities:
+
+    * 45 functions appear in udp.c and modified in udplite.c
+    * 26 are used with trivial modifications (sed/perl could do this)
+    * 10 are used with minor changes (structure / sockopt names)
+    *  8 require real modifications (in control flow and algorithm)
+    *  1 function is missing in udplite.c (no equivalent of udp_check())
+
+  A summary of this analysis can be found on
+    http://www.erg.abdn.ac.uk/users/gerrit/udp-lite/udplite-comparison.html
+
+  Further similarities include structure identifiers, hence udp_seq_afinfo is
+  e.g. already reused in UDP-Lite; as are several UDP constants.
+
+  However, I doubt whether merging will make things better. In a lot of cases
+  the code is functionally identical but depends and operates on global data
+  structures and locks which are exported as kernel symbols:
+    * udp(lite)_hash
+    * udp(lite)_hash_lock
+    * udp(lite)_port_rover
+    * udp(lite)_statistics
+  Hence it would be necessary to rename these globals apart in both source code
+  files, which would lead to a lot of #ifdefs in udp.c and introduce a fragile
+  dependency between both. Any change made in udp.c would thus immediately
+  propagate to udplite.c - manual revision remains inevitable.
+
+  2) MIB Standardisation
+
+  A MIB for UDP-Lite does not (yet) exist. For someone who is familiar with
+  SNMP/ASN.1 it would be an easy  task to turn the above variable definitions
+  into a MIB - in the same manner as per e.g. RFC 2013.  Anyone interested in
+  helping with this work should contact us at <gerrit@erg.abdn.ac.uk>.
+
+